"Jim Kelley" wrote in message
...


H. Adam Stevens, NQ5H wrote:
"Jim Kelley" wrote in message
...
deletia....


I have a question. If a loading coil only makes a physically short
antenna look like it's an electrical quarter wavelength reactively, why
does its position along the radiator make such an apparent difference in
performance?

73, Jim AC6XG



My first reaction is to point out that this was (is?) a question on the
Extra exam.

I think you may be right.

Now how can I explain qualitatively why this is?

Start with the answer to the exam question? :-)

Consider an end-fed wire antenna.
An electromagnetic wave goes through the conduction electrons down to
the
end and reflects back.
At 1/4 wavelength, the reflected wave is exactly in phase with the
source so
the load looks minimal and resistive, loss plus radiation. As the
antenna
gets shorter the radiation resistance gets lower and the reflected wave
gets
back to the feed point sooner (becomes capacitive). We need to add
inductance to slow down the wave so it gets back in phase.

Is that the controversial phase shift?

We cannot, alas,
raise the radiation resistance; this is a short antenna. If I place the
inductor at the feed point all the current must flow through it,
maximizing
loss. If I place it at the top little current flows through it,
minimizing
effectiveness. If I distribute it the antenna's resonance is broader,
but at
what cost? Lower Q. The signal strength is less. So I make the coil as
short
as I can, put it in the middle and it's juuust right.

73, H. NQ5H

Sounds like you're describing a sort of 'current drop'. Is I^2R loss
entirely responsible for this drop? It seems that the phase shift you
described earlier would have to cause a change in the standing wave
pattern along the radiator. If the loading coil was at the feedpoint,
then the maximum current would appear only at the feedpoint. Above the
coil, the currents would be out of phase, as you described, because of
the shortened radiator, and the maximum available current would not flow
along any point on the radiator. Moving the coil higher would allow
maximum current to flow along at least the lower portion of the
radiator. Loss is certainly a factor, but I can't see how it is the
entire explanation for the rather pronounced effect. Hence my question.

73, Jim AC6XG

Hi Jim
Clearly an entire explanation would require a rigorous solution to Maxwell's
equations, but you state it better than I did.
The current below the loading coil is as if the antenna were full length,
max power radiated; the voltage above the loading coil is as if the antenna
were full length. And you're right, moving the coil away from the current
max (feed point) reduces I^2R losses in the coil. Are there E^2/R losses in
the coil if we mount it at the top?
Looks like if we make the boundary conditions at the ends of the antenna as
if it were full-length it works best.
Remember the boundary conditions; the current is max at the feedpoint and
zero at the end (can't go anywhere).
The empirical fact is a lumped L in the center of the antenna works best and
one can "sort of" intuitively see why placing the coil at either end has
problems. Hence I use a 4 foot screwdriver and a 4 foot whip. The antenna at
resonance on 40 and 80 is about 20 ohms which I match with a toroidal
autoformer. Then 2 feet of coax to the TS480HX.
73, H.
NQ5H