[TowerTalk] EFHW core

[K9MA]

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 at sdellington.us