EFHW clarification

[Dan Presley]

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
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[Elecraft mailing list]

I would suggest, a good coax choke also.  Cheap easy and good insurance.  The real problem of the 9:1 un un is that IF the transformer is not balanced, there will be CMC to deal with. 

Mel, K6KBE


      From: Dan Presley <[hidden email]>
 To: K2 <[hidden email]>
 Sent: Wednesday, February 8, 2017 4:14 PM
 Subject: [Elecraft] EFHW clarification
   
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
[hidden email]


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[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
> [hidden email]
>
>
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--
Scott  K9MA

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[k6dgw]

Guess I missed that in your post, sorry.  Any conductor of any length
will radiate, resonant or not ... provided you can get your TX to feed
power into it.  In all cases, the far end of the conductor is an open
circuit [High E, low I, High Z] The impedance at the feed end will be a
complex number.  If the conductor is resonant, the reactance will be
zero.  If it is not resonant, it will be non-zero and either inductive
or capacitive, and one more component your ATU has to deal with.  
Elecraft ATU's [even the simplest KX1] can deal with a fairly wide range
of complex impedances ... with a 9:1 transformer, you should be OK.

Yes, the outside surface of the coax shield will act as a counterpoise.  
Even without coax [direct feed], the radio chassis, headphone cable,
battery, and you will usually suffice although hanging a few feet of
wire off your lap from the shield of the BNC may help.

If you do use coax, and you really think you need a common mode choke,
put it at the rig, not the transformer.  Otherwise you don't have much
counterpoise. [:-)

Hope this is closer to answering your questions

73,

Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County


On 2/8/2017 4:14 PM, 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
> [hidden email]
>

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