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Whip antennas with coils
A long time ago there was a arguement both on this group and on E ham
with respect to the function of a coil with respect to replacing missing degrees of a wavelength. Both Tom W8ji and Roy W7EL did experiments which apparently justified an account written up by Tom under the subject. You should look it up and read it. Tom states that the double helix is totally resistive and antenna elements should be straight reffering to spacial distance., He then came to the astonishing conclusion that for maximum efficiency radiators should be straight and for some reason dismissed the double helix to give strength to his claim that radiators must be straight which is fraudulent. A straight radiator generates a external magnetic field which introduces losses whereas the double helix supplies all applied current to the generation of radiation. None of the experts on either group challenged challenged the statement allegedly supported by both Tom and Roy and it still remains in print to this day Art |
Whip antennas with coils
On Aug 28, 9:23*pm, Art Unwin wrote:
A long time ago there was a arguement both on this group and on E ham with respect to the function of a coil with respect to replacing missing degrees of a wavelength. Both Tom W8ji and Roy W7EL did experiments which apparently justified an account written up by Tom under the subject. You should look it up and read it. Tom states that the double helix is totally resistive and antenna elements should be straight reffering to spacial distance., He then came to the astonishing conclusion that for maximum efficiency radiators should be straight and for some reason dismissed the double helix to give strength to his claim that radiators must be straight which is fraudulent. A straight radiator generates a external magnetic field which introduces losses whereas the double helix supplies all applied current to the generation of radiation. None of the experts on either group challenged challenged the statement allegedly supported by both Tom and Roy and it still remains in print to this day Art I continued on reading Toms many dissertations on radiation and antennas when I came across a lecture he gave at Dayton regarding small antennas. Again he pushed his personal intuition with respect to small antennas and spacial distance and received no rebuttals from the audience or the ARRL . In addition he always spouts his pet sentence on radiation is by the acceleration of charge but never states what the charge carrier is as he is still wedded to the idea of wave carriers which has no justification what so ever. We desperately need the help from physics majors to challenge the spread of old wives tales that are propagated without technical justification and sucked in by the new generation of which the hobby is now created or we lose the credibility obtained by hams who foraged before us with respect to the state of the art. Art Unwin |
Whip antennas with coils
On 8/28/2010 10:28 PM, Art Unwin wrote:
On Aug 28, 9:23 pm, Art wrote: A long time ago there was a arguement both on this group and on E ham with respect to the function of a coil with respect to replacing missing degrees of a wavelength. Both Tom W8ji and Roy W7EL did experiments which apparently justified an account written up by Tom under the subject. You should look it up and read it. Tom states that the double helix is totally resistive and antenna elements should be straight reffering to spacial distance., He then came to the astonishing conclusion that for maximum efficiency radiators should be straight and for some reason dismissed the double helix to give strength to his claim that radiators must be straight which is fraudulent. A straight radiator generates a external magnetic field which introduces losses whereas the double helix supplies all applied current to the generation of radiation. None of the experts on either group challenged challenged the statement allegedly supported by both Tom and Roy and it still remains in print to this day Art I continued on reading Toms many dissertations on radiation and antennas when I came across a lecture he gave at Dayton regarding small antennas. Again he pushed his personal intuition with respect to small antennas and spacial distance and received no rebuttals from the audience or the ARRL . In addition he always spouts his pet sentence on radiation is by the acceleration of charge but never states what the charge carrier is as he is still wedded to the idea of wave carriers which has no justification what so ever. We desperately need the help from physics majors to challenge the spread of old wives tales that are propagated without technical justification and sucked in by the new generation of which the hobby is now created or we lose the credibility obtained by hams who foraged before us with respect to the state of the art. Art Unwin Thanks for those. I needed a good laugh. tom K0TAR |
Whip antennas with coils
On Aug 29, 2:23*am, Art Unwin wrote:
A long time ago there was a arguement both on this group and on E ham with respect to the function of a coil with respect to replacing missing degrees of a wavelength. Both Tom W8ji and Roy W7EL did experiments which apparently justified an account written up by Tom under the subject. You should look it up and read it. Tom states that the double helix is totally resistive and antenna elements should be straight reffering to spacial distance., He then came to the astonishing conclusion that for maximum efficiency radiators should be straight and for some reason dismissed the double helix to give strength to his claim that radiators must be straight which is fraudulent. A straight radiator generates a external magnetic field which introduces losses whereas the double helix supplies all applied current to the generation of radiation. None of the experts on either group challenged challenged the statement allegedly supported by both Tom and Roy and it still remains in print to this day Art so? many incorrect things are in print, do you believe all of them? i think your double helixes are missing a few turns here and there. |
Whip antennas with coils
On Aug 29, 3:28*am, Art Unwin wrote:
On Aug 28, 9:23*pm, Art Unwin wrote: A long time ago there was a arguement both on this group and on E ham with respect to the function of a coil with respect to replacing missing degrees of a wavelength. Both Tom W8ji and Roy W7EL did experiments which apparently justified an account written up by Tom under the subject. You should look it up and read it. Tom states that the double helix is totally resistive and antenna elements should be straight reffering to spacial distance., He then came to the astonishing conclusion that for maximum efficiency radiators should be straight and for some reason dismissed the double helix to give strength to his claim that radiators must be straight which is fraudulent. A straight radiator generates a external magnetic field which introduces losses whereas the double helix supplies all applied current to the generation of radiation. None of the experts on either group challenged challenged the statement allegedly supported by both Tom and Roy and it still remains in print to this day Art I continued on reading Toms many dissertations on radiation and antennas when I came across a lecture he gave at Dayton regarding small antennas. Again he pushed his personal intuition with respect to small antennas and spacial distance and received no rebuttals from the audience or the ARRL . In addition he always spouts his pet sentence on radiation is by the acceleration of charge but never states what the charge carrier is as he is still wedded to the idea of wave carriers which has no justification what so ever. We desperately need the help from physics majors to challenge the spread of old wives tales that are propagated without technical justification and sucked in by the new generation of which the hobby is now created or we lose the credibility obtained by hams who foraged before us with respect to the state of the art. Art Unwin its all up to you art, i suggest you write the definitive expose on antenna myths... it should find a good spot in the humor section of the book stores. |
Whip antennas with coils
On Aug 28, 9:23*pm, Art Unwin wrote:
A long time ago there was a arguement both on this group and on E ham with respect to the function of a coil with respect to replacing missing degrees of a wavelength. Both Tom W8ji and Roy W7EL did experiments which apparently justified an account written up by Tom under the subject. I am completely ignorant of your "double helix" so I cannot comment on that subject. I always thought a double helix was a strand of DNA. On the subject of the number of degrees occupied by a loading coil, both Tom and Roy made an incorrect assumption that rendered their conclusions at best moot, and at worst false. Roy reported no measurable phase shift in the current through a loading coil and seemed to support the misconception that the phase shift is proportional to the delay through the coil, which it is not. Both Tom and Roy seemed to assume that the antenna current changes phase by one electrical degree for each physical degree of antenna. But EZNEC easily proves that to be an incorrect assumption for the current in a standing-wave antenna which includes dipoles, monopoles, and loaded mobile antennas. The following applies to 1/2WL dipoles and 1/4WL monopoles including loaded mobile antennas. THE PHASE OF THE NET CURRENT IN A STANDING-WAVE ANTENNA CHANGES VERY LITTLE BETWEEN THE FEEDPOINT AND THE TIP END OF THE ANTENNA. *The phase of that current cannot be used to calculate the delay through a wire or through a loading coil.* The phase of the net current in a thin-wire 1/4WL monopole changes by ~3 degrees over a 90 degree physical length. The same is true for a loaded mobile antenna. Anyone assuming that signal delay is proportional to current phase shift in a standing wave antenna is off by a magnitude or more. Tom's reported 3 ns delay through his coil is about 12% of the actual value of the delay at 4MHz. The following inductance calculator can be used to estimate the delay through a loading coil. The parameter to use is the "Beta = rad/m, Axial propagation factor of n=0 sheath helix waveguide mode at design frequency[1,8]' http://hamwaves.com/antennas/inductance.html The rad/m of the axial propagation factor can be used to calculate the number of degrees occupied by the specified loading coil and the velocity factor of the loading coil. The 100T, 10" coil that Tom used for his "measurements" has a delay of ~30 degrees at 4 MHz, virtually unrelated to the phase-shift in the standing-wave current that he reported. Delay is NOT proportional to current phase-shift on a standing-wave antenna and certainly not for the loading coils on a standing wave antenna. However, delay is proportional to current phase- shift on a traveling-wave antenna. -- 73, Cecil, w5dxp.com |
Whip antennas with coils
On Aug 29, 4:45*pm, Cecil Moore wrote:
On Aug 28, 9:23*pm, Art Unwin wrote: A long time ago there was a arguement both on this group and on E ham with respect to the function of a coil with respect to replacing missing degrees of a wavelength. Both Tom W8ji and Roy W7EL did experiments which apparently justified an account written up by Tom under the subject. I am completely ignorant of your "double helix" so I cannot comment on that subject. I always thought a double helix was a strand of DNA. On the subject of the number of degrees occupied by a loading coil, both Tom and Roy made an incorrect assumption that rendered their conclusions at best moot, and at worst false. Roy reported no measurable phase shift in the current through a loading coil and seemed to support the misconception that the phase shift is proportional to the delay through the coil, which it is not. Both Tom and Roy seemed to assume that the antenna current changes phase by one electrical degree for each physical degree of antenna. But EZNEC easily proves that to be an incorrect assumption for the current in a standing-wave antenna which includes dipoles, monopoles, and loaded mobile antennas. The following applies to 1/2WL dipoles and 1/4WL monopoles including loaded mobile antennas. THE PHASE OF THE NET CURRENT IN A STANDING-WAVE ANTENNA CHANGES VERY LITTLE BETWEEN THE FEEDPOINT AND THE TIP END OF THE ANTENNA. *The phase of that current cannot be used to calculate the delay through a wire or through a loading coil.* The phase of the net current in a thin-wire 1/4WL monopole changes by ~3 degrees over a 90 degree physical length. The same is true for a loaded mobile antenna. Anyone assuming that signal delay is proportional to current phase shift in a standing wave antenna is off by a magnitude or more. Tom's reported 3 ns delay through his coil is about 12% of the actual value of the delay at 4MHz. The following inductance calculator can be used to estimate the delay through a loading coil. The parameter to use is the "Beta = rad/m, Axial propagation factor of n=0 sheath helix waveguide mode at design frequency[1,8]' http://hamwaves.com/antennas/inductance.html The rad/m of the axial propagation factor can be used to calculate the number of degrees occupied by the specified loading coil and the velocity factor of the loading coil. The 100T, 10" coil that Tom used for his "measurements" has a delay of ~30 degrees at 4 MHz, virtually unrelated to the phase-shift in the standing-wave current that he reported. Delay is NOT proportional to current phase-shift on a standing-wave antenna and certainly not for the loading coils on a standing wave antenna. However, delay is proportional to current phase- shift on a traveling-wave antenna. -- 73, Cecil, w5dxp.com but of course we all know that a standing wave is a figment of your instrumentation! |
Whip antennas with coils
On Aug 28, 9:23*pm, Art Unwin wrote:
A long time ago there was a arguement both on this group and on E ham with respect to the function of a coil with respect to replacing missing degrees of a wavelength. The purpose and function of a loading coil used with an electrically short antenna is to offset the capacitive reactance of the short radiating section. Otherwise it will not accept much power from a transmitter or deliver much power to a receiver, due to a very high mismatch to common types of transmission line connected to its terminals. Check this using a simple NEC model. The model will have ~ the same radiation resistance, radiation pattern, and gain in dBi whether or not a loading coil at the feedpoint makes the antenna system resonant, other things equal. J. Kraus states in the link below that the radiation resistance of a loaded vertical (for instance) is nearly the same as that of an unloaded vertical of the same height. That a loading coil contributes "missing degrees" to resonate a short antenna is rather a specious concept. http://i62.photobucket.com/albums/h8...ndVertical.gif RF |
Whip antennas with coils
On 8/29/2010 9:45 AM, Cecil Moore wrote:
I am completely ignorant of your "double helix" so I cannot comment on that subject. I always thought a double helix was a strand of DNA. 73, Cecil, w5dxp.com On the helix coil, of any helical wound loading coil, or even a linear loaded monopole/dipole, you can wind another helix, in between the turns of the first helix, both ends are open and not connected. This tends to expand the antennas' bandwidth, "normalize" ant impedance, etc. That is the only thing I can figure he is referring to ... Regards, JS |
Whip antennas with coils
On 8/28/2010 7:23 PM, Art Unwin wrote:
... Art Having locked myself away for months, wife will claim years, and searching for the "best" shortened stealth antenna(s), I have been forced to employ coils. In any loaded antenna (hide the JD!) the coil is best at the bottom with a .62 L/D ratio. I currently use DLM's from Robert Vincents' patent, it allows me to break the laws ... others research and mileage may vary. The loading coil adds +J to compensate the -J from a short antenna ... however, old hams taught me that the coil adds electrical degrees, God bless them ... Regards, JS |
Whip antennas with coils
On Aug 29, 2:46*pm, K1TTT wrote:
but of course we all know that a standing wave is a figment of your instrumentation! Tom and Roy both measured *net* current - they did not use a directional coupler. If they had used a directional coupler to measure the current, they would have measured ~30 degrees shift in both the forward current and reflected current through an 80m loading coil. Let's talk about the net current in a 1/4WL lossless shorted stub. Have you never looked at that equation? What is the phase-shift in the net current from end to end in that stub? Exactly how do you rationalize zero degrees phase-shift in the current in a stub known to be 90 degrees in length? -- 73, Cecil, w5dxp.com |
Whip antennas with coils
On Aug 29, 3:04*pm, Richard Fry wrote:
The purpose and function of a loading coil used with an electrically short antenna is to offset the capacitive reactance of the short radiating section. Uh Richard, how does it accomplish that feat without a phase shift? Aren't -jX and +jX, 180 degrees out of phase? Exactly how is a -jX offset without a 180 degree phase shift? In an electrical 1/4WL standing-wave antenna, like an 80m mobile antenna, the phase shift between the forward wave at the feedpoint and the incident reflected wave at the feedpoint is obviously 180 degrees. Exactly how is that accomplished without a phase shift in the loading coil? There is a logical intuitive way to arrive at the phase shift through a loading coil. Would you like to hear about it? -- 73, Cecil, w5dxp.com |
Whip antennas with coils
On Aug 29, 5:34*pm, Cecil Moore wrote:
On Aug 29, 3:04*pm, Richard Fry wrote: The purpose and function of a loading coil used with an electrically short antenna is to offset the capacitive reactance of the short radiating section. Uh Richard, (clip) If the loading coil used to resonate an electrically short vertical really contributed '"electrical degrees" arising from some attribute(s) of the coil that made the short antenna system the full electrical equivalent of an unloaded, 1/4-wave vertical, then please explain why the loaded version does not have the radiation resistance, and typically the radiation efficiency of the unloaded version. RF |
Whip antennas with coils
On Aug 29, 5:54*pm, Richard Fry wrote:
If the loading coil used to resonate an electrically short vertical really contributed '"electrical degrees" arising from some attribute(s) of the coil that made the short antenna system the full electrical equivalent of an unloaded, 1/4-wave vertical, then please explain why the loaded version does not have the radiation resistance, and typically the radiation efficiency of the unloaded version. The radiation resistance and radiation efficiency of a short antenna, like any other antenna, depends upon the *physical* length of the antenna. Nothing can be done about that fact of physics where bigger is generally better. Short resonant antennas have a lower radiation resistance and therefore lower efficiency. Please do not confuse radiation resistance and antenna efficiency with feedpoint impedance where the reflected wave must arrive 180/360 deg in phase with the forward wave for the feedpoint impedance to be resistive and resonant. There is simply no other possibility. The *feedpoint impedance* of a standing-wave antenna depends upon the *electrical* length of the antenna. If it is resistive, the reflected wave has undergone at least a 180 degree phase shift referenced to the forward wave. Otherwise, the feedpoint impedance would not be purely resistive. Make no mistake, a typical loaded mobile antenna is 90 degrees long and part of that 90 degrees is furnished by the loading coil. Note that I said "part", not *all* of the "missing" degrees. W8JI is correct about approximately half of the phase shift between the coil and the stinger. He is 100% wrong about the other half of the phase shift which occurs within the coil. Here's a question for you: If the feedpoint impedance of a loaded standing-wave (mobile) antenna is purely resistive, how could the reflected wave arriving at the feedpoint have undergone anything except a 180 degree phase shift? Why is the feedpoint impedance of a resonant short loaded antenna usually less than that of a 1/4WL antenna? Because the radiation resistance is lower and the I^2*R losses are lower. But all resonant shortened monopoles are 90 degrees in electrical length. Anyone arguing against that fact of physics is just ignorant of how standing- wave antennas work. That includes some otherwise knowledgeable "gurus", incapable of admitting a mortal mistake, who post to this newsgroup. -- 73, Cecil, w5dxp.com |
Whip antennas with coils
On 8/29/2010 1:04 PM, Richard Fry wrote: The purpose and function of a loading coil used with an electrically short antenna is to offset the capacitive reactance of the short radiating section. Otherwise it will not accept much power from a transmitter or deliver much power to a receiver, due to a very high mismatch to common types of transmission line connected to its terminals. . . . Difficulty in getting power to an antenna is due to the mismatch between the transmitter and the impedance it sees, rather than between the transmission line and antenna. As a simple example, consider a 75 ohm dipole connected to a transmitter through a half wavelength of 600 ohm transmission line. The transmitter sees 75 ohms. Most transmitters will deliver full power to a load of that impedance and, except for line loss, all that power is delivered to the antenna in spite of a 12:1 mismatch between the transmitter and transmission line (assuming a 50 ohm output transmitter) and 8:1 mismatch between the transmission line and antenna. If you change the transmission line impedance to 75 ohms, the transmitter can't tell the difference -- it still sees 75 ohms and delivers the same amount of power, even though the line and antenna are now perfectly matched. Roy Lewallen, W7EL |
Whip antennas with coils
"Cecil Moore" wrote ... Here's a question for you: If the feedpoint impedance of a loaded standing-wave (mobile) antenna is purely resistive, how could the reflected wave arriving at the feedpoint have undergone anything except a 180 degree phase shift? There are the two possibilities: See: http://paws.kettering.edu/~drussell/...t/reflect.html 1.Reflection from a HARD boundary "at a fixed (hard) boundary, the displacement remains zero and the reflected wave changes its polarity (undergoes a 180o phase change) " 2. Reflection from a SOFT boundary " at a free (soft) boundary, the restoring force is zero and the reflected wave has the same polarity (no phase change) as the incident wave " So if the feedpoint is in distance 1/4 WL from the end you have 0 or 180 degree phase shift. Which case is in antennas? S* Why is the feedpoint impedance of a resonant short loaded antenna usually less than that of a 1/4WL antenna? Because the radiation resistance is lower and the I^2*R losses are lower. But all resonant shortened monopoles are 90 degrees in electrical length. Anyone arguing against that fact of physics is just ignorant of how standing- wave antennas work. That includes some otherwise knowledgeable "gurus", incapable of admitting a mortal mistake, who post to this newsgroup. -- 73, Cecil, w5dxp.com |
Whip antennas with coils
On Aug 29, 6:34*pm, Cecil Moore wrote:
The *feedpoint impedance* of a standing-wave antenna depends upon the *electrical* length of the antenna. If it is resistive, the reflected wave has undergone at least a 180 degree phase shift referenced to the forward wave. Otherwise, the feedpoint impedance would not be purely resistive. etc However an assumption might be taken from some posts here that a short vertical radiator loaded to resonance is the full electrical equivalent of an unloaded, resonant vertical of about 1/4-wavelength, while it is not. That is my point. RF |
Whip antennas with coils
On Aug 29, 10:38*pm, Roy Lewallen wrote:
Difficulty in getting power to an antenna is due to the mismatch between the transmitter and the impedance it sees, rather than between the transmission line and antenna. As a simple example, consider a 75 ohm dipole connected to a transmitter through a half wavelength of 600 ohm transmission line. /etc Rather than using an example of a balanced antenna having reasonably high radiation resistance and zero or low reactance at its input terminals, let us consider a base-fed 10 foot whip at 3.8 MHz -- which is more along the lines of this thread. Without using a loading coil, the input Z of that whip is about 0.6 -j 1250 ohms. The SWR that this antenna input Z presents to unmatched 50 to 600 ohm transmission line ranges from 52,167:1 to 5,340:1. Not much power will be transferred through such a match, which is the reason for the statements in my quote which you referred to. RF |
Whip antennas with coils
On Aug 30, 5:12 am, Richard Fry wrote:
On Aug 29, 6:34 pm, Cecil Moore wrote: However an assumption might be taken from some posts here that a short vertical radiator loaded to resonance is the full electrical equivalent of an unloaded, resonant vertical of about 1/4-wavelength, while it is not. That is my point. The short vertical radiator loaded to resonance *IS* the full *electrical* length of an unloaded, resonant vertical of about 1/4WL, which is related to the feedpoint impedance. It is NOT the full *physical* length which is related to radiation resistance and efficiency. The feedpoint impedance of any electrically long 90 degree standing- wave antenna, including resonant loaded mobile antennas, is: Zfp = (Vfor-Vref)/(Ifor+Iref) on the antenna, not on the feedline. The reflected voltage has undergone a 180 degree phase shift. The reflected current has undergone a 360 degree phase shift. Part of the phase shift occurs in the loading coil. A typical resonant mobile antenna is *electrically* 90 degrees long. If it was less than 90 degrees long *electrically* it would exhibit capacitive reactance at the feedpoint. Let's discuss a base-loaded configuration which is less complicated than a center-loaded configuration. (1) The delay through the loading coil is part of that 90 degrees. (2) The delay through the stinger is part of that 90 degrees. (3) The phase shift at the coil to stinger junction is part of the 90 degrees. Tom, W8JI, assumes a lumped inductor for calculating the phase shift at the coil to stinger junction but a 75m bugcatcher loading coil is NOT a lumped inductor - it is a distributed network existing in the real world with an associated real-world delay through the coil. The other rail of the argument assumes all of the "missing degrees" come from the coil and none from the coil to stinger junction. Both sides are wrong. All three phase shift components listed above exist in a base-loaded mobile antenna. (There are four phase shift components in a center-loaded mobile antenna. Degrees of electrical length are actually lost at the low Z0 base section to high Z0 loading coil junction. That's why the inductance (coil delay) has to increase for center-loaded configurations.) Interestingly enough, a base-loaded mobile antenna functions like the dual-Z0 stubs covered on my web page and can be analyzed in the same manner: http://www.w5dxp.com/shrtstub.htm Here is a simplified approximate representation of what a base-loaded mobile antenna looks like electrically: FP------Z01=5000 ohms------+------Z02=500 ohms------ The Z01 portion is the base loading coil and the Z02 portion is the stinger. The Z0 of the loading coil can be obtained from the inductance calculator at: http://hamwaves.com/antennas/inductance.html -- 73, Cecil, w5dxp.com |
Whip antennas with coils
On Aug 30, 5:44*am, Richard Fry wrote:
... let us consider a base-fed 10 foot whip at 3.8 MHz ... Without using a loading coil, the input Z of that whip is about 0.6 -j 1250 ohms. A 10 foot whip at 3.8 MHz is about 0.0386 wavelength or about 14 degrees. That's about -j4.0 on a Smith Chart. Can we say that -j1250/ Z0 = -j4.0? such that the Z0 characteristic impedance of the whip at that input Z is ~312.5 ohms? -- 73, Cecil, w5dxp.com |
Whip antennas with coils
On 8/30/2010 3:12 AM, Richard Fry wrote:
On Aug 29, 6:34 pm, Cecil wrote: The *feedpoint impedance* of a standing-wave antenna depends upon the *electrical* length of the antenna. If it is resistive, the reflected wave has undergone at least a 180 degree phase shift referenced to the forward wave. Otherwise, the feedpoint impedance would not be purely resistive. etc However an assumption might be taken from some posts here that a short vertical radiator loaded to resonance is the full electrical equivalent of an unloaded, resonant vertical of about 1/4-wavelength, while it is not. That is my point. RF In the above "equivalent" is used in an interesting way. If equivalent is used as a term of efficiency, you are absolutely correct. If you are saying my transmitter still knows the difference between a "straight" 1/4 wave and an "equivalent loaded" antenna, the use becomes false ... i.e., when I dip the loaded antenna with my GDO, and the antenna is properly constructed, it tells me I have the "true" equivalent of a 1/4 wave. Regards, JS |
Whip antennas with coils
On 8/30/2010 3:44 AM, Richard Fry wrote: On Aug 29, 10:38 pm, Roy wrote: Difficulty in getting power to an antenna is due to the mismatch between the transmitter and the impedance it sees, rather than between the transmission line and antenna. As a simple example, consider a 75 ohm dipole connected to a transmitter through a half wavelength of 600 ohm transmission line. /etc Rather than using an example of a balanced antenna having reasonably high radiation resistance and zero or low reactance at its input terminals, let us consider a base-fed 10 foot whip at 3.8 MHz -- which is more along the lines of this thread. Without using a loading coil, the input Z of that whip is about 0.6 -j 1250 ohms. The SWR that this antenna input Z presents to unmatched 50 to 600 ohm transmission line ranges from 52,167:1 to 5,340:1. Not much power will be transferred through such a match, which is the reason for the statements in my quote which you referred to. RF Power will indeed be transferred through such a match. Using your antenna as an example, suppose that a transmitter with output Z of 50 ohms is connected to a tuner that transforms its output impedance to 0.6 + j1250 ohms. Connect the output of the tuner to a half wavelength 600 ohm transmission line to the antenna. The transmitter will see 50 + j0 ohms, the antenna will see an impedance of 0.6 + j1250 ohms, and full power will be transferred. Power transfer has nothing to do with the SWR on the line or the match between the line and antenna. In practice, the line loss will increase some due to the very high SWR, but the loss increase won't be much if the matched line loss is low. I chose a half wavelength for simplicity, but it's not necessary. Other lengths of line will transform the antenna impedance to different values. All that's necessary is to readjust the tuner accordingly to match the different impedance. Amateurs have successfully been using this method to feed nonresonant and multi-band antennas for decades. Roy Lewallen, W7EL |
Whip antennas with coils
On Aug 30, 3:19*pm, Roy Lewallen wrote:
Using your antenna as an example, suppose that a transmitter with output Z of 50 ohms is connected to a tuner that transforms its output impedance to 0.6 + j1250 ohms. /etc Note that my post stated "UNMATCHED." RF |
Whip antennas with coils
Roy,
The mention of reactance means we are talking in the frequency domain, and a steady state solution is being discussed. Roy Lewallen wrote in : .... Power will indeed be transferred through such a match. Using your antenna as an example, suppose that a transmitter with output Z of 50 ohms is connected to a tuner that transforms its output impedance to 0.6 + j1250 ohms. Connect the output of the tuner to a half wavelength Does the use of "output impedance" here mean that he transmitter can be validly represented by a Thevenin equivalent circuit, and that "output impedance" is the Thevenin equivalent source impedance. Without getting into that arguable postion and reinforcing the notion that a transmitter rated for a nominal 50 ohm load has a source impedance of 50+j0, you could say: .... Using your antenna as an example, suppose that a transmitter designed to operate into a load Z of 50+j0 ohms is connected to a tuner that transforms the antenna (0.5-j1250) to its preferred load impedance (50 +j0). Connect ... 600 ohm transmission line to the antenna. The transmitter will see 50 + j0 ohms, the antenna will see an impedance of 0.6 + j1250 ohms, and full power will be transferred. Power transfer has nothing to do with the SWR on the line or the match between the line and antenna. In practice, the line loss will increase some due to the very high SWR, but the loss increase won't be much if the matched line loss is low. I chose a half wavelength for simplicity, but it's not necessary. Other lengths of line will transform the antenna impedance to different values. All that's necessary is to readjust the tuner accordingly to match the different impedance. And: All that's necessary is to readjust the tuner accordingly to deliver the transmitter its rated load impedance. Amateurs have successfully been using this method to feed nonresonant and multi-band antennas for decades. Roy Lewallen, W7EL As you will have noted, some band the term 'match' around with abandon, and it means different things in different contexts, and to different readers. Take a transmitter designed for a 50+j0 load, connected by an electrical half wave of 70 ohm coax to a 50 ohm load. Is it 'matched'? Well, from the information, it is correctly loaded, it is designed for a 50+j0 load, and it has a 50+j0 (approximately) load. We don't actually know the source impedance, and even if we did, it this case, whether the transmitter is 'matched' to the line, and whether the line is 'matched' to the load is unimportant. 'Output impedance' is another term that is used differently, some use it to mean the equivalent source impedance, some to mean the required load, and some insist the foregoing are naturally the same, or will be if the transmitter is 'matched' for maximum power output. Owen |
Whip antennas with coils
On 8/30/2010 3:19 PM, Richard Fry wrote:
On Aug 30, 3:19 pm, Roy wrote: Using your antenna as an example, suppose that a transmitter with output Z of 50 ohms is connected to a tuner that transforms its output impedance to 0.6 + j1250 ohms. /etc Note that my post stated "UNMATCHED." RF I did. You stated: Without using a loading coil, the input Z of that whip is about 0.6 -j 1250 ohms. The SWR that this antenna input Z presents to unmatched 50 to 600 ohm transmission line ranges from 52,167:1 to 5,340:1. In my example, the antenna is not matched to the transmission line. Nor, for that matter, is the transmitter matched to the transmission line. My point is that power transfer doesn't depend on either of these points being matched. Roy Lewallen, W7EL |
Whip antennas with coils
On Aug 29, 8:45*pm, John Smith wrote:
On 8/28/2010 7:23 PM, Art Unwin wrote: ... Art Having locked myself away for months, wife will claim years, and searching for the "best" shortened stealth antenna(s), I have been forced to employ coils. *In any loaded antenna (hide the JD!) the coil is best at the bottom with a .62 L/D ratio. I currently use DLM's from Robert Vincents' patent, it allows me to break the laws ... others research and mileage may vary. The loading coil adds +J to compensate the -J from a short antenna ... however, old hams taught me that the coil adds electrical degrees, God bless them ... Regards, JS this one sounds like a religious argument to me... to phase shift or not to phase shift, that is the degrees of shortening... weather tis nobler to count the degrees or accept the time delay traveling waves, one way or another there is still high voltage at the top and high current at the bottom, and never the twain shall meet. |
Whip antennas with coils
On 8/30/2010 4:40 PM, K1TTT wrote:
... this one sounds like a religious argument to me... to phase shift or not to phase shift, that is the degrees of shortening... weather tis nobler to count the degrees or accept the time delay traveling waves, one way or another there is still high voltage at the top and high current at the bottom, and never the twain shall meet. From your posts here, over many subjects, I would deduce everything here sounds religious to you. Or, as if it comes from evil spirits. Or, it comes from an oracle sniffing the vapors of petroleum. Or, some drug which has affected the mind ... etc. Regards, JS |
Whip antennas with coils
On Aug 30, 6:36*pm, "J. Mc Laughlin" wrote:
* * I add a vote for avoiding the arguable position about the "output Z" of a transmitter (note, transmitter) and instead using words similar to those used by Mr. Owen Duffy: * a transmitter designed to operate into a load Z of 50+j0 ohms I'll drink to that. Consider a transmitter connected to an antenna tuner by a piece of 50 ohm coax where the SWR is an ideal 1:1 on that short piece of coax. There are no reflected waves on the coax. Therefore, no reflected energy reaches the source. Energy transfer is a one-way process on that flat piece of coax. Except for efficiency, the source impedance doesn't matter. Whether the source impedance is 10 ohms, or 50 ohms, or 200 ohms, dissipative or non-dissipative, reactive or resistive - just doesn't matter. All that matters is the voltage delivered to the 50 ohm Z0-match at the tuner input. -- 73, Cecil, w5dxp.com |
Whip antennas with coils
On Aug 30, 5:52*pm, Roy Lewallen wrote:
In my example, the antenna is not matched to the transmission line. Nor, for that matter, is the transmitter matched to the transmission line. My point is that power transfer doesn't depend on either of these points being matched. Roy: My post showing very high input SWR at the base of an unloaded, base- driven, 10 foot vertical on 3.8 MHz described an UNMATCHED system resulting from its connection to transmission lines of typical impedance values. It did not include matching networks, whether located at the base of the vertical radiator, the output connector of the transmitter, or wherever. Then you posted, "Using your antenna as an example, suppose that a transmitter with output Z of 50 ohms is connected to a tuner that transforms its output impedance to 0.6 + j1250 ohms. ... The transmitter will see 50 + j0 ohms, the antenna will see an impedance of 0.6 + j1250 ohms, and full power will be transferred." That configuration you posted is a MATCHED system, and its performance does not disprove the accuracy of my post. RF |
Whip antennas with coils
On 8/31/2010 3:09 PM, Richard Fry wrote:
On Aug 30, 5:52 pm, Roy wrote: In my example, the antenna is not matched to the transmission line. Nor, for that matter, is the transmitter matched to the transmission line. My point is that power transfer doesn't depend on either of these points being matched. Roy: My post showing very high input SWR at the base of an unloaded, base- driven, 10 foot vertical on 3.8 MHz described an UNMATCHED system resulting from its connection to transmission lines of typical impedance values. It did not include matching networks, whether located at the base of the vertical radiator, the output connector of the transmitter, or wherever. Then you posted, "Using your antenna as an example, suppose that a transmitter with output Z of 50 ohms is connected to a tuner that transforms its output impedance to 0.6 + j1250 ohms. ... The transmitter will see 50 + j0 ohms, the antenna will see an impedance of 0.6 + j1250 ohms, and full power will be transferred." That configuration you posted is a MATCHED system, and its performance does not disprove the accuracy of my post. RF So you're saying that the mismatch between the impedance of an antenna and the transmission line connected to it doesn't inhibit power flow when there's a matching network anywhere in the system. But it does interfere with power flow when there's no matching network? What if the antenna is 50 ohms and the transmission line is a half wavelength of 600 ohms, for a 12:1 mismatch? There's no matching network. The transmitter sees 50 ohms. The antenna sees 50 ohms. What will interfere with the power flow? Roy Lewallen, W7EL |
Whip antennas with coils
On Aug 31, 5:25*pm, Roy Lewallen wrote:
What if the antenna is 50 ohms and the transmission line is a half wavelength of 600 ohms, for a 12:1 mismatch? There's no matching network. The transmitter sees 50 ohms. The antenna sees 50 ohms. What will interfere with the power flow ? This, and your other what ifs describe a system *different* than the one in my post. The performance of the systems you described does not apply to the system I described, and vise-versa.. RF |
Whip antennas with coils
In message , Roy Lewallen
writes On 8/31/2010 3:09 PM, Richard Fry wrote: On Aug 30, 5:52 pm, Roy wrote: In my example, the antenna is not matched to the transmission line. Nor, for that matter, is the transmitter matched to the transmission line. My point is that power transfer doesn't depend on either of these points being matched. Roy: My post showing very high input SWR at the base of an unloaded, base- driven, 10 foot vertical on 3.8 MHz described an UNMATCHED system resulting from its connection to transmission lines of typical impedance values. It did not include matching networks, whether located at the base of the vertical radiator, the output connector of the transmitter, or wherever. Then you posted, "Using your antenna as an example, suppose that a transmitter with output Z of 50 ohms is connected to a tuner that transforms its output impedance to 0.6 + j1250 ohms. ... The transmitter will see 50 + j0 ohms, the antenna will see an impedance of 0.6 + j1250 ohms, and full power will be transferred." That configuration you posted is a MATCHED system, and its performance does not disprove the accuracy of my post. RF So you're saying that the mismatch between the impedance of an antenna and the transmission line connected to it doesn't inhibit power flow when there's a matching network anywhere in the system. But it does interfere with power flow when there's no matching network? What if the antenna is 50 ohms and the transmission line is a half wavelength of 600 ohms, for a 12:1 mismatch? There's no matching network. The transmitter sees 50 ohms. The antenna sees 50 ohms. What will interfere with the power flow? In my simplistic world, this is how I understand things: Provided the TX is followed by a tuner/matcher, which matches whatever is attached to the tuner output to 50 ohms at the tuner input, the TX will be happy. The power loss in the feeder is essentially a function of its inherent loss (when matched) per unit length, and the SWR on it. The SWR is a function of the mismatch between the load on the antenna end of the feeder, and the feeder characteristic impedance. The greater the SWR and the longer the feeder, the higher will be the loss on the feeder. 'Lossless' feeder have no loss, regardless of length and SWR. However, with real-world feeders, the losses rise increasingly rapidly with increasing SWR. The impedance looking into the tuner end of the feeder is the impedance of the load, transformed by length of the feeder, and is also modified by the loss of the feeder. The higher the feeder loss, the closer the impedance seen at the tuner end will approach the characteristic impedance of the feeder. [Long lengths of lossy feeder - maybe with a nominal termination on the far end - can make good dummy loads at VHF and UHF.] -- Ian |
Whip antennas with coils
Ian Jackson wrote in
: .... In my simplistic world, this is how I understand things: Provided the TX is followed by a tuner/matcher, which matches whatever is attached to the tuner output to 50 ohms at the tuner input, the TX will be happy. "Happey" eh! The power loss in the feeder is essentially a function of its inherent loss (when matched) per unit length, and the SWR on it. Wrong. The SWR is a function of the mismatch between the load on the antenna end of the feeder, and the feeder characteristic impedance. The Well, the SWR at the load end is a function of Zl and Zo (both complex quantities)... but the 'notional' SWR varies along the line as accounted for by the complex propagation coefficient, in most practical cases at HF, SWR decreases smoothly from load to source. greater the SWR and the longer the feeder, the higher will be the loss on the feeder. 'Lossless' feeder have no loss, regardless of length and SWR. However, with real-world feeders, the losses rise increasingly rapidly with increasing SWR. See your earlier misconception regarding loss being a simple function of SWR. The impedance looking into the tuner end of the feeder is the impedance of the load, transformed by length of the feeder, and is also modified by the loss of the feeder. You got that right. The higher the feeder loss, the closer the impedance seen at the tuner end will approach the characteristic impedance of the feeder. [Long lengths of lossy feeder - maybe with a nominal termination on the far end - can make good dummy loads at VHF and UHF.] Yes, but is it of practical application in a transmit scenario? If the input impedance due to a severly mismatched load is at all close to Zo, then you have lost most of the transmitter power in the line. The "make a good dummy load" recipe doesn't address the power rating, especially where most of the power is dissipated in a very short length of cable. Owen |
Whip antennas with coils
Owen Duffy wrote in news:Xns9DE6CD1DCDA37nonenowhere@
61.9.134.55: "Happey" eh! How did that e get in there... "Happy", a new dumbed down attribute of a transmitter. Everyone loves a happy transmittter. What happens if they are not happy? Well, everyone knows you will let the smoke out of them (AKA reflected power is dissipated in the PA and kills transmitters). Yep, a simple world... even if wrong. Owen |
Whip antennas with coils
On Sep 1, 4:11*am, Ian Jackson
wrote: The impedance looking into the tuner end of the feeder is the impedance of the load, transformed by length of the feeder, and is also modified by the loss of the feeder. That's the result of the 50 ohm Z0-match achieved at the input of the tuner which is responsible for the transmitter's "happiness". A Z0- match causes the incident reflected energy to be redistributed back toward the antenna through a combination of reflection and destructive interference. -- 73, Cecil, w5dxp.com |
Whip antennas with coils
In message , Owen Duffy
writes Ian Jackson wrote in : ... In my simplistic world, this is how I understand things: Provided the TX is followed by a tuner/matcher, which matches whatever is attached to the tuner output to 50 ohms at the tuner input, the TX will be happy. "Happey" eh! Your later correction is noted! The power loss in the feeder is essentially a function of its inherent loss (when matched) per unit length, and the SWR on it. Wrong. OK, the loss may not be uniform if the SWR is greater at the load end. But when is the power loss* NOT a function of its matched loss per unit length, and the SWR on it. [*Should I have said 'in dB'? ] The SWR is a function of the mismatch between the load on the antenna end of the feeder, and the feeder characteristic impedance. The Well, the SWR at the load end is a function of Zl and Zo (both complex quantities)... but the 'notional' SWR varies along the line as accounted for by the complex propagation coefficient, in most practical cases at HF, SWR decreases smoothly from load to source. greater the SWR and the longer the feeder, the higher will be the loss on the feeder. 'Lossless' feeder have no loss, regardless of length and SWR. However, with real-world feeders, the losses rise increasingly rapidly with increasing SWR. See your earlier misconception regarding loss being a simple function of SWR. I didn't say 'simple'. I said 'essentially'. The impedance looking into the tuner end of the feeder is the impedance of the load, transformed by length of the feeder, and is also modified by the loss of the feeder. You got that right. Yes, I know I did. The higher the feeder loss, the closer the impedance seen at the tuner end will approach the characteristic impedance of the feeder. [Long lengths of lossy feeder - maybe with a nominal termination on the far end - can make good dummy loads at VHF and UHF.] Yes, but is it of practical application in a transmit scenario? As a termination, I've used a fairly long piece of coax plus, for good luck, a low-power termination hung on the end. If the input impedance due to a severly mismatched load is at all close to Zo, then you have lost most of the transmitter power in the line. Quite. The "make a good dummy load" recipe doesn't address the power rating, especially where most of the power is dissipated in a very short length of cable. Oh, indeed. But I have to confess that I've realised that I'm suddenly unsure of where, on a long, lossy, feeder with a mismatched load, most of the absolute power is actually lost. I'm assuming that it is at the TX end, where the power is greatest - even though that's where the SWR is best. Please advise! -- Ian |
Whip antennas with coils
On Sep 1, 10:29 am, Ian Jackson
wrote: Oh, indeed. But I have to confess that I've realised that I'm suddenly unsure of where, on a long, lossy, feeder with a mismatched load, most of the absolute power is actually lost. I'm assuming that it is at the TX end, where the power is greatest - even though that's where the SWR is best. Please advise! It seems logical that the highest I^2*R losses would be where the standing-wave current is maximum and the highest dielectric losses wold be where the the standing-wave voltage is maximum. As Owen is fond of pointing out, the locations of those points are very important. If a current maximum point exists at the load and a voltage maximum point exists at the source, the losses at the load are probably higher than the losses at the source on HF. If a voltage maximum point exists at the load and a current maximum point exists at the source, the losses at the load are probably lower than the losses at the source on HF (assuming that losses due to SWR are mostly I^2*R losses on HF). -- 73, Cecil, w5dxp.com |
Whip antennas with coils
On 9/1/2010 9:20 AM, Cecil Moore wrote:
... It seems logical that the highest I^2*R losses would be where the standing-wave current is maximum and the highest dielectric losses wold be where the the standing-wave voltage is maximum. As Owen is fond of pointing out, the locations of those points are very important. If a current maximum point exists at the load and a voltage maximum point exists at the source, the losses at the load are probably higher than the losses at the source on HF. If a voltage maximum point exists at the load and a current maximum point exists at the source, the losses at the load are probably lower than the losses at the source on HF (assuming that losses due to SWR are mostly I^2*R losses on HF). -- 73, Cecil, w5dxp.com Uhhh, sorry to pose this question to you here but, doesn't a "tuner" really just "shorten" and "lengthen" the feed line? I mean, not physically, of course. But, it would seem to me that, this is exactly what my xmitter and ant are "seeing." Regards, JS |
Whip antennas with coils
On Sep 1, 11:43*am, John Smith wrote:
Uhhh, sorry to pose this question to you here but, doesn't a "tuner" really just "shorten" and "lengthen" the feed line? *I mean, not physically, of course. *But, it would seem to me that, this is exactly what my xmitter and ant are "seeing." If all the tuner did was shorten and lengthen the feedline, there would be only one purely resistive low resistance available on the SWR circle on the Smith Chart and it wouldn't be exactly 50 ohms. So a tuner does one other thing - it transforms the non-50 ohm resistive impedance to a 50 ohm resistive impedance. It is not only the equivalent of shortening and lengthening the transmission line but also performs an N:1 transformer function. Of course, it does that seamlessly, i.e. it is not actually a two-step process. When I vary the length of my ladder-line to obtain system resonance, I have to be satisfied with purely resistive impedances between 35 ohms and 85 ohms. I cannot achieve a perfect 50 ohms on all HF bands with my matching method. That would require the addition of an actual N:1 transformer which is certainly possible but probably not worth the effort. Since 35-85 ohms is perfectly acceptable to my SC-500 amp, I don't need a high-power tuner. And since the SC-500 is spec'ed to handle an SWR of 6:1, I doubt that an 35-85 ohm load makes it "unhappy". If my SC-500 has ever been "unhappy", I failed to observe that "unhappiness" but maybe I am just oblivious to such? :) -- 73, Cecil, w5dxp.com |
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