"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.
Now how can I explain qualitatively why this is?
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. 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