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Circular polarization... does it have to be synchronous??
Group:
Warning... this could be mind blowing! Conventionally electromagnetic wave 'polarization' refers to the relative physical spatial orientation of the electric field vector (E) of an electromagnetic wave. It is commonly understood that polarization of electromagnetic waves may be either linear or circular. Linear Polarization (LP): Of course waves that are linearly polarized may have any arbitrary orientation angle (theta) with respect to a reference frame such as the earth's surface. For example most common linear amateur antennas produce and/or respond to waves of linear polarization, and these antennas produce either either horizonally or vertically polarized waves depending upon the orientation of the (linear) antenna with respect to the earth's surface (ground). As examples; a 1/2 wave length dipole for 10 meters hung at 30 feet between two trees of equal height produces a largely horizonally polarized wave and, a 2 meter 1/4 wave dipole mounted in the center of the roof of an automobile produces a largely vertically polarized wave. Of course as electromagnetic waves are propagated throughout an environment are never purely orientated and usually contain an ensemble of many orientations, because the waves are reflected from the ground, trees, buildings, mountains, bridges, moving vehicles, and sometimes propogated through moving and anisotropic media such as the ionosphere, etc... and so the multiple reflection surfaces at various angles to the earth's surface and/or refractions and Faraday rotations will conspire to "mix up" the original orientation of the E vector of a purely linear transmitted wave and usually produces a quite mixed polarization at distance from an emitting antenna. Malus' Law {I = Io [cos(theta)]^^2} describes the response of a linearly polarized receiving antenna to waves arriving at a polarization angle theta relative to the receiving antenna's preferred orientation. i.e a horizontally polarized antenna will produce maximum response to horizontally polarized waves and a minimum response (zero) to a vertically polarized wave and vice versa. Of course in practice, because of the multipath reflections and refractions the 'cross response' is never exactly zero or maximum as predicted by Malus Law. Just the same it is preferable to have the orientation of a receiving antenna 'aligned' with that of a particular transmitting antenna. In the HF region it is difficult for hams to "rotate" the orientation of their receiving antennas to maximize signal pickup based upon polarization, and so most hams are forced to take whatever response their relatively fixed antennas produce to the relatively unknown orientation of received waves. In military or commercial installations, where money and space may not be an issue, either electronically or mechanically derived spatial antenna polarization diversity can be utilized to maximize received signal strength based upon arriving polarization. Polarization diversity receivers... Circular Polarization (CP): Circular polarization describes the condition when an electromagnetic wave is spinning or rotating with around its direction of transmission. That is the electric vector (E) of a circularly polarized electromagnetic wave is rotating with an angular velocity as the wave travels through space. This is in contrast to the E vector of a linearly polarized wave which merely oscillates in one linear direction. Just as with linear polarization (horizontal and vertical) there are two different distinctly possible orientations for circularily polarized waves, these are known as Right Hand Circular Polarization (RHCP) and Left Hand Circular Polarization (LHCP). There are actually two well known conventions used to label R and L CP depending upon the community of interest, namely physics/optics and electrical/electronics. Usually electronics folks refer the direction of rotation to the rotation of the E vector around the direction of travel from a transmitting antenna, whilst the optical physicists refer the rotation of E around the direction of travel towards a receiving lens. It's the same as the definition of up and down, it's all in the eye of the beholder. Regardless there are two orientations for CP Apparently circular polarization is less commonly known and understood than linear (horizontal/vertical) polarization especially among hams. There exist RHCP antennas and there are LHCP antennas. Perhaps one of the easiest forms of CP antennas for hams to understand are the axial mode helix antennas first discovered/studied by the great radio astronomer/ham John Kraus W8JK. Axial mode helix antennas may be "wound" with either a right hand thread or a left hand thread. Again Malus Law applies, in an easily applied modified form and so... RHCP receiving antennas respond to RHCP waves and LHCP receiving antennas respond to LHCP waves. A purely RHCP antenna will produce zero response to an LHCP wave, etc... An interesting effect happens upon reflection of CP waves. An RHCP wave reflected from a perfectly reflecting surface returns (is echoed) as a LHCP wave! CP propagation is often used in Satellite communications where a satellite may use both RHCP and LHCP transmitting antennas on the same frequency for communicating independently with two different ground stations using R and L CP antennas on the same frequency. CP frequency diversity doubles channel capacity! Yet another common form of CP antenna uses crossed linear antennas fed with a 90 degree (Pi/2) phase difference excitation. As far as I know all currently known CP antennas such as axial mode helixes and crossed 90 degree linear arrays produce CP waves where the angular velocity of rotation is one revolution per cycle of the RF carrier, or in other words one radian of circular rotation for each radian of frequency transmitted. In other words most well known CP antennas produce ONLY synchronous CP, where the angular velocity of rotation of the E vector is synchronized exactly with the frequency of the wave being transmitted. I believe that the well known and understood situation of purely synchronous CP is NOT necessesarily the only form of CP. Warning... The following may be an invention! Consider the case of a linear antenna, say a dipole, fed from a feed line over rotating slip rings, such that the antenna can be rotated while it is transmitting. Now transmit on that dipole antenna whilst mechanically spinning it clockwise [RHCP?] (with a mechanical motor of some kind). The dipole antenna is linear and thuse emits linear polariztion, except it is mechanically spinning, and so the E vector emanating from the antenna will be rotating with respect to its direction of travel. In this case the angular velocity of the motor that spins the linear antenna need not be synchronous with the frequency being radiated. For example we could mechanically spin the antenna at 330 rpm while transmitting a carrier of 1 GHz. This would most certainly produce circular polarization. For is not the E vector spinning at 330 revs! In fact the astute newsreader may note that we need not use a motor to rotate the antenna. In fact, I can propose several ways of "electronically" rotating the linear antenna at any arbitrary angular velocity, not necessarily synchronous with the transmitted frequency and so produce a so-called non-synchronous CP at any desired rate of rotation. Clearly, according to Malus Law, the maximum response to the non-synchronous CP received waves from this 'rotating' antenna contraption would be from a similarily rotating receiving antenna! Question? What would be the response of an axial mode helix antenna or say crossed 90 degree fed dipoles or any other "synchronous" CP antenna to such a non-synchronous wave produces by a rotating antenna? Would the response of a syncrhronous axial mode helix be less than that of a sympathetically rotating receiving antenna? What? Thoughts, comments? -- Pete K1PO -- Indialantic By-the-Sea, FL |
Circular polarization... does it have to be synchronous??
Peter O. Brackett wrote:
. . . It is commonly understood that polarization of electromagnetic waves may be either linear or circular. Then some education is in order. Electromagnetic waves are elliptically polarized. The two extreme special cases of this are linear and circular (with axial ratio of zero -- or infinite depending on your choice of definition -- and one respectively). There are an infinite number of other possible elliptical polarizations with different axial ratios. Linear Polarization (LP): Of course waves that are linearly polarized may have any arbitrary orientation angle (theta) with respect to a reference frame such as the earth's surface. For example most common linear amateur antennas produce and/or respond to waves of linear polarization, and these antennas produce either either horizonally or vertically polarized waves depending upon the orientation of the (linear) antenna with respect to the earth's surface (ground). Of course linear polarization can have any orientation, not just vertical or horizontal. And even those terms lose meaning when away from the Earth. However, it's often convenient to mathematically separate waves into two superposed components of horizontal and vertical polarization. As examples; a 1/2 wave length dipole for 10 meters hung at 30 feet between two trees of equal height produces a largely horizonally polarized wave and, a 2 meter 1/4 wave dipole mounted in the center of the roof of an automobile produces a largely vertically polarized wave. The polarization of the dipole signal will be purely horizontal only directly broadside. The signal off the ends are purely vertically polarized, and in other directions neither horizontal nor vertical. Of course as electromagnetic waves are propagated throughout an environment are never purely orientated and usually contain an ensemble of many orientations, because the waves are reflected from the ground, trees, buildings, mountains, bridges, moving vehicles, and sometimes propogated through moving and anisotropic media such as the ionosphere, etc... and so the multiple reflection surfaces at various angles to the earth's surface and/or refractions and Faraday rotations will conspire to "mix up" the original orientation of the E vector of a purely linear transmitted wave and usually produces a quite mixed polarization at distance from an emitting antenna. By "mixed" polarization, I assume you mean a single polarization which is neither horizontal nor vertical and can be described as a "mixture" of a purely horizontal and a purely vertical wave. Malus' Law {I = Io [cos(theta)]^^2} describes the response of a linearly polarized receiving antenna to waves arriving at a polarization angle theta relative to the receiving antenna's preferred orientation. i.e a horizontally polarized antenna will produce maximum response to horizontally polarized waves and a minimum response (zero) to a vertically polarized wave and vice versa. Of course in practice, because of the multipath reflections and refractions the 'cross response' is never exactly zero or maximum as predicted by Malus Law. It's also difficult to get the polarizations of the antennas exactly right. Just the same it is preferable to have the orientation of a receiving antenna 'aligned' with that of a particular transmitting antenna. In the HF region it is difficult for hams to "rotate" the orientation of their receiving antennas to maximize signal pickup based upon polarization, and so most hams are forced to take whatever response their relatively fixed antennas produce to the relatively unknown orientation of received waves. There's no advantage at HF of having the antenna orientations the same if the path is via the ionosphere. In military or commercial installations, where money and space may not be an issue, either electronically or mechanically derived spatial antenna polarization diversity can be utilized to maximize received signal strength based upon arriving polarization. Polarization diversity receivers... Circular Polarization (CP): . . . Again Malus Law applies, in an easily applied modified form and so... RHCP receiving antennas respond to RHCP waves and LHCP receiving antennas respond to LHCP waves. A purely RHCP antenna will produce zero response to an LHCP wave, etc... Interesting. Can you work an example for us? I'm curious as to what you use for theta in the "law's" equation. An interesting effect happens upon reflection of CP waves. An RHCP wave reflected from a perfectly reflecting surface returns (is echoed) as a LHCP wave! Only if it strikes the surface directly head-on. Otherwise you get an elliptically polarized wave. The axial ratio depends on the angle of incidence and, if the reflector isn't perfectly conducting, on the impedance of the surface. CP propagation is often used in Satellite communications where a satellite may use both RHCP and LHCP transmitting antennas on the same frequency for communicating independently with two different ground stations using R and L CP antennas on the same frequency. CP frequency diversity doubles channel capacity! I think you mean that polarization (not frequency) diversity doubles channel capacity. Yet another common form of CP antenna uses crossed linear antennas fed with a 90 degree (Pi/2) phase difference excitation. As far as I know all currently known CP antennas such as axial mode helixes and crossed 90 degree linear arrays produce CP waves where the angular velocity of rotation is one revolution per cycle of the RF carrier, or in other words one radian of circular rotation for each radian of frequency transmitted. In other words most well known CP antennas produce ONLY synchronous CP, where the angular velocity of rotation of the E vector is synchronized exactly with the frequency of the wave being transmitted. That is, in fact, the definition of circular or elliptical polarization. I believe that the well known and understood situation of purely synchronous CP is NOT necessesarily the only form of CP. It's the only one which fits the definition. If you choose to rotate the polarization at some other rate, you should call it something else. Warning... The following may be an invention! Consider the case of a linear antenna, say a dipole, fed from a feed line over rotating slip rings, such that the antenna can be rotated while it is transmitting. Now transmit on that dipole antenna whilst mechanically spinning it clockwise [RHCP?] (with a mechanical motor of some kind). The dipole antenna is linear and thuse emits linear polariztion, except it is mechanically spinning, and so the E vector emanating from the antenna will be rotating with respect to its direction of travel. In this case the angular velocity of the motor that spins the linear antenna need not be synchronous with the frequency being radiated. For example we could mechanically spin the antenna at 330 rpm while transmitting a carrier of 1 GHz. This would most certainly produce circular polarization. For is not the E vector spinning at 330 revs! Sorry, it doesn't. An unavoidable side effect of the synchronicity change is that the amplitude of the E field still changes at a 1 GHz rate, going through a complete cycle from max to zero to max to zero to max each nanosecond. A circularly polarized wave doesn't change amplitude with time. A non-circular elliptical wave changes amplitude but not fully to zero each cycle. Circularly polarized waves have many characteristics and particular relationships to linearly polarized waves. The waves you're producing don't have some of these characteristics, like the constant amplitude. Your method doesn't produce circularly polarized waves even though the polarization does indeed change with time. In fact the astute newsreader may note that we need not use a motor to rotate the antenna. In fact, I can propose several ways of "electronically" rotating the linear antenna at any arbitrary angular velocity, not necessarily synchronous with the transmitted frequency and so produce a so-called non-synchronous CP at any desired rate of rotation. Clearly, according to Malus Law, the maximum response to the non-synchronous CP received waves from this 'rotating' antenna contraption would be from a similarily rotating receiving antenna! Question? What would be the response of an axial mode helix antenna or say crossed 90 degree fed dipoles or any other "synchronous" CP antenna to such a non-synchronous wave produces by a rotating antenna? Because a circularly polarized antenna responds equally well to all orientations of linear polarization, the normal helix wouldn't be aware of the polarization rotation -- unless the polarization rotation was fast enough to be nearly synchronous. Would the response of a syncrhronous axial mode helix be less than that of a sympathetically rotating receiving antenna? No. What? Thoughts, comments? Sorry, I didn't find it "mind-blowing". -- Pete K1PO -- Indialantic By-the-Sea, FL Roy Lewallen, W7EL |
Circular polarization... does it have to be synchronous??
"Peter O. Brackett" wrote in message ... What would be the response of an axial mode helix antenna or say crossed 90 degree fed dipoles or any other "synchronous" CP antenna to such a non-synchronous wave produces by a rotating antenna? the same as for a linearly polarized wave. since the rotation frequency is much lower than the carrier frequency (unless you are considering elf transmissions) during any time period consisting of several cycles of the carrier it would appear stationary to the antenna. Would the response of a syncrhronous axial mode helix be less than that of a sympathetically rotating receiving antenna? it wouldn't matter. now if there were two linearly polarized antennas rotating such that their polarizations stayed in sync that would at least reduce the fading caused by one rotating and the other being stationary. but only if the path between them didn't produce any rotation or randomization of the polarization, so essentially only for short paths with no reflective multi-path or other effects. seems like more trouble than its worth... what would you gain from it anyway? |
Circular polarization... does it have to be synchronous??
Roy:
Thanks for your well thought out responses. See my comments below interspersed with snippings of your response. [snip] "Roy Lewallen" wrote in message treetonline... Peter O. Brackett wrote: . . . It is commonly understood that polarization of electromagnetic waves may be either linear or circular. Then some education is in order. Electromagnetic waves are elliptically polarized. The two extreme special cases of this are linear and circular (with axial ratio of zero -- or infinite depending on your choice of definition -- and one respectively). There are an infinite number of other possible elliptical polarizations with different axial ratios. [snip] I agree. My statement was not quite precise. I should have stated something like, "it is commonly understood that polarization of waves may be categorized as being either linear or elliptical, and in the elliptical category the special case of circular polarization occurs whenever the major and minor axes of the elliptical polarization are equal." [snip] Of course linear polarization can have any orientation, not just vertical or horizontal. And even those terms lose meaning when away from the Earth. However, it's often convenient to mathematically separate waves into two superposed components of horizontal and vertical polarization. [snip] Agreed! [snip] The polarization of the dipole signal will be purely horizontal only directly broadside. The signal off the ends are purely vertically polarized, and in other directions neither horizontal nor vertical. [snip] Agreed! It is relatively difficult, and perhaps even impossible to arrange the physical configuration of an antenna such that it emits (or receives) wave of purely one category of polarization. In practice though many antennas concentrate a major part of their emissions in one polariztion form. [snip] By "mixed" polarization, I assume you mean a single polarization which is neither horizontal nor vertical and can be described as a "mixture" of a purely horizontal and a purely vertical wave. [snip] No. What I meant by "mixed" was that, just as with daylight for example, the field contains many polarization orientations. In fact usually outside in daylight most of the light we see with our eyes contains very nearly an equal distribution of all polariztions. An exception in the sky's light is perpedicular to the suns rays where because of upper atmospheric conditions light becomes slightly polarized. It is claimed that some people can actually "see" this polarized light differently than normal light. (Haider's Brush) Of course many people know that reflected light, for example from the surface of a lake, becomes highly polarized. This is the reason that "Polaroid" sunglasses are used by sportsmen and others to reduce perceived glare from reflective surfaces. That said, mixed polarization, is also largely the case of HF waves received over ionospheric paths. In other words HF waves received over long distances will contain a wide distribution of linear and perhaps circular polarizations. Thus rendering the use of single polarized antennas relatively useless at HF by amateurs. Unless of course one is prepared to pay the significant price in space and equipment to implement a polarization diversity receiving system. [snip] It's also difficult to get the polarizations of the antennas exactly right. [snip] Agreed! [snip] There's no advantage at HF of having the antenna orientations the same if the path is via the ionosphere. [snip] True for a single antenna and receiver, which is the usual case for a ham, see my remarks above. However if one is willing to pay the price for several antennas and synchronous receiving systems then receiving gains can often be obtained by the exploitation of polarization diversity. [snip] Interesting. Can you work an example for us? I'm curious as to what you use for theta in the "law's" equation. [snip] Theta is just the relative orientation of the polarization of the transmitting and receiving antennas, or in the case of an optical polarimeter, the relative orientations of the polarizing and analyzing polarizer. Theta is commonly illustrated in undergraduate optical laboratories and science experiment kits, using a couple of pieces of "Polaroid" film with the polarization angle marked on the film by a notch or other marking. When the two films are aligned with their polariztion direction perpendicular there is no light propagation, i.e. theta is 90 degrees, and when they are aligned with theta equal to zero then light is propagated. In the case of dipole antennas, theta is zero when two antennas are co-linear and theta is 90 degrees when the antennas are perpendicular. [snip] Only if it strikes the surface directly head-on. Otherwise you get an elliptically polarized wave. The axial ratio depends on the angle of incidence and, if the reflector isn't perfectly conducting, on the impedance of the surface. [snip] Agreed! A very intersting optical phenomena to observe is to look at a mirror through an optical circular polarizer (polarizer in tandem with a 1/4 wave retarder) which renders the "image" of the circular polarizer to be black. i.e. the optical circular polarizer eliminates the reflection. This technique is widely used to eliminate reflections from information displays that must operate in high sunlight with good sunlight readability. High quality high transmissivity optical circular polarizers are relatively expensive, and so one does not find such technology applied to consumer displays like computer monitors, TV sets or IPhones, however optical circular polarizers are widely used by the military for eliminating sunlight reflections from their (expensive) information displays. [snip] CP propagation is often used in Satellite communications where a satellite may use both RHCP and LHCP transmitting antennas on the same frequency for communicating independently with two different ground stations using R and L CP antennas on the same frequency. CP frequency diversity doubles channel capacity! I think you mean that polarization (not frequency) diversity doubles channel capacity. [snip] Yep that's exactly what I meant, but my fingers did not type it that way. Thanks! [snip] angular velocity of rotation is one revolution per cycle of the RF carrier, or in other words one radian of circular rotation for each radian of frequency transmitted. In other words most well known CP antennas produce ONLY synchronous CP, where the angular velocity of rotation of the E vector is synchronized exactly with the frequency of the wave being transmitted. That is, in fact, the definition of circular or elliptical polarization. [snip] Agreed, both you and I and thousands of others know that. [smile] [snip] I believe that the well known and understood situation of purely synchronous CP is NOT necessesarily the only form of CP. It's the only one which fits the definition. If you choose to rotate the polarization at some other rate, you should call it something else. [snip] Definition! Gosh where is Cecil when you need him? The only problem with definitions is that there are so many of them! --------------------------------------------------------------------------------------------- "When I use a word, Humpty Dumpty said in a rather scornful tone, "It means just what I chose it to mean - neither more nor less." "The question is," said Alice, "whether you can make words mean so many different things." "The question is," said Humpty Dumpty, "which is to be Master - that's all." -- Lewis Caroll, from Through the Looking Glass -------------------------------------------------------------------------------------------- [grin] [snip] Sorry, it doesn't. An unavoidable side effect of the synchronicity change is that the amplitude of the E field still changes at a 1 GHz rate, going through a complete cycle from max to zero to max to zero to max each nanosecond. A circularly polarized wave doesn't change amplitude with time. A non-circular elliptical wave changes amplitude but not fully to zero each cycle. [snip] Here there is a bit of fuzziness... I agree that the E field of a wave is always changing at the RF carrier frequency since it is an AC waveform. Alternating current is always changing! And so a 1 GHz carrier will always have an E field that oscillates back and forth at the carrier (center?) frequency when analyzed by a (linear) polarimeter. I disagree with you that a circular polarized wave has a constant E field. Even in the case of a purely circularly polarized the E field still oscillates at the carrier (center?) frequency when analyzed by a linear polarizer. i.e. if a purely CP wave is received on a linear polarized antenna the detected E field (Volts per meter) will be observed to be oscillating at the carrier frequency. However if received on a purely CP responding antenna this oscillating E fileld will appear to be constant. The E field vector can be considered to be similar to the image of a spoke on a rolling wheel. The radius of the spoke is constant, but it's projection on the ground over which the wheel is rolling will always be oscillating in length. [snip] Circularly polarized waves have many characteristics and particular relationships to linearly polarized waves. The waves you're producing don't have some of these characteristics, like the constant amplitude. Your method doesn't produce circularly polarized waves even though the polarization does indeed change with time. [snip] I beg to disagree. The waves that I am describing are exactly the same. Consider if the mechanical motor that spins my linear antenna spins at exactly the carrier frequency. There would be then no way to tell the difference between the two. [snip] Because a circularly polarized antenna responds equally well to all orientations of linear polarization, the normal helix wouldn't be aware of the polarization rotation -- unless the polarization rotation was fast enough to be nearly synchronous. [snip] Heh, heh... what would you consider to be "fast enough"? Would the rate of spin have to be 99-44/100 percent of the synchronous frequency? Or would it have to be closer than that? At what magic spin frequency would the two be indistinguisable. FWIW... I can propose a scheme that will electronically rotate the linear antenna at any desired frequency, at least up to the accuracy of modern atomic clock standards. [snip] Sorry, I didn't find it "mind-blowing". [snip] Roy, I don't belive you have thought about it hard enough yet, for clearly this idea has already "blown" your mind! For did you not state above that a circular carrier wave has a constant amplitude? A radio wave with constant aplitude, indeed! Something must be blown! At zero frequency, how would a constant wave propagate? This assumption/view that zero frequency wave can propagate is akin to Cecil's view that there are no reflections at DC. I don't mean to be facitious and I am quite serious about all of this. Just because no one has ever considered non-synchronous circular polariztion before does not mean that it doesn't exist, or that it may not be useful. Me? I have already thought of several potential uses for non-synchronous circular polarization. How about polariztion frequency modulation? Or... how about polariztion phase modulation? Or... Got you thinking yet? Thanks again for your clearly interesting comments and feedback. More thoughts, comments? -- Pete K1PO -- Indialantic By-the-Sea, FL |
Circular polarization... does it have to be synchronous??
Dave:
[snip] it wouldn't matter. now if there were two linearly polarized antennas rotating such that their polarizations stayed in sync that would at least reduce the fading caused by one rotating and the other being stationary. but only if the path between them didn't produce any rotation or randomization of the polarization, so essentially only for short paths with no reflective multi-path or other effects. seems like more trouble than its worth... what would you gain from it anyway? [snip] A better understanding of circular polarization? The design of a new polariztion locked loop, akin to a phase locked loop, but... More? .. .. .. Thanks! -- Pete K1PO -- Indialantic By-the-Sea, FL |
Circular polarization... does it have to be synchronous??
On Dec 6, 10:11*am, "Peter O. Brackett"
wrote: Roy: Thanks for your well thought out responses. See my comments below interspersed with snippings of your response. [snip]"Roy Lewallen" wrote in message treetonline... Peter O.. Brackett wrote: . . . It is commonly understood that polarization of electromagnetic waves may be either linear or circular. Then some education is in order. Electromagnetic waves are elliptically polarized. The two extreme special cases of this are linear and circular (with axial ratio of zero -- or infinite depending on your choice of definition -- and one respectively). There are an infinite number of other possible elliptical polarizations with different axial ratios. [snip] I agree. *My statement was not quite precise. I should have stated something like, "it is commonly understood that polarization of waves may be categorized as being either linear or elliptical, and in the elliptical category the special case of circular polarization occurs whenever the major and minor axes of the elliptical polarization are equal." [snip] Of course linear polarization can have any orientation, not just vertical or horizontal. And even those terms lose meaning when away from the Earth. However, it's often convenient to mathematically separate waves into two superposed components of horizontal and vertical polarization. [snip] Agreed! [snip] The polarization of the dipole signal will be purely horizontal only directly broadside. The signal off the ends are purely vertically polarized, and in other directions neither horizontal nor vertical. [snip] Agreed! *It is relatively difficult, and perhaps even impossible to arrange the physical configuration of an antenna such that it emits (or receives) wave of purely one category of polarization. In practice though many antennas concentrate a major part of their emissions in one polariztion form. [snip] By "mixed" polarization, I assume you mean a single polarization which is neither horizontal nor vertical and can be described as a "mixture" of a purely horizontal and a purely vertical wave. [snip] No. *What I meant by "mixed" was that, just as with daylight for example, the field contains many polarization orientations. *In fact usually outside in daylight most of the light we see with our eyes contains very nearly an equal distribution of all polariztions. *An exception in the sky's light is perpedicular to *the suns rays where because of upper atmospheric conditions light becomes slightly polarized. *It is claimed that some people can actually "see" this polarized light differently than normal light. (Haider's Brush) *Of course many people know that reflected light, for example from the surface of a lake, becomes highly polarized. *This is the reason that "Polaroid" sunglasses are used by sportsmen and others to reduce perceived glare from reflective surfaces. That said, mixed polarization, is also largely the case of HF waves received over ionospheric paths. *In other words HF waves received over long distances will contain a wide distribution of linear and perhaps circular polarizations. *Thus rendering the use of single polarized antennas relatively useless at HF by amateurs. *Unless of course one is prepared to pay the significant price in space and equipment to implement a polarization diversity receiving system. [snip] It's also difficult to get the polarizations of the antennas exactly right. [snip] Agreed! [snip] There's no advantage at HF of having the antenna orientations the same if the path is via the ionosphere. [snip] True for a single antenna and receiver, which is the usual case for a ham, see my remarks above. However if one is willing to pay the price for several antennas and synchronous receiving systems then receiving gains can often be obtained by the exploitation of polarization diversity. *[snip] Interesting. Can you work an example for us? I'm curious as to what you use for theta in the "law's" equation. [snip] Theta is just the relative orientation of the polarization of the transmitting and receiving antennas, or in the case of an optical polarimeter, the relative orientations of the polarizing and analyzing polarizer. Theta is commonly illustrated in undergraduate optical laboratories and science experiment kits, using a couple of pieces of *"Polaroid" film with the polarization angle marked on the film by a notch or other marking. *When the two films are aligned with their polariztion direction perpendicular there is no light propagation, i.e. theta is 90 degrees, and when they are aligned with theta equal to zero then light is propagated. In the case of dipole antennas, theta is zero when two antennas are co-linear and theta is 90 degrees when the antennas are perpendicular. [snip] Only if it strikes the surface directly head-on. Otherwise you get an elliptically polarized wave. The axial ratio depends on the angle of incidence and, if the reflector isn't perfectly conducting, on the impedance of the surface. [snip] Agreed! A very intersting optical phenomena to observe is to look at a mirror through an optical circular polarizer (polarizer in tandem with a 1/4 wave retarder) which renders the "image" of the circular polarizer to be black. *i.e. the optical circular polarizer eliminates the reflection. *This technique is widely used to eliminate reflections from information displays that must operate in high sunlight with good sunlight readability. *High quality high transmissivity optical circular polarizers are relatively expensive, and so one does not find such technology applied to consumer displays like computer monitors, TV sets or IPhones, however optical circular polarizers are widely used by the military for eliminating sunlight reflections from their (expensive) information displays. [snip] CP propagation is often used in Satellite communications where a satellite may use both RHCP and LHCP transmitting antennas on the same frequency for communicating independently with two different ground stations using R and L CP *antennas on the same frequency. *CP frequency diversity doubles channel capacity! I think you mean that polarization (not frequency) diversity doubles channel capacity. [snip] Yep that's exactly what I meant, but my fingers did not type it that way. Thanks! [snip] angular velocity of rotation is one revolution per cycle of the RF carrier, or in other words one radian of circular rotation for each radian of frequency transmitted. *In other words most well known CP antennas produce ONLY synchronous CP, where the angular velocity of rotation of the E vector is synchronized exactly with the frequency of the wave being transmitted. That is, in fact, the definition of circular or elliptical polarization.. [snip] Agreed, both you and I and thousands of others know that. *[smile] [snip] I believe that the well known and understood situation of purely synchronous CP is NOT necessesarily the only form of CP. It's the only one which fits the definition. If you choose to rotate the polarization at some other rate, you should call it something else. [snip] Definition! *Gosh where is Cecil when you need him? *The only problem with definitions is that there are so many of them! --------------------------------------------------------------------------------------------- "When I use a word, Humpty Dumpty said in a rather scornful tone, "It means just what I chose it to mean - neither more nor less." "The question is," said Alice, "whether you can make words mean so many different things." "The question is," said Humpty Dumpty, "which is to be Master - that's all." * * * * * * * * *-- Lewis Caroll, from Through the Looking Glass -------------------------------------------------------------------------------------------- [grin] [snip] Sorry, it doesn't. An unavoidable side effect of the synchronicity change is that the amplitude of the E field still changes at a 1 GHz rate, going through a complete cycle from max to zero to max to zero to max each nanosecond. A circularly polarized wave doesn't change amplitude with time. A non-circular elliptical wave changes amplitude but not fully to zero each cycle. [snip] Here there is a bit of fuzziness... I agree that the E field of a wave is always changing at the RF carrier frequency since it is an AC waveform. *Alternating current is always changing! *And so a 1 GHz carrier will always have an E field that oscillates back and forth at the carrier (center?) frequency when analyzed by a (linear) polarimeter. I disagree with you that a circular polarized wave has a constant E field.. Even in the case of a purely circularly polarized the E field still oscillates at the carrier (center?) frequency when analyzed by a linear polarizer. i.e. if a purely CP wave is received on a linear polarized antenna the detected E field (Volts per meter) will be observed to be oscillating at the carrier frequency. *However if received on a purely CP responding antenna this oscillating E fileld will appear to be constant. The E field vector can be considered to be similar to the image of a spoke on a rolling wheel. *The radius of the spoke is constant, but it's projection on the ground over which the wheel is rolling will always be oscillating in length. [snip] Circularly polarized waves have many characteristics and particular relationships to linearly polarized waves. The waves you're producing don't have some of these characteristics, like the constant amplitude. Your method doesn't produce circularly polarized waves even though the polarization does indeed change with time. [snip] I beg to disagree. *The waves that I am describing are exactly the same.. Consider if the mechanical motor that spins my linear antenna spins at exactly the ... read more » It was stated above that the purely horizontal polarisation will occur when the dipole is broadside This is not correct Using an optimiser and inserting a one liner where all dimensions are different allows for the design to conform to Maxwell laws in their entirety, which means the inclusion of the "weak" force required for equilibrium Regards Art |
Circular polarization... does it have to be synchronous??
"Art Unwin" wrote in message ... the inclusion of the "weak" force required for equilibrium leave it up to art to take a perfectly good premise and insert utter idiocy into it. next he'll be saying that since the magical levitating weak force neutrinos are jumping off the antenna at an angle to the element that the polarization is caused by them. how about it art, can you make your levitating neutrinos rotate in different directions with left or right hand circular antennas?? |
Circular polarization... does it have to be synchronous??
On Dec 6, 10:37*am, "Dave" wrote:
"Art Unwin" wrote in message ... the inclusion of the "weak" force required for equilibrium leave it up to art to take a perfectly good premise and insert utter idiocy into it. *next he'll be saying that since the magical levitating weak force neutrinos are jumping off the antenna at an angle to the element that the polarization is caused by them. *how about it art, can you make your levitating neutrinos rotate in different directions with left or right hand circular antennas?? You can have diversity with respect to all polarizations except circular where you only have the choice of one. If you believe that antenna programs are utter idiocy then that will be inline with your general attitude. I am sure that some have taken up my suggestion to check for themselves instead of resorting to knee jerk reactions with out foundation. One more fool like you on this newsgroup changes little Art |
Circular polarization... does it have to be synchronous??
"Art Unwin" wrote in message ... You can have diversity with respect to all polarizations except circular where you only have the choice of one. why can't you do lhcp and rhcp diversity? If you believe that antenna programs are utter idiocy then that will be inline with your general attitude. I am sure that some have taken up my suggestion to check for themselves instead of resorting to knee jerk reactions with out foundation. on the contrary, i believe antenna programs and understand how they work, at one time i wrote one of my own that did well on designing phased vertical arrays... and not a single reference to the weak force in it at all! nor will you find any of the existing antenna modeling programs that use the weak force. which kind of contradicts your whole rant, you say you believe in the modeling programs and that they give results that agree with your corrupted weak force model, and yet they don't use the weak force at all... never have, and never will. nor can you state where the weak force is included in Maxwell's equations, which of course all the modeling programs are based on. so that just leaves you hanging by your magical equilibrium levitating diamagnetic neutrinos... which you still haven't explained how they work with my ferromagnetic radiators. |
Circular polarization... does it have to be synchronous??
On Sat, 06 Dec 2008 18:46:16 GMT, "Dave" wrote the
lamentations of a weak mind struggling with the high concepts of an infinitely Byzantine theory from the laboratories of Ærthur: on the contrary, i believe antenna programs and understand how they work, at one time i wrote one of my own that did well on designing phased vertical arrays... and not a single reference to the weak force in it at all! It is singularly impossible for them to have not included the weak force - whose total contribution to the resulting -um- results registers in the 13th digit to the right of the decimal point. Dismissing this immense revelation is like arguing that a drowning man is immune from the effects of a drunk ****ing into the ocean. nor will you find any of the existing antenna modeling programs that use the weak force. op. cit. which kind of contradicts your whole rant, That well may be seeing that Ærthur practices a self reinforcing argument that exhibits that quality of Æquilibrium: damned if you do, and damned if you do it again. you say you believe in the modeling programs and that they give results that agree with your corrupted weak force model, A corrupted weak force, the wæk force? and yet they don't use the weak force at all... Of course they do (op. cit.) never have, and never will. Always has and always will (I already said that didn't I? (which is what op. cit. mæns in Lat.)) nor can you state where the weak force is included in Maxwell's equations, Ærthur, while rooting in the library stacks of an ancient university located on the banks of a great (but not grand) lake, he discovered them in the margins (long neglected as flyspecks on the page due to their singular characteristic out 13 places to the right). Patents are pænding, so watch your step. As we are taxpayers, supporting inventors on the dole, it should be our full right to be able to examine these hidden documents, but Ærthur continues to suppress their access. which of course all the modeling programs are based on. so that just leaves you hanging by your magical equilibrium levitating diamagnetic neutrinos... which you still haven't explained how they work with my ferromagnetic radiators. The only thing he hasn't explained is the beneficial prosperities of the color of the color-coded wire. Just as all resistors look the same except for the colors - and we are all perfectly aware that not all resistors are the same - hence it is a color thing. (Lest we diverge into the side topic of wæk resistance, aka Unpedance.) 73's Richard Clark, KB7QHC |
Circular polarization... does it have to be synchronous??
Richard:
[snip] "Richard Clark" wrote in message ... On Sat, 06 Dec 2008 18:46:16 GMT, "Dave" wrote the lamentations of a weak mind struggling with the high concepts of an infinitely Byzantine theory from the laboratories of Ærthur: on the contrary, i believe antenna programs and understand how they work, at one time i wrote one of my own that did well on designing phased vertical arrays... and not a single reference to the weak force in it at all! .. .. .. 73's Richard Clark, KB7QHC [snip] Hmmmm you guys are just to sceptical of poor Art's "different" biases. The one eyed man in the land of the blind, indeed. Have ya'll considered that Art may not be fully occupying our own four-space and may in fact be operating in several of modern string theory's higher dimensions. After all, modern we now know as explained by John Moffat [1], that from the view of modern Physicists unfettered by actual observation and experiment that there may be at least 11 of those dimensions available to someone of Art's calibre and that perhaps... just perhaps, we "flatladers" may not even be able to comprehend Art's machinations from our own puny four space viewpoint. All that said... we've got to get around to viewing emag fields from the viewpoint of circular components. The universe may well be better understood when viewed by circular polarization rather than by rectilinear polarization. No? [1] John W. Moffat, "Reinventing Gravity", HarperCollins Publishers, New York, 2008. ISBN: 978-0-06-117088-1. May be found at LC under LCC QC178.M64 2008. Cheers! -- Pete K1PO -- Indialantic By-the-Sea, FL |
Circular polarization... does it have to be synchronous??
On Sat, 6 Dec 2008 17:10:39 -0500, "Peter O. Brackett"
wrote: "Reinventing Gravity", I prefer the original over ersatz. 73's Richard Clark, KB7QHC |
Circular polarization... does it have to be synchronous??
Peter O. Brackett wrote:
. . . By "mixed" polarization, I assume you mean a single polarization which is neither horizontal nor vertical and can be described as a "mixture" of a purely horizontal and a purely vertical wave. [snip] No. What I meant by "mixed" was that, just as with daylight for example, the field contains many polarization orientations. In fact usually outside in daylight most of the light we see with our eyes contains very nearly an equal distribution of all polariztions. An exception in the sky's light is perpedicular to the suns rays where because of upper atmospheric conditions light becomes slightly polarized. It is claimed that some people can actually "see" this polarized light differently than normal light. (Haider's Brush) Of course many people know that reflected light, for example from the surface of a lake, becomes highly polarized. This is the reason that "Polaroid" sunglasses are used by sportsmen and others to reduce perceived glare from reflective surfaces. That said, mixed polarization, is also largely the case of HF waves received over ionospheric paths. In other words HF waves received over long distances will contain a wide distribution of linear and perhaps circular polarizations. Thus rendering the use of single polarized antennas relatively useless at HF by amateurs. Unless of course one is prepared to pay the significant price in space and equipment to implement a polarization diversity receiving system. There is only one E field associated with a wave and, if linearly polarized, it has only one orientation or polarization. It's not like incoherent light, but akin to a laser. There is no "mixture" of polarizations in an EM wave. . . . True for a single antenna and receiver, which is the usual case for a ham, see my remarks above. However if one is willing to pay the price for several antennas and synchronous receiving systems then receiving gains can often be obtained by the exploitation of polarization diversity. Actually, you don't want synchronous receivers, or else you get a single effective polarization just as though the antennas were combined into a phased array. For spacial or polarization diversity, you need intentionally non-coherent receivers. [snip] Interesting. Can you work an example for us? I'm curious as to what you use for theta in the "law's" equation. [snip] Theta is just the relative orientation of the polarization of the transmitting and receiving antennas, or in the case of an optical polarimeter, the relative orientations of the polarizing and analyzing polarizer. Theta is commonly illustrated in undergraduate optical laboratories and science experiment kits, using a couple of pieces of "Polaroid" film with the polarization angle marked on the film by a notch or other marking. When the two films are aligned with their polariztion direction perpendicular there is no light propagation, i.e. theta is 90 degrees, and when they are aligned with theta equal to zero then light is propagated. In the case of dipole antennas, theta is zero when two antennas are co-linear and theta is 90 degrees when the antennas are perpendicular. So in your equation, what are theta for RHP and LHP, since you've said that the equation applies to circular polarization? . . . [snip] angular velocity of rotation is one revolution per cycle of the RF carrier, or in other words one radian of circular rotation for each radian of frequency transmitted. In other words most well known CP antennas produce ONLY synchronous CP, where the angular velocity of rotation of the E vector is synchronized exactly with the frequency of the wave being transmitted. That is, in fact, the definition of circular or elliptical polarization. [snip] Agreed, both you and I and thousands of others know that. [smile] Then why are you calling your non-synchronous system "circular polarization"? Definition! Gosh where is Cecil when you need him? The only problem with definitions is that there are so many of them! --------------------------------------------------------------------------------------------- "When I use a word, Humpty Dumpty said in a rather scornful tone, "It means just what I chose it to mean - neither more nor less." "The question is," said Alice, "whether you can make words mean so many different things." "The question is," said Humpty Dumpty, "which is to be Master - that's all." -- Lewis Caroll, from Through the Looking Glass -------------------------------------------------------------------------------------------- [grin] That's a great attitude for a politician, philosopher, or biblical scholar. But engineers and scientists depend on universally understood technical terms in order to communicate. I'm free to say that my car gets a gas mileage of 30 miles/hour and weighs 420 miles. But it wouldn't be a smart thing to do if I intend to convey information. [snip] Sorry, it doesn't. An unavoidable side effect of the synchronicity change is that the amplitude of the E field still changes at a 1 GHz rate, going through a complete cycle from max to zero to max to zero to max each nanosecond. A circularly polarized wave doesn't change amplitude with time. A non-circular elliptical wave changes amplitude but not fully to zero each cycle. [snip] Here there is a bit of fuzziness... I agree that the E field of a wave is always changing at the RF carrier frequency since it is an AC waveform. Alternating current is always changing! And so a 1 GHz carrier will always have an E field that oscillates back and forth at the carrier (center?) frequency when analyzed by a (linear) polarimeter. I disagree with you that a circular polarized wave has a constant E field. Even in the case of a purely circularly polarized the E field still oscillates at the carrier (center?) frequency when analyzed by a linear polarizer. i.e. if a purely CP wave is received on a linear polarized antenna the detected E field (Volts per meter) will be observed to be oscillating at the carrier frequency. However if received on a purely CP responding antenna this oscillating E fileld will appear to be constant. The E field vector can be considered to be similar to the image of a spoke on a rolling wheel. The radius of the spoke is constant, but it's projection on the ground over which the wheel is rolling will always be oscillating in length. When you receive a circularly polarized wave on a linearly polarized antenna, you're seeing only the component of the wave that's linearly polarized in the orientation of the antenna. This is exactly the same process as filtering a complex waveform. You've removed part of the field and are observing what's left after the filtering process, then drawing conclusions about the original waveform based on those observations, much like listening to a concert orchestra through a long pipe and deciding that orchestral sound is very ringy and limited in tonal range. It would benefit you to gain a bit of education about circularly polarized waves. You'll find that a circularly polarized wave can be created from (or broken into) two linearly polarized waves oriented at right angles and in phase quadrature. So each of the components has a time-varying amplitude, but the sum, which is the circularly polarized wave, has a constant amplitude but time-varying orientation. Your linear antenna filters out one of the components, leaving you to observe only the other. [snip] Circularly polarized waves have many characteristics and particular relationships to linearly polarized waves. The waves you're producing don't have some of these characteristics, like the constant amplitude. Your method doesn't produce circularly polarized waves even though the polarization does indeed change with time. [snip] I beg to disagree. The waves that I am describing are exactly the same. Consider if the mechanical motor that spins my linear antenna spins at exactly the carrier frequency. There would be then no way to tell the difference between the two. That's right, in that case you would be producing circularly polarized waves. But only with a synchronous spin speed. As soon as you separate the rotational speed from the wave's oscillation, you have something else with different characteristics, e.g., a time varying amplitude. [snip] Because a circularly polarized antenna responds equally well to all orientations of linear polarization, the normal helix wouldn't be aware of the polarization rotation -- unless the polarization rotation was fast enough to be nearly synchronous. [snip] Heh, heh... what would you consider to be "fast enough"? Would the rate of spin have to be 99-44/100 percent of the synchronous frequency? Or would it have to be closer than that? At what magic spin frequency would the two be indistinguisable. FWIW... I can propose a scheme that will electronically rotate the linear antenna at any desired frequency, at least up to the accuracy of modern atomic clock standards. What you'll end up with is amplitude modulation with the modulating frequency being the beat note between your spinning speed and the wave frequency. This creates sidebands. You'll see this when the sidebands are within the bandwidth of the helix. Outside that, the helix will filter off the sidebands and you'll just see the "carrier" -- the original wave with no modulation. [snip] Sorry, I didn't find it "mind-blowing". [snip] Roy, I don't belive you have thought about it hard enough yet, for clearly this idea has already "blown" your mind! If you say so. For did you not state above that a circular carrier wave has a constant amplitude? Yes, I did. Circularly polarized, that is. A radio wave with constant aplitude, indeed! Something must be blown! At zero frequency, how would a constant wave propagate? Here's a really neat little trick you might want to add to your bag -- superposition. As I mentioned, you can create a circularly polarized wave from two linearly polarized waves. The linearly polarized waves are of course normally time-varying. As long as the propagation medium is linear (such as air), superposition says you can split the circularly polarized wave apart into two linearly polarized waves, study and analyze how they propagate, then add the two components back together again after the propagation. This is, incidentally, a very simple way to see what happens when a circularly polarized wave reflects from a surface -- analyze the linear components separately and add the results. This assumption/view that zero frequency wave can propagate is akin to Cecil's view that there are no reflections at DC. No, it isn't. I don't mean to be facitious and I am quite serious about all of this. Just because no one has ever considered non-synchronous circular polariztion before does not mean that it doesn't exist, or that it may not be useful. Me? I have already thought of several potential uses for non-synchronous circular polarization. How about polariztion frequency modulation? Or... how about polariztion phase modulation? Or... Got you thinking yet? Sorry, I don't recall having stopped thinking. If I have, this isn't the way to get me started. Thanks again for your clearly interesting comments and feedback. More thoughts, comments? -- Pete K1PO -- Indialantic By-the-Sea, FL That's about all I can do at this end. I can't make you actually pick up a text and learn about circularly polarized waves, and until you do, you'll have some fundamental misconceptions about them. Guess I'm one of those folks who someone described recently as "having the common sense educated out of me". It's served me well, since it's enabled me able to spend a career designing a wide variety of state of the art electronic circuits and antennas, successfully mass produced, which work as designed. But I know it's not for everyone. Roy Lewallen, W7EL |
Circular polarization... does it have to be synchronous??
On Dec 6, 12:46*pm, "Dave" wrote:
"Art Unwin" wrote in message ... You can have diversity with respect to all polarizations except circular where you only have the choice of one. why can't you do lhcp and rhcp diversity? If you believe that antenna programs are utter idiocy then that will be inline with your general attitude. I am sure that some have taken up my suggestion to check for themselves instead of resorting to knee jerk reactions with out foundation. on the contrary, i believe antenna programs and understand how they work, at one time i wrote one of my own that did well on designing phased vertical arrays... and not a single reference to the weak force in it at all! *nor will you find any of the existing antenna modeling programs that use the weak force. *which kind of contradicts your whole rant, you say you believe in the modeling programs and that they give results that agree with your corrupted weak force model, and yet they don't use the weak force at all.... never have, and never will. *nor can you state where the weak force is included in Maxwell's equations, which of course all the modeling programs are based on. *so that just leaves you hanging by your magical equilibrium levitating diamagnetic neutrinos... which you still haven't explained how they work with my ferromagnetic radiators. I explained ferro magnetism and antennas a long time ago where the weak force becomes swamped You should be able to come to your own conclusions when evatuating the effect on the Tank Circuit With respect to the weak force action it was that addition to Maxwells laws that provided equilibrium. Kraus gave an example of it when he empirically created pitch angle with respect to other parameters without a full understanding of what created it. In this Universe there is no such thing as a straight line tho a helicoptor can simulate it with two rotors at right angles to create equilibrium the same as a gyroscope or a Sedgeman. The Universe is contained within an arbitrary border in equilibrium, you can't get away from that. The pitch angle that Kraus uses is a creation of the weak force which thus forbids parallelism in antenna arrays. If your antenna that you are bragging about contains parallelism between elements and or the ground surface then you are NOT obtaining maximum radiation but in fact you are increasing your losses. You really have a long way to go with respect to antennas and the answers you search for are not to be found in Snakesphere that is muddied to prevent understanding. As far as antenna programs not using the weak force, that is stupid as it is what is termed as the "displacement" current a guess arrived at based on the units required But rarely do hams use computer programs as initially designed around Maxwell but instead use a modification of such in following Yagi and Uda planar design which is an aproximation. All you have to do is to provide a one liner to a optimiser to realise you are stating a load of crap and have reached a point where you cannot handle the truth as it reveals exactly who and what you are. Some day a knoweledgable person will arrive on this group and ram a computer sample down your throat and expose you and the others as just talking heads. Most of you are like a high school student who wondered into a post graduate lecture room where all appeared as a torrent of babble until the time you grew up, if you ever did. Have a great week end Art |
Circular polarization... does it have to be synchronous??
Roy:
[snip] That's a great attitude for a politician, philosopher, or biblical scholar. But engineers and scientists depend on universally understood technical terms in order to communicate. I'm free to say that my car gets a gas mileage of 30 miles/hour and weighs 420 miles. But it wouldn't be a smart thing to do if I intend to convey information. [snip] Sorry, it doesn't. An unavoidable side effect of the synchronicity change is that the amplitude of the E field still changes at a 1 GHz rate, going through a complete cycle from max to zero to max to zero to max each nanosecond. A circularly polarized wave doesn't change amplitude with time. A non-circular elliptical wave changes amplitude but not fully to zero each cycle. [snip] Here there is a bit of fuzziness... I agree that the E field of a wave is always changing at the RF carrier frequency since it is an AC waveform. Alternating current is always changing! And so a 1 GHz carrier will always have an E field that oscillates back and forth at the carrier (center?) frequency when analyzed by a (linear) polarimeter. I disagree with you that a circular polarized wave has a constant E field. Even in the case of a purely circularly polarized the E field still oscillates at the carrier (center?) frequency when analyzed by a linear polarizer. i.e. if a purely CP wave is received on a linear polarized antenna the detected E field (Volts per meter) will be observed to be oscillating at the carrier frequency. However if received on a purely CP responding antenna this oscillating E fileld will appear to be constant. The E field vector can be considered to be similar to the image of a spoke on a rolling wheel. The radius of the spoke is constant, but it's projection on the ground over which the wheel is rolling will always be oscillating in length. When you receive a circularly polarized wave on a linearly polarized antenna, you're seeing only the component of the wave that's linearly polarized in the orientation of the antenna. This is exactly the same process as filtering a complex waveform. You've removed part of the field and are observing what's left after the filtering process, then drawing conclusions about the original waveform based on those observations, much like listening to a concert orchestra through a long pipe and deciding that orchestral sound is very ringy and limited in tonal range. It would benefit you to gain a bit of education about circularly polarized waves. You'll find that a circularly polarized wave can be created from (or broken into) two linearly polarized waves oriented at right angles and in phase quadrature. So each of the components has a time-varying amplitude, but the sum, which is the circularly polarized wave, has a constant amplitude but time-varying orientation. Your linear antenna filters out one of the components, leaving you to observe only the other. [snip] Yes indeed, we must be talking at cross purposes, since we seem to have no disagreement on any of the above. I don't see where we differ at all! [snip] Would the rate of spin have to be 99-44/100 percent of the synchronous frequency? Or would it have to be closer than that? At what magic spin frequency would the two be indistinguisable. [snip] I would repeat the above question in a slightly different way... How much frequency, or for that matter phase, difference must there be between the mechanical spin frequency and the carrier frequency before you could tell the difference between your "conventionally defined" circular polarization and my definition? If my antenna was spining with an angular velocity within say, 0.000000000005% of the carrier frequency, would that do it? Or perhaps my spin rate would have to be closer to the carrier frequency than that? If so, then how close does it have to be to qualify to be called circular polarization under (your) traditional/conventional definition? [snip] What you'll end up with is amplitude modulation with the modulating frequency being the beat note between your spinning speed and the wave frequency. This creates sidebands. You'll see this when the sidebands are within the bandwidth of the helix. Outside that, the helix will filter off the sidebands and you'll just see the "carrier" -- the original wave with no modulation. [snip] Hmmm... Yes, I agree and that's partially correct, but some of the above paragraph is somewhat "fuzzy" to say the least. That helix must be a very sharp [brick wall???] filter, no? Let's get real here, no practical implementation of any kind of physical filtering mechanism can filter with infinitely sharp transition bands. It just doesn't happen in nature. [snip] Here's a really neat little trick you might want to add to your bag -- superposition. As I mentioned, you can create a circularly polarized wave from two linearly polarized waves. The linearly polarized waves are of course normally time-varying. As long as the propagation medium is linear (such as air), superposition says you can split the circularly polarized wave apart into two linearly polarized waves, study and analyze how they propagate, then add the two components back together again after the propagation. This is, incidentally, a very simple way to see what happens when a circularly polarized wave reflects from a surface -- analyze the linear components separately and add the results. [snip] Heh, heh... Superposition is not a 'trick' it is a well known principle and Roy, I agree with all of the above! What's your point? Bringing up superposition is fine, but you seem to raise the concept of superposition simply as a digression here, not as a means of disproving my assertion that mechanically spinning a linear antenna is tantamount to conventional circular polarization. [snip] That's about all I can do at this end. I can't make you actually pick up a text and learn about circularly polarized waves, and until you do, you'll have some fundamental misconceptions about them. [snip] Hmmm... that was a cheap shot! Unfortunately I agree, YOU cannot make me pick up a text. However, I can make myself do so myself, and... it may (or may not) interest you to know that I have done so on many occasions. In fact I have picked up several such texts, addressing such subject matter authored by Physicists and Engineers ranging over subjects as diverse as radio frequency antennas and optics. Would it impress you if I sent you a picture of my personal library of several hundred volumes, which contains perhaps a dozen or more textbooks on electromagnetics. Since I have been examined on these subjects at graduate degree levels by the faculty at several duly accredited Universities it seems that there is some evidence that I may have read and understood at least a few paragraphs from those texts that I "picked up"! [smile] [snip] Guess I'm one of those folks who someone described recently as "having the common sense educated out of me". It's served me well, since it's enabled me able to spend a career designing a wide variety of state of the art electronic circuits and antennas, successfully mass produced, which work as designed. But I know it's not for everyone. [snip] Hmmm... I too have spent (wasted?) most of several decades designing electronic products and equipment for international markets sold in more than 40 countries with at total sales volume exceeding $5BB dollars. And it seems in today's world that if you combine that Engineering experience with $2.50 you can buy a cup of coffee at Starbucks! Now that we have suitably set the stage, lets get back to the common sense Engineering question at hand! All I need is a number! Perhaps I should regurgitate the statement of Lord Kelvin about knowledge that dear departed Reg used to quote. You know... the one about quantifying things, the one that says you know nothing unless you can put a number to it! Do I really need to do that here? Reggie dear friend, are you watching from above? Roy, please answer the following common sense Engineering questions, just how close must the angular velocity of my spinning antenna be to the carrier frequency before YOU will allow it to be called circular polarization? A simple numerical value in percentage form would do fine! [smile] -- Pete K1PO -- Indialantic By-the-Sea, FL |
Circular polarization... does it have to be synchronous??
On Sun, 7 Dec 2008 00:22:05 -0500, "Peter O. Brackett"
wrote: On Sat, 06 Dec 2008 15:49:26 -0800, Roy Lewallen wrote: Guess I'm one of those folks who someone described recently as "having the common sense educated out of me". Roy, please answer the following common sense Engineering questions, And I thought Abbott and Costello were dead - but evidently not their "Who's on First?" routine. :-/ 73's Richard Clark, KB7QHC |
Circular polarization... does it have to be synchronous??
Peter O. Brackett wrote:
. . . Yes indeed, we must be talking at cross purposes, since we seem to have no disagreement on any of the above. I don't see where we differ at all! For starters, a circularly polarized wave, as universally understood, has an E field which is constant in amplitude, rotates in synchronism with the rotational frequency of the field, and has a particular relationship to constituent linearly polarized components. The field you're generating doesn't, yet you're calling it "circularly polarized". [snip] Would the rate of spin have to be 99-44/100 percent of the synchronous frequency? Or would it have to be closer than that? At what magic spin frequency would the two be indistinguisable. [snip] I would repeat the above question in a slightly different way... How much frequency, or for that matter phase, difference must there be between the mechanical spin frequency and the carrier frequency before you could tell the difference between your "conventionally defined" circular polarization and my definition? Any difference at all. If there's even a tiny difference, the E field will change in amplitude with time. If it's perfectly synchronous it won't. The rate at which it changes with time is the difference between the field rotation frequency and the frequency of the generated signal. If they're synchronous, the difference is zero, and no change in amplitude with time. If my antenna was spining with an angular velocity within say, 0.000000000005% of the carrier frequency, would that do it? If by "it" you mean make the difference non-discernible, the answer is no. See above. Or perhaps my spin rate would have to be closer to the carrier frequency than that? See above. If so, then how close does it have to be to qualify to be called circular polarization under (your) traditional/conventional definition? They have to be identical. See above. The question you posed earlier was different, involving detection of the difference with a particular kind of antenna. Like the linear antenna you used in another example, it filters the signal which alters its properties. So my answer to this new question is different. [snip] What you'll end up with is amplitude modulation with the modulating frequency being the beat note between your spinning speed and the wave frequency. This creates sidebands. You'll see this when the sidebands are within the bandwidth of the helix. Outside that, the helix will filter off the sidebands and you'll just see the "carrier" -- the original wave with no modulation. [snip] Hmmm... Yes, I agree and that's partially correct, but some of the above paragraph is somewhat "fuzzy" to say the least. That helix must be a very sharp [brick wall???] filter, no? No. Let's get real here, no practical implementation of any kind of physical filtering mechanism can filter with infinitely sharp transition bands. It just doesn't happen in nature. That's not required, although I see it's how you've interpreted my use of "bandwidth". There is no such brick wall rejection region. [snip] Here's a really neat little trick you might want to add to your bag -- superposition. As I mentioned, you can create a circularly polarized wave from two linearly polarized waves. The linearly polarized waves are of course normally time-varying. As long as the propagation medium is linear (such as air), superposition says you can split the circularly polarized wave apart into two linearly polarized waves, study and analyze how they propagate, then add the two components back together again after the propagation. This is, incidentally, a very simple way to see what happens when a circularly polarized wave reflects from a surface -- analyze the linear components separately and add the results. [snip] Heh, heh... Superposition is not a 'trick' it is a well known principle and Roy, I agree with all of the above! What's your point? You don't believe that a wave with constant amplitude E field can propagate. My point is that the constant E field amplitude circularly polarized wave can be made of the sum of two time-varying waves. Each of these waves can propagate. If you're familiar with superposition it should be obvious that the original wave can be split into those components, each component and its propagation can be analyzed separately, and the results summed at the far end of the path. That's how a CP wave having a constant amplitude can propagate. Bringing up superposition is fine, but you seem to raise the concept of superposition simply as a digression here, not as a means of disproving my assertion that mechanically spinning a linear antenna is tantamount to conventional circular polarization. No, it was brought up to demonstrate how a wave having a constant amplitude E field can propagate. You had used the argument that a circularly polarized wave can't propagate because its E field has a constant amplitude, as support for your incorrect assertion that the amplitude of the E field of a circularly polarized varies with time. A circularly polarized wave has a constant amplitude E field, which can be easily demonstrated from the equations describing it. It propagates. Your pseudo-circularly polarized wave doesn't have a constant amplitude E field, which is only one way it differs from a circularly polarized wave. [snip] That's about all I can do at this end. I can't make you actually pick up a text and learn about circularly polarized waves, and until you do, you'll have some fundamental misconceptions about them. [snip] Hmmm... that was a cheap shot! Unfortunately I agree, YOU cannot make me pick up a text. However, I can make myself do so myself, and... it may (or may not) interest you to know that I have done so on many occasions. In fact I have picked up several such texts, addressing such subject matter authored by Physicists and Engineers ranging over subjects as diverse as radio frequency antennas and optics. Would it impress you if I sent you a picture of my personal library of several hundred volumes, which contains perhaps a dozen or more textbooks on electromagnetics. Since I have been examined on these subjects at graduate degree levels by the faculty at several duly accredited Universities it seems that there is some evidence that I may have read and understood at least a few paragraphs from those texts that I "picked up"! [smile] I'm impressed, but it's not apparent to me why, with those resources available, you're having trouble finding how the amplitude of the circularly polarized wave E field varies with time, or applying superposition to discover how it propagates. Choose one or two of your texts which has the equations for circularly polarized waves. Chances are good that I have the same text, and if you'd like I can show you how to derive the instantaneous E field amplitude from the equations. But I'm afraid you would have to pick it up to find the equations. But if you can do that, you might be able to write the equations describing your signal, and then the differences between it and the CP equations should become obvious. [snip] Guess I'm one of those folks who someone described recently as "having the common sense educated out of me". It's served me well, since it's enabled me able to spend a career designing a wide variety of state of the art electronic circuits and antennas, successfully mass produced, which work as designed. But I know it's not for everyone. [snip] Hmmm... I too have spent (wasted?) most of several decades designing electronic products and equipment for international markets sold in more than 40 countries with at total sales volume exceeding $5BB dollars. And it seems in today's world that if you combine that Engineering experience with $2.50 you can buy a cup of coffee at Starbucks! Now that we have suitably set the stage, lets get back to the common sense Engineering question at hand! All I need is a number! Oh, if that's all you need, 42 is always a good choice. Perhaps I should regurgitate the statement of Lord Kelvin about knowledge that dear departed Reg used to quote. You know... the one about quantifying things, the one that says you know nothing unless you can put a number to it! Do I really need to do that here? Reggie dear friend, are you watching from above? Roy, please answer the following common sense Engineering questions, just how close must the angular velocity of my spinning antenna be to the carrier frequency before YOU will allow it to be called circular polarization? It must be exactly the same. A simple numerical value in percentage form would do fine! 0. |
Circular polarization... does it have to be synchronous??
Peter O. Brackett wrote: Now transmit on that dipole antenna whilst mechanically spinning it clockwise [RHCP?] (with a mechanical motor of some kind). The dipole antenna is linear and thuse emits linear polariztion, except it is mechanically spinning, and so the E vector emanating from the antenna will be rotating with respect to its direction of travel. In this case the angular velocity of the motor that spins the linear antenna need not be synchronous with the frequency being radiated. For example we could mechanically spin the antenna at 330 rpm while transmitting a carrier of 1 GHz. This would most certainly produce circular polarization. For is not the E vector spinning at 330 revs! Andy writes: It sounds to me like you are describing the technique for generating an aircraft VOR signal, which has been in use for well over 50 years. The VOR band is 108 - 117 Mhz, and the antenna is a cardoid pattern that is rotated mechanically at a 30 hz rate. At a distant point this results in a 30 hz amplitude modulation of the received signal, which is one of the components used in the signal processing for the receiver to determine the direction to or from the ground VOR station. Simply rotating the antenna does not result in circular polarization, but rather it changes the field strength of the radiated signal at a point in space.... The received signal is therefore modulated in amplitude as the pattern passes a singular distant point in space..... I just wanted to throw this in the mix, since rotating the antenna has been around for a long time. Of course it can be done electronically now, but the initial systems were simply turned by a motor. Andy W4OAH , ex- aircraft nav system designer....long retired. |
Circular polarization... does it have to be synchronous??
Andy:
Hey, thanks for your input. I know about VOR systems and other similar systems such as TACAN. Indeed they do use rotating antennas. However VOR and TACAN use rotating antennas in the same way as rotating PPI radar antennas, that is they emit linearly polarized waves whilst rotating the direction of highest directivity/gain. They do not emit circular polariztion as such. Rather they emit linear polarization whilst aiming or directing the 'beam' of linear polarized waves as they rotate. Sort of like rotating a flashlight, or the beam of a searchlight or coastal lighthouse. I'm not sure that anyone yet (that includes Roy Lewalen) has fully understood exactly what I was trying to convey. I'm afraid that the true nature of circular polarization is not well understood by many. Perhaps opitical scientists understand circular polarization best, if only because most of the important 'applications' of circular polarization are in the field of optics rather than radio. -- Pete K1PO -- Indialantic, By-the-Sea, FL "AndyS" wrote in message ... Peter O. Brackett wrote: Now transmit on that dipole antenna whilst mechanically spinning it clockwise [RHCP?] (with a mechanical motor of some kind). The dipole antenna is linear and thuse emits linear polariztion, except it is mechanically spinning, and so the E vector emanating from the antenna will be rotating with respect to its direction of travel. In this case the angular velocity of the motor that spins the linear antenna need not be synchronous with the frequency being radiated. For example we could mechanically spin the antenna at 330 rpm while transmitting a carrier of 1 GHz. This would most certainly produce circular polarization. For is not the E vector spinning at 330 revs! Andy writes: It sounds to me like you are describing the technique for generating an aircraft VOR signal, which has been in use for well over 50 years. The VOR band is 108 - 117 Mhz, and the antenna is a cardoid pattern that is rotated mechanically at a 30 hz rate. At a distant point this results in a 30 hz amplitude modulation of the received signal, which is one of the components used in the signal processing for the receiver to determine the direction to or from the ground VOR station. Simply rotating the antenna does not result in circular polarization, but rather it changes the field strength of the radiated signal at a point in space.... The received signal is therefore modulated in amplitude as the pattern passes a singular distant point in space..... I just wanted to throw this in the mix, since rotating the antenna has been around for a long time. Of course it can be done electronically now, but the initial systems were simply turned by a motor. Andy W4OAH , ex- aircraft nav system designer....long retired. |
Circular polarization... does it have to be synchronous??
Peter O. Brackett wrote: Sort of like rotating a flashlight, or the beam of a searchlight or coastal lighthouse. Andy comments: Exactly right !!! And a good analogy.... Consider this then: A patch antenna, circularly polarized, mounted at the end of a motor shaft, rotating in the opposite direction of the polarization... ..... at a speed equal to the frequency... Does the polarization "unravel" and emit a linear, non-rotating polarization ? Is this the sort of principle that you were trying to convey ?? If this is the case, any discrepancy in the motor, say 1 hz out of 10 Mhz , would result in an Efield rotating at a 1 hz rate.... and the receiving antenna would have to be very very very long in order to fully receive the polarized wave....... I think.... And if the motor shaft and the frequency were identical, the Efield would be linear, stable, and non-rotating..... This is getting beyond my personal antenna expertise, but I still find it interesting....... Please pardon my lack of understanding, .... if I still don't "get" it.... Andy W4OAH |
Circular polarization... does it have to be synchronous??
Something just occurred to me. I did get to thinking.
My previous answers were wrong. Peter's spinning antenna wouldn't produce a circularly polarized wave (as universally defined) even if it was synchronous with the wave frequency. As I've said, a circularly polarized wave has constant E field amplitude; Peter's wave would have a time-varying amplitude. If it were synchronous, the nulls and peaks would always occur at the same places in the rotation cycle, so they would occur at fixed angles relative to a rotational reference point. If non-synchronous, the nulls and peaks would rotate at the beat frequency. It seems to me that the way to mechanically generate a circularly polarized wave would be to rotate a source of *static* E field, for example, a short dipole with constant applied DC voltage at the feedpoint. That should produce a circularly polarized wave with the frequency being the rotational frequency of the dipole. At any point in space, the E field would change with time, and would propagate, and it would look exactly like a circularly polarized wave broadside to the rotation plane. If the scheme works and radiation is occurring, then power must be going into the antenna, which in turn means it's drawing current that's in phase with the applied voltage. When stopped, no current will flow, but when rotating, it does. So how does the antenna know it's rotating? How about this -- if you instantaneously move the antenna into some position, a static E field appears there, and propagates outward at the speed of light. Closer in than the leading edge of the propagating wave, the field is static. When we rotate the dipole to a new position, it moves through the field from its previous position, which induces a current in it. Hence the current. It's fundamentally a generator, with the field being in the air. I'd be willing to bet a moderate sum that if you did apply a DC voltage to a dipole and rotated it, you'd see an alternating current with a frequency equal to the frequency of rotation, and a circularly polarized wave broadside to the antenna. I suspect that the current and the radiated field increase in amplitude with rotational speed, so you might have to get it going really fast before you can detect the effects. Now there's some food for thought. Roy Lewallen, W7EL |
Circular polarization... does it have to be synchronous??
In article tonline, Roy
Lewallen wrote: Then some education is in order. Electromagnetic waves are elliptically polarized. The two extreme special cases of this are linear and circular (with axial ratio of zero -- or infinite depending on your choice of definition -- and one respectively). There are an infinite number of other possible elliptical polarizations with different axial ratios. Hello, and that's quite correct, Roy. Having read the OP's statements and others in this thread I would like to recommend that one step back from antennas for a moment in order to examine the generation of an ellipse (representing the locus of points of a rotating E (or H) field. The parametric equations take the form x(t) = A*cos(2*pi*f*t) and y(t) = B*cos(2*pi*f*t + phi). (These equations are of the same form that generate the familiar Lissajous patterns except that for Lissajous the x and y values differ in frequeny.) While polarization is a convenient concept in electromagnetic wave propagaion there's no reason that we couldnt just treat it as the superposition of two separate Ex (or Hx) and Ey (or Hy) waves. Of course we have to pay attention to amplitude and phase relationships. I think investing some time with this math (it's not all that difficult) will provide one with insight into the concept of polarization and perhaps head off some misconception. If anyone is interested and has Mathcad, I've got a worksheet that allows one to vary these parameters, plots the resulting ellipse (or circle or line) and also calculates ellipticity (axial ratio) and eccentricity. Sincerely, and 73s from N4GGO, John Wood (Code 5550) e-mail: Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5337 |
Circular polarization... does it have to be synchronous??
J. B. Wood wrote:
I think investing some time with this math (it's not all that difficult) will provide one with insight into the concept of polarization and perhaps head off some misconception. If anyone is interested and has Mathcad, I've got a worksheet that allows one to vary these parameters, plots the resulting ellipse (or circle or line) and also calculates ellipticity (axial ratio) and eccentricity. Sincerely, and 73s from N4GGO, All I have to know is that Circular Polarization always helps when one end of the path is prone to random polarizations, even with the 3 dB power loss. |
Circular polarization... does it have to be synchronous??
"Roy Lewallen" wrote in message treetonline... Something just occurred to me. I did get to thinking. My previous answers were wrong. Peter's spinning antenna wouldn't produce a circularly polarized wave (as universally defined) even if it was synchronous with the wave frequency. As I've said, a circularly polarized wave has constant E field amplitude; Peter's wave would have a time-varying amplitude. If it were synchronous, the nulls and peaks would always occur at the same places in the rotation cycle, so they would occur at fixed angles relative to a rotational reference point. If non-synchronous, the nulls and peaks would rotate at the beat frequency. It seems to me that the way to mechanically generate a circularly polarized wave would be to rotate a source of *static* E field, for example, a short dipole with constant applied DC voltage at the feedpoint. That should produce a circularly polarized wave with the frequency being the rotational frequency of the dipole. At any point in space, the E field would change with time, and would propagate, and it would look exactly like a circularly polarized wave broadside to the rotation plane. If the scheme works and radiation is occurring, then power must be going into the antenna, which in turn means it's drawing current that's in phase with the applied voltage. When stopped, no current will flow, but when rotating, it does. So how does the antenna know it's rotating? How about this -- if you instantaneously move the antenna into some position, a static E field appears there, and propagates outward at the speed of light. Closer in than the leading edge of the propagating wave, the field is static. When we rotate the dipole to a new position, it moves through the field from its previous position, which induces a current in it. Hence the current. It's fundamentally a generator, with the field being in the air. I'd be willing to bet a moderate sum that if you did apply a DC voltage to a dipole and rotated it, you'd see an alternating current with a frequency equal to the frequency of rotation, and a circularly polarized wave broadside to the antenna. I suspect that the current and the radiated field increase in amplitude with rotational speed, so you might have to get it going really fast before you can detect the effects. Now there's some food for thought. Roy Lewallen, W7EL A source of endless coffee-time debates where I used to work! No, the current into the rotating dipole would be DC and the means of rotation at the radio frequency would take the place of the 'transmitter'. If the current were alternating then the radiated electric field would be discontinuous but it isn't; it has constant magnitude. Between two such systems separated by many wavelengths, if there were no anisotropic material around, reciprocity would apply and a means of conveying DC by radio would be created! However, intriguing and amusing as this analogy might be I wonder if it really has any practical value. For real mechanical rotating parts the frequency would be limited to something rather low like the tens of kHz at which Alexanderson alternators work, and then the wavelength would be so long that it would probably be impossible to construct an efficient radiator*. The quickest moving antenna I've encountered was a commutated plasma antenna, using a construction similar to a 'dekatron' tube, but even then the length of the radiator was so small that SHF would be needed to achieve worthwhile radiation efficiency* and the maximum commutation speed was limited to a few MHz by the time it takes to establish the plasma at each step in the commutation cycle. *(Of course, the conventional principles of radiation resistance vs. loss resistance may need 'massaging' to bring them into line with the concept of creating transverse waves by rotating a dipole connected to a battery!) Chris |
Circular polarization... does it have to be synchronous??
"christofire" wrote in message ... "Roy Lewallen" wrote in message treetonline... Something just occurred to me. I did get to thinking. My previous answers were wrong. Peter's spinning antenna wouldn't produce a circularly polarized wave (as universally defined) even if it was synchronous with the wave frequency. As I've said, a circularly polarized wave has constant E field amplitude; Peter's wave would have a time-varying amplitude. If it were synchronous, the nulls and peaks would always occur at the same places in the rotation cycle, so they would occur at fixed angles relative to a rotational reference point. If non-synchronous, the nulls and peaks would rotate at the beat frequency. It seems to me that the way to mechanically generate a circularly polarized wave would be to rotate a source of *static* E field, for example, a short dipole with constant applied DC voltage at the feedpoint. That should produce a circularly polarized wave with the frequency being the rotational frequency of the dipole. At any point in space, the E field would change with time, and would propagate, and it would look exactly like a circularly polarized wave broadside to the rotation plane. If the scheme works and radiation is occurring, then power must be going into the antenna, which in turn means it's drawing current that's in phase with the applied voltage. When stopped, no current will flow, but when rotating, it does. So how does the antenna know it's rotating? How about this -- if you instantaneously move the antenna into some position, a static E field appears there, and propagates outward at the speed of light. Closer in than the leading edge of the propagating wave, the field is static. When we rotate the dipole to a new position, it moves through the field from its previous position, which induces a current in it. Hence the current. It's fundamentally a generator, with the field being in the air. I'd be willing to bet a moderate sum that if you did apply a DC voltage to a dipole and rotated it, you'd see an alternating current with a frequency equal to the frequency of rotation, and a circularly polarized wave broadside to the antenna. I suspect that the current and the radiated field increase in amplitude with rotational speed, so you might have to get it going really fast before you can detect the effects. Now there's some food for thought. Roy Lewallen, W7EL A source of endless coffee-time debates where I used to work! No, the current into the rotating dipole would be DC and the means of rotation at the radio frequency would take the place of the 'transmitter'. If the current were alternating then the radiated electric field would be discontinuous but it isn't; it has constant magnitude. Between two such systems separated by many wavelengths, if there were no anisotropic material around, reciprocity would apply and a means of conveying DC by radio would be created! However, intriguing and amusing as this analogy might be I wonder if it really has any practical value. For real mechanical rotating parts the frequency would be limited to something rather low like the tens of kHz at which Alexanderson alternators work, and then the wavelength would be so long that it would probably be impossible to construct an efficient radiator*. The quickest moving antenna I've encountered was a commutated plasma antenna, using a construction similar to a 'dekatron' tube, but even then the length of the radiator was so small that SHF would be needed to achieve worthwhile radiation efficiency* and the maximum commutation speed was limited to a few MHz by the time it takes to establish the plasma at each step in the commutation cycle. *(Of course, the conventional principles of radiation resistance vs. loss resistance may need 'massaging' to bring them into line with the concept of creating transverse waves by rotating a dipole connected to a battery!) Chris Hi Chris I am not smart enough to analyze the effects of rotating a dipole with DC applied to it, but I have doubts that it would create a "far field". Did you guys ever figure out how the "DC dipole" generates a Far Field? Jerry KD6JDJ |
Circular polarization... does it have to be synchronous??
christofire wrote:
A source of endless coffee-time debates where I used to work! No, the current into the rotating dipole would be DC and the means of rotation at the radio frequency would take the place of the 'transmitter'. If the current were alternating then the radiated electric field would be discontinuous but it isn't; it has constant magnitude. Between two such systems separated by many wavelengths, if there were no anisotropic material around, reciprocity would apply and a means of conveying DC by radio would be created! Now that I think about it, you're right -- the current would have to be DC, so there would be only DC power into the dipole. Interesting that you and your co-workers thought of and debated this. I've given it less than an hour of thought since it popped into my head, so you've had a lot more time to work out the details. Sounds like it might work something like I described, then. However, intriguing and amusing as this analogy might be I wonder if it really has any practical value. For real mechanical rotating parts the frequency would be limited to something rather low like the tens of kHz at which Alexanderson alternators work, and then the wavelength would be so long that it would probably be impossible to construct an efficient radiator*. The quickest moving antenna I've encountered was a commutated plasma antenna, using a construction similar to a 'dekatron' tube, but even then the length of the radiator was so small that SHF would be needed to achieve worthwhile radiation efficiency* and the maximum commutation speed was limited to a few MHz by the time it takes to establish the plasma at each step in the commutation cycle. I can't see where this could possibly be of any practical use. For me it was simply a mind exercise spurred by Peter's musings, resulting from wondering just how a mechanical system could be made to generate a CP wave. *(Of course, the conventional principles of radiation resistance vs. loss resistance may need 'massaging' to bring them into line with the concept of creating transverse waves by rotating a dipole connected to a battery!) Indeed. And it seems there wouldn't be any skin effect, then, with only DC going to the wire. And what about current distribution on the dipole? Roy Lewallen, W7EL |
Circular polarization... does it have to be synchronous??
"Roy Lewallen" wrote in message treetonline... christofire wrote: A source of endless coffee-time debates where I used to work! No, the current into the rotating dipole would be DC and the means of rotation at the radio frequency would take the place of the 'transmitter'. If the current were alternating then the radiated electric field would be discontinuous but it isn't; it has constant magnitude. Between two such systems separated by many wavelengths, if there were no anisotropic material around, reciprocity would apply and a means of conveying DC by radio would be created! Now that I think about it, you're right -- the current would have to be DC, so there would be only DC power into the dipole. Interesting that you and your co-workers thought of and debated this. I've given it less than an hour of thought since it popped into my head, so you've had a lot more time to work out the details. Sounds like it might work something like I described, then. However, intriguing and amusing as this analogy might be I wonder if it really has any practical value. For real mechanical rotating parts the frequency would be limited to something rather low like the tens of kHz at which Alexanderson alternators work, and then the wavelength would be so long that it would probably be impossible to construct an efficient radiator*. The quickest moving antenna I've encountered was a commutated plasma antenna, using a construction similar to a 'dekatron' tube, but even then the length of the radiator was so small that SHF would be needed to achieve worthwhile radiation efficiency* and the maximum commutation speed was limited to a few MHz by the time it takes to establish the plasma at each step in the commutation cycle. I can't see where this could possibly be of any practical use. For me it was simply a mind exercise spurred by Peter's musings, resulting from wondering just how a mechanical system could be made to generate a CP wave. *(Of course, the conventional principles of radiation resistance vs. loss resistance may need 'massaging' to bring them into line with the concept of creating transverse waves by rotating a dipole connected to a battery!) Indeed. And it seems there wouldn't be any skin effect, then, with only DC going to the wire. And what about current distribution on the dipole? Roy Lewallen, W7EL Hi Roy I have problems with believing there will be any current in either dipole. What am I missing? Jerry KD6JDJ |
Circular polarization... does it have to be synchronous??
Jerry wrote:
I am not smart enough to analyze the effects of rotating a dipole with DC applied to it, but I have doubts that it would create a "far field". Did you guys ever figure out how the "DC dipole" generates a Far Field? Jerry KD6JDJ It requires energy to create a far field, since the far field is a form of energy. I explained why I thought power might be consumed by the antenna -- current would flow due to coupling with the field still present from previous positions (although I mentioned alternating current while Chris correctly pointed out that it would have to be DC). I don't see any problem with conversion of the DC into AC. It's done all the time with spinning magnets -- look at the alternator in your car for example. And in times of yore, RF was generated directly with high speed alternators. The principle is very similar to, if not exactly the same as, the scheme I described. The whole thing is just a mental exercise to help gain a better understanding of the nature of a circularly polarized field. Roy Lewallen, W7EL |
Circular polarization... does it have to be synchronous??
"Roy Lewallen" wrote in message ... Jerry wrote: I am not smart enough to analyze the effects of rotating a dipole with DC applied to it, but I have doubts that it would create a "far field". Did you guys ever figure out how the "DC dipole" generates a Far Field? Jerry KD6JDJ It requires energy to create a far field, since the far field is a form of energy. I explained why I thought power might be consumed by the antenna -- current would flow due to coupling with the field still present from previous positions (although I mentioned alternating current while Chris correctly pointed out that it would have to be DC). I don't see any problem with conversion of the DC into AC. It's done all the time with spinning magnets -- look at the alternator in your car for example. And in times of yore, RF was generated directly with high speed alternators. The principle is very similar to, if not exactly the same as, the scheme I described. The whole thing is just a mental exercise to help gain a better understanding of the nature of a circularly polarized field. Roy Lewallen, W7EL Indeed, and I would add that the spinning dipole fed with a constant voltage appears the same as a stationary dipole fed with an alternating voltage with respect to any chosen linear polarisation. I was once told of a method of measuring the radiation patterns of large installed antennas by 'flying' near to them a small metal rod rotating about an axis that passes perpendicularly through the middle of the length of the rod. By detecting, synchronously with rotation of the rod, changes in the terminal VSWR (or reflection co-efficient for voltage) the near-field radiation pattern could be assessed (i.e. an impression of the aperture current distribution) from which the far-field patterns could be derived by Fourier transform in the normal way (acknowledgement is due to the late Dick Manton). There is a range of 3D angles over which the axis can vary without upsetting the measurement. I don't know if this was ever implemented, e.g. to measure the patterns of a television transmitting antenna - a helicopter carrying a measuring receiver is used in the far field nowadays. Chris |
Circular polarization... does it have to be synchronous??
"Jerry" wrote in message ... "Roy Lewallen" wrote in message treetonline... christofire wrote: A source of endless coffee-time debates where I used to work! No, the current into the rotating dipole would be DC and the means of rotation at the radio frequency would take the place of the 'transmitter'. If the current were alternating then the radiated electric field would be discontinuous but it isn't; it has constant magnitude. Between two such systems separated by many wavelengths, if there were no anisotropic material around, reciprocity would apply and a means of conveying DC by radio would be created! Now that I think about it, you're right -- the current would have to be DC, so there would be only DC power into the dipole. Interesting that you and your co-workers thought of and debated this. I've given it less than an hour of thought since it popped into my head, so you've had a lot more time to work out the details. Sounds like it might work something like I described, then. However, intriguing and amusing as this analogy might be I wonder if it really has any practical value. For real mechanical rotating parts the frequency would be limited to something rather low like the tens of kHz at which Alexanderson alternators work, and then the wavelength would be so long that it would probably be impossible to construct an efficient radiator*. The quickest moving antenna I've encountered was a commutated plasma antenna, using a construction similar to a 'dekatron' tube, but even then the length of the radiator was so small that SHF would be needed to achieve worthwhile radiation efficiency* and the maximum commutation speed was limited to a few MHz by the time it takes to establish the plasma at each step in the commutation cycle. I can't see where this could possibly be of any practical use. For me it was simply a mind exercise spurred by Peter's musings, resulting from wondering just how a mechanical system could be made to generate a CP wave. *(Of course, the conventional principles of radiation resistance vs. loss resistance may need 'massaging' to bring them into line with the concept of creating transverse waves by rotating a dipole connected to a battery!) Indeed. And it seems there wouldn't be any skin effect, then, with only DC going to the wire. And what about current distribution on the dipole? Roy Lewallen, W7EL Hi Roy I have problems with believing there will be any current in either dipole. What am I missing? Jerry KD6JDJ That's understandable. Chris |
Circular polarization... does it have to be synchronous??
"Roy Lewallen" wrote in message ... Jerry wrote: I am not smart enough to analyze the effects of rotating a dipole with DC applied to it, but I have doubts that it would create a "far field". Did you guys ever figure out how the "DC dipole" generates a Far Field? Jerry KD6JDJ It requires energy to create a far field, since the far field is a form of energy. I explained why I thought power might be consumed by the antenna -- current would flow due to coupling with the field still present from previous positions (although I mentioned alternating current while Chris correctly pointed out that it would have to be DC). I don't see any problem with conversion of the DC into AC. It's done all the time with spinning magnets -- look at the alternator in your car for example. And in times of yore, RF was generated directly with high speed alternators. The principle is very similar to, if not exactly the same as, the scheme I described. The whole thing is just a mental exercise to help gain a better understanding of the nature of a circularly polarized field. Roy Lewallen, W7EL Hi Roy When you write "current would flow due to coupling with the field still present from previous positions", do you submit that more power is required to rotate a dipole with no DC on it than one with DC on it? I will respectfully submit that a car alternator doesnt so much spin a magmetic field as it Rotates the field past a conductor. A car alternator is a lumpy magnetic field that is spun past stationary coils of wire. There is no misunderstanding about inductive coupling of close by conductors. My question related to far field "radiation". I am aware that my understanding of far Field radiation is very limited, so i dont propose that i have answers. I do have question about generating a far field by spinning a DC excited dipole. Jerry KD6JDJ |
Circular polarization... does it have to be synchronous??
"christofire" wrote in message ... "Jerry" wrote in message ... "Roy Lewallen" wrote in message treetonline... christofire wrote: A source of endless coffee-time debates where I used to work! No, the current into the rotating dipole would be DC and the means of rotation at the radio frequency would take the place of the 'transmitter'. If the current were alternating then the radiated electric field would be discontinuous but it isn't; it has constant magnitude. Between two such systems separated by many wavelengths, if there were no anisotropic material around, reciprocity would apply and a means of conveying DC by radio would be created! Now that I think about it, you're right -- the current would have to be DC, so there would be only DC power into the dipole. Interesting that you and your co-workers thought of and debated this. I've given it less than an hour of thought since it popped into my head, so you've had a lot more time to work out the details. Sounds like it might work something like I described, then. However, intriguing and amusing as this analogy might be I wonder if it really has any practical value. For real mechanical rotating parts the frequency would be limited to something rather low like the tens of kHz at which Alexanderson alternators work, and then the wavelength would be so long that it would probably be impossible to construct an efficient radiator*. The quickest moving antenna I've encountered was a commutated plasma antenna, using a construction similar to a 'dekatron' tube, but even then the length of the radiator was so small that SHF would be needed to achieve worthwhile radiation efficiency* and the maximum commutation speed was limited to a few MHz by the time it takes to establish the plasma at each step in the commutation cycle. I can't see where this could possibly be of any practical use. For me it was simply a mind exercise spurred by Peter's musings, resulting from wondering just how a mechanical system could be made to generate a CP wave. *(Of course, the conventional principles of radiation resistance vs. loss resistance may need 'massaging' to bring them into line with the concept of creating transverse waves by rotating a dipole connected to a battery!) Indeed. And it seems there wouldn't be any skin effect, then, with only DC going to the wire. And what about current distribution on the dipole? Roy Lewallen, W7EL Hi Roy I have problems with believing there will be any current in either dipole. What am I missing? Jerry KD6JDJ That's understandable. Chris Hi Chris Tell me, did you guys ever decide that there would be a far field generated by the spinning dipole with DC on it? I dont refer to the inductive field. Maybe there is no way to separate Far Field from any condition where an inductive field is generated. Jerry KD6JDJ |
Circular polarization... does it have to be synchronous??
"Jerry" wrote in message ... - snip - I have problems with believing there will be any current in either dipole. What am I missing? Jerry KD6JDJ That's understandable. Chris Hi Chris Tell me, did you guys ever decide that there would be a far field generated by the spinning dipole with DC on it? I dont refer to the inductive field. Maybe there is no way to separate Far Field from any condition where an inductive field is generated. Jerry KD6JDJ Jerry, I think you're right - in the far field there is spherical spreading of power without regard to separate magnetic and electric components that an antenna, of whatever form, might produce. Of course the radiated power incident on any surface can be represented by an equivalent value of electric or magnetic field strength but this is on strict understanding that the counterpart (magnetic or electric) component is present with the requisite field strength (E/H = Zo = 377 ohms in free space) and PFD = E2/Zo. The answer to your first question is 'yes - hypothetically' there 'would be a far field generated by the spinning dipole with DC on it' but this shouldn't be taken as a recipe for some wacky rotating machine. As I outlined earlier, there is probably little practical application for this interesting analogy because if it were ever put into practice it would probably be hopelessly inefficient and transformation of Maxwell's equations into an inertial frame spinning at the radio frequency is hard, to say the least! As has been suggested, it's probably best to take the concept no further than an interesting thought exercise - if you don't understand that, don't worry, you're not missing much Chris |
Circular polarization... does it have to be synchronous??
"christofire" wrote in message ... "Jerry" wrote in message ... - snip - I have problems with believing there will be any current in either dipole. What am I missing? Jerry KD6JDJ That's understandable. Chris Hi Chris Tell me, did you guys ever decide that there would be a far field generated by the spinning dipole with DC on it? I dont refer to the inductive field. Maybe there is no way to separate Far Field from any condition where an inductive field is generated. Jerry KD6JDJ Jerry, I think you're right - in the far field there is spherical spreading of power without regard to separate magnetic and electric components that an antenna, of whatever form, might produce. Of course the radiated power incident on any surface can be represented by an equivalent value of electric or magnetic field strength but this is on strict understanding that the counterpart (magnetic or electric) component is present with the requisite field strength (E/H = Zo = 377 ohms in free space) and PFD = E2/Zo. The answer to your first question is 'yes - hypothetically' there 'would be a far field generated by the spinning dipole with DC on it' but this shouldn't be taken as a recipe for some wacky rotating machine. As I outlined earlier, there is probably little practical application for this interesting analogy because if it were ever put into practice it would probably be hopelessly inefficient and transformation of Maxwell's equations into an inertial frame spinning at the radio frequency is hard, to say the least! As has been suggested, it's probably best to take the concept no further than an interesting thought exercise - if you don't understand that, don't worry, you're not missing much Chris Hi Chris Thanks for the reply. Yeah, I never ascribed any practical use to the "CP by spinning". But, there are some fundamentally good thoughts generated here. For instance, I can easily see why two dipoles rotating at the same rate and rotational direction will couple *nothing*. Thats like trying to receive RHCP with a LHCP antenna. I have lived a long time without understanding Poynting and Maxwell (almost 100 years older than me), I wouldnt want to change that now. Jerry KD6JDJ |
Circular polarization... does it have to be synchronous??
Richard Clark wrote:
Ærthur, while rooting in the library stacks of an ancient university located on the banks of a great (but not grand) lake, That would be good old Miskatonic U in Arkham? - 73 d eMike N3LI - |
Circular polarization... does it have to be synchronous??
"christofire" wrote in message ... "Jerry" wrote in message ... - snip - I have problems with believing there will be any current in either dipole. What am I missing? Jerry KD6JDJ That's understandable. Chris Hi Chris Tell me, did you guys ever decide that there would be a far field generated by the spinning dipole with DC on it? I dont refer to the inductive field. Maybe there is no way to separate Far Field from any condition where an inductive field is generated. Jerry KD6JDJ Jerry, I think you're right - in the far field there is spherical spreading of power without regard to separate magnetic and electric components that an antenna, of whatever form, might produce. Of course the radiated power incident on any surface can be represented by an equivalent value of electric or magnetic field strength but this is on strict understanding that the counterpart (magnetic or electric) component is present with the requisite field strength (E/H = Zo = 377 ohms in free space) and PFD = E2/Zo. The answer to your first question is 'yes - hypothetically' there 'would be a far field generated by the spinning dipole with DC on it' but this shouldn't be taken as a recipe for some wacky rotating machine. As I outlined earlier, there is probably little practical application for this interesting analogy because if it were ever put into practice it would probably be hopelessly inefficient and transformation of Maxwell's equations into an inertial frame spinning at the radio frequency is hard, to say the least! As has been suggested, it's probably best to take the concept no further than an interesting thought exercise - if you don't understand that, don't worry, you're not missing much Chris Hi Chris I am having a block in my learning. As I understand it, this would actually happen if it could be performed. A spinning dipole would require more power to spin it if it had DC on it than if it had no DC on it. And, actually, it would require no power to keep the dipole spinning since there would be that theoritical vacuum around it. But, once you apply the DC, power would be required to keep it spinning. That amount of added power would be determined by the amount of DC applied. Do you confirm that this is true? My question relates to my ignorance about what there is in the "vacuum" to cause "drag". Jerry KD6JDJ |
Circular polarization... does it have to be synchronous??
"Jerry" wrote in message ... - snip - I am having a block in my learning. As I understand it, this would actually happen if it could be performed. Yes A spinning dipole would require more power to spin it if it had DC on it than if it had no DC on it. Yes - as I said, the means of spinning the dipole would be the counterpart to the 'transmitter' And, actually, it would require no power to keep the dipole spinning since there would be that theoritical vacuum around it. If you say so - you're specifying a hypothetical zero-friction system which is but one of several possible scenarios. But, once you apply the DC, power would be required to keep it spinning. That amount of added power would be determined by the amount of DC applied. Do you confirm that this is true? Certainly work would need to be done to spin the dipole and create the outgoing wave by virtue of its rotation. I suppose it follows that the strength of the outgoing wave would be proportional to the applied voltage but I'm not certain that a greater voltage would require more mechanical work to spin the dipole - you may be right but I'm not certain I can confirm this from what I think I know! My question relates to my ignorance about what there is in the "vacuum" to cause "drag". Jerry KD6JDJ I'm afraid I had taken very little account of causes of mechanical drag. As noted before, this was a thought experiment - the sort of thing that can reach a useful conclusion (i.e. 'not likely' in this case!) without requiring detailed examination of what may be 'second-order' influences. Chris |
Circular polarization... does it have to be synchronous??
-almighty snip-
My question relates to my ignorance about what there is in the "vacuum" to cause "drag". Jerry KD6JDJ Just research 'solar sailing' if you want to read about a phenomenon that involves 'drag' in a vacuum on account of a flux of photons. It's used to help keep satellites 'on station' whilst saving hydrazine. Chris |
Circular polarization... does it have to be synchronous??
"christofire" wrote in message ... "Jerry" wrote in message ... - snip - I am having a block in my learning. As I understand it, this would actually happen if it could be performed. Yes A spinning dipole would require more power to spin it if it had DC on it than if it had no DC on it. Yes - as I said, the means of spinning the dipole would be the counterpart to the 'transmitter' And, actually, it would require no power to keep the dipole spinning since there would be that theoritical vacuum around it. If you say so - you're specifying a hypothetical zero-friction system which is but one of several possible scenarios. But, once you apply the DC, power would be required to keep it spinning. That amount of added power would be determined by the amount of DC applied. Do you confirm that this is true? Certainly work would need to be done to spin the dipole and create the outgoing wave by virtue of its rotation. I suppose it follows that the strength of the outgoing wave would be proportional to the applied voltage but I'm not certain that a greater voltage would require more mechanical work to spin the dipole - you may be right but I'm not certain I can confirm this from what I think I know! My question relates to my ignorance about what there is in the "vacuum" to cause "drag". Jerry KD6JDJ I'm afraid I had taken very little account of causes of mechanical drag. As noted before, this was a thought experiment - the sort of thing that can reach a useful conclusion (i.e. 'not likely' in this case!) without requiring detailed examination of what may be 'second-order' influences. Chris Hi Chris I accept as valid, your statement that the dipole with DC will radiate a far field when spun. I have a mental block related to questioning what makes it harder to spin when the DC is increased. Yes, I do consider the media in which the dipole is spinning creates no friction. I do wonder what makes it harder to spin when the DC voltage is increased. Jerry |
Circular polarization... does it have to be synchronous??
Hmmm... never been there???
-- Pete K1PO "Michael Coslo" wrote in message ... Richard Clark wrote: Ærthur, while rooting in the library stacks of an ancient university located on the banks of a great (but not grand) lake, That would be good old Miskatonic U in Arkham? - 73 d eMike N3LI - |
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