[Elecraft] EFHW clarification

[K9MA]

The low current in the counterpoise or ground system is the main 
advantage of an EFHW over a random wire.  (A random wire "close" to a 
half wave will have a similar radiation pattern, but much higher ground 
system current.)

The feedpoint impedance of the EFHW can be 1500 to 5000 Ohms. With a 9:1 
transformer, that means the SWR will be, at best, somewhere between 3:1 
and 11:1.  Coax is very lossy at such high SWR, so you wouldn't want to 
use more than a few feet of it.  Better, if possible, is to run the wire 
and transformer right to the radio, with just a few inches of coax.  On 
20 meters and above, you may have to slightly adjust the wire length, 
probably shorter,  to compensate for the stray capacitance of 
transformer.  If the ATU won't match it, try shortening the wire a bit.  
That same length will probably work on the lower band, but you may have 
to look for a compromise.

While the coax shield, radio chassis, etc. will serve as a counterpoise, 
a few feet of wire on the ground opposite the antenna may be slightly 
better.  Really cheap insurance, both in money and weight.  Because the 
current is so low, a balun should not be necessary.

As someone else pointed out, another advantage of the EFHW is that you 
don't need a feedline, which saves some weight.  In a way, the near end 
of the antenna IS the feedline, as it doesn't radiate much until the 
current gets higher a fraction of a wavelength away.  Even on the second 
harmonic, where the first high current point is only 1/4 the way to the 
end of the wire, there's another one at the 3/4 point.  Even if the 
first one isn't radiating very effectively, about half the power goes to 
the second, higher one.

73,

Scott  K9MA


On 2/8/2017 18:14, Dan Presley wrote:
> Thanks for the replies so far. Just to clarify-I plan to use the built in auto tuners in my KX2 and 3, so hopefully they can handle the job. I already have a good 9:1 transformer which should work. The primary question is the advantage to the EFHW as opposed to a true random length (not a halfwave at desired freq) with a counterpoise. It sounds like the high current point is a quarter wave from the feedpoint which could be an advantage from the random length. I’ve also seen a variety of ideas on the proper length of coax to use with an EFHW, which as I understand will act as a counterpoise. I now have one of the nice lightweight SOTA poles which would be good with whatever wire I go with. Waiting for some decent weather in the Pacific NW  :))
>
>
> Dan Presley  N7CQR
> n7cqr at arrl.net
>
>
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-- 
Scott  K9MA

k9ma at sdellington.us