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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 |
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