[TowerTalk] EFHW core

[K9MA]

On 9/21/2020 7:49 PM, Richard (Rick) Karlquist wrote:
> On 9/21/2020 4:27 PM, K9MA wrote:
>> It's a multiple of a half wave on all those bands, which means the 
>> feedpoint impedance is very high, typically 2,000 to 5,000 Ohms. A good 
>
> "all"?
>
> If it were about 65 feet long, it could be a half wave on 40 CW, and a
> full wave on 20 low phone, 1.5 wl on 15 high phone and 2 wavelengths 
> on 10 FM. (The resonant frequency increases by a factor a little in
> excess of the harmonic number.)  However, even that doesn't explain 
> how it is supposed to work on 30, 17, and 12 meters or cover 10 CW.
>
> You might as well call it an end fed random wire.
>
> Rick N6RK

It's certainly not close to a half wave multiple on 30, 17, and 12 
meters. On the other bands, it doesn't have to be exactly resonant, just 
close enough that the impedance is high enough that it works with a 
minimal counterpoise. With a really good 8:1 transformer (nearly ideal 
over the frequency range), one should be able to get the SWR within 
range of a typical internal ATU, say below 3:1. It might take some 
adjustment of the wire length to find the best compromise for all the bands.

I use mine mainly on 40 and 20, so I modeled it to determine the optimal 
length for those CW bands. (21.3 meters of AWG 26.) I use a very small 
homebrew tuner with that antenna, and it has a fairly narrow range. That 
length, however, is for a wire supported at one point about 10 meters 
high, about 5 meters from the far end, sort of a lopsided inverted V. 
The feedpoint is near ground level and the far end has a long line on it 
so it can be as high as possible. With a single support, that 
configuration, according to the model, was about optimal for those 
bands. In my case, a feedline is precluded due to weight limitations. (I 
carry the antenna on a bicycle.)

With a wider range tuner, a wire that length will  work as a random wire 
on the other bands, but a better ground system will be necessary to make 
it efficient and prevent RF burns.

73,

Scott K9MA


-- 
Scott  K9MA

k9ma at sdellington.us