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#1
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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 |
#2
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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! |
#3
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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 |
#4
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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 |
#5
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Whip antennas with coils
On Aug 29, 12:45*pm, 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. Could be a self-replicating antenna... |
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