Bruce Perens K6BP
I just checked out an end-fed antenna for portable low-power HF ham radio. It has no counterpoise, the instructions don't even talk about a counterpoise, and the designer warns that if you use a "ground grid" you may detune the antenna!
I looked at another brand, and it's the same. That one is sold by DX Engineering.
Electrically, there _must_ be a counterpoise. What is it with these antennas? It's the shield of your feed-line, your radio, everything connected to your radio, and you.
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Bruce Perens K6BP
,,,
This is going to cause RF feedback when you go to run digital modes with their low-voltage audio input. And it won't work as efficiently as it could.
ftg
@BrucePerens_K6BP
Most use the coax feeding it, as apparently 0.05 wavelengths is optimal (according to some simulations I cannot make google find right now), so doing it with the coax feedline is easy.
The end-feds people build here in Finland all seem to include a ferrite choke in the feedline a slight distance from the antenna impedance transformer.
Bruce Perens K6BP
@ftg - This fellow has done a credible study: https://www.aa5tb.com/efha.html
and he has references at the end.
Apparently 0.05 wavelength is the length necessary to provide the correct impedance. I believe that their impedance is correct and their SWR is sufficiently low, but I bet the radiation resistance is horrible.
Like those small loop antennas that are really only 5% efficient, you can still work the world.
ftg
@BrucePerens_K6BP
Could be that.
And with the small loops the selectivity and ease of nulling out local interference also helps.
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