It s hard to be smart here without tons of experimentation. Danny Horvat of
myantennas.com seems to have done it. First he selected cores that do not heat
much yet require few turns. Second he found a type of winding that make 9:1
transformation efficient with very little losses. Third, he put a capacitor on
input to cancel stray capacitances, such that does not get burnt. Forth, he put
a coil a few feet from the end to move the resonance lower for higher
frequencies. Fifth, for KW, one needs an extra choke at the end of the
feed-line to avoid a “hot” amp. A few turns of coax on a large snap-in toroid
are good enough. His boxes take a KW though not 100% duty.
One EFHW box carries 80-10 and 40-10, just different wires. It is a miracle
for such a simple box to cover so many bands. I compared his EFHW to an OCF,
and not much difference except EFHW covers more bands. What is even more
surprising, his EFHW-80-10 seems to work even on 160 at 500W without burning.
Ignacy NO9E
> On Sep 21, 2020, at 19:27, K9MA <k9ma@sdellington.us> 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 transformer
> with a turns ratio of about 8:1 should get it within range of the internal
> ATUs of most radios. Making a transformer that works over that frequency
> range is not trivial: If the core is big enough to avoid saturation on 40
> meters, the stray capacitance may be to high on 10.
>
> As far as the core, it depends on the size of the core(s), material, number
> of turns, impedance, and frequency. There's a simple equation for that,
> found in any magnetics design reference.
>
> https://www.mag-inc.com/Design/Design-Guides
>
> This will tell you the minimum core size and minimum number of turns for a
> given voltage. Figuring out the effect of stray capacitance isn't so easy.
>
> I think a better approach is to use a simple L-network tuner. With a switched
> inductor with just a single properly positioned tap for band, the SWR should
> be well within the internal ATU range. The only tricky part is finding a
> variable capacitor with a high enough voltage rating, about 2500 V at 500 W.
> The L and C values can be determined experimenatlly at low power with small
> components.
>
> 73,
> Scott K9MA
>
>
>
>
> On 9/21/2020 5:48 PM, Richard (Rick) Karlquist wrote:
>> How can an antenna be a "half wave" on 40 thru 10 meters
>> all at the same time?
>>
>> Rick N6RK
>>
>> On 9/21/2020 3:44 PM, Tom Osborne Sr. wrote:
>>> Hi All
>>>
>>> Today I put together an EFHW antenna for 40-10. In checking it out, the
>>> SWR was good and all looked like it was going to work. Made one contact
>>> with it, running around 500 watts, and the core got really hot. I am
>>> assuming for around 600 watts, 1 core isn't enough.
>>>
>>> I have some more cores and was wondering, for 600 watts. How many should I
>>> use, 2 or 3? I could put 3 together and make 1 core.
>>>
>>> Running low power it works fine and doesn't get hot. 73
>>> Tom W7WHY
>>> _______________________________________________
>>>
>>>
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>
>
> --
> Scott K9MA
>
> k9ma@sdellington.us
>
> _______________________________________________
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