Take a look at this link by AA6TB where he describes a counterpoise for
an EFHW.
http://www.aa5tb.com/efha.html (see Figure 5)
At least in his version, which I believe is the "standard" configuration
for one, the additional 0.05 wavelength wire is indeed added to the
length of the antenna. All it has done is move the secondary of the
transformer away from the end of the wire to get a lower impedance ...
just as I described below. I don't understand why this is so difficult
to grasp.
Unless the counterpoise you are talking about is connected somewhere
else in the system? Primary side of the transformer? Somewhere else?
If so I'd like to see a link to a drawing of it and I'd like to have
somebody explain how it supposedly works because it makes no sense to me
to put it there.
Otherwise, you are indeed adding length to the antenna on the far side
of the matching network ... just exactly as if it was an off-center fed
dipole.
Dave AB7E
On 1/13/2018 4:55 PM, Bill Johnson wrote:
The 3.5 ft aren't added. On the transformer, the low gnd side is where the .05' wire is
added. It takes care of static and stray capacitance. I cannot remember the fellow who
has written volumes about EFHW's and there is clearly a need for this. Absent a ground
this works extremely well for portable end feds. The research was done in the field not
on a model. There appears to be some misunderstanding, Dave. EFHW's use a wound
transformer to match/reduce the impedance seen on the end of the wire. I had written
about this in a previous post with very poor sentence structure, using "smart
phone".
73,
Bill
K9YEQ
-----Original Message-----
From: elecraft-boun...@mailman.qth.net
[mailto:elecraft-boun...@mailman.qth.net] On Behalf Of David Gilbert
Sent: Saturday, January 13, 2018 12:22 PM
To: elecraft@mailman.qth.net
Subject: Re: [Elecraft] EFHW vs Off-Center Fed Dipole
You can use whatever terms you want, but physically and electrically you are turning the
EFHW into an off-center fed dipole when you add the "counterpoise" wire beyond
the feedpoint. If you don't agree with that you are merely adding to the confusion.
If you add 3.5 feet to a half wave 40 meter dipole it already is no longer a
true half wave anyway, and you might as well just feed the half wave antenna
3.5 feet from one end.
Don't believe me? I modeled a half wave 40m dipole at 70 feet in
EZNEC+. At 67 feet long and fed at the center the feedpoint impedance
was 66 - j4 ohms. When I simply moved the feedpoint out to 3.5 feet from one end
(keeping the total length at 67 feet) the feedpoint impedance became 1509 - j1202. I
then added 3.5 feet to the antenna (total of 70.5 feet) and fed it 3.5 feet from one end,
which in your world would be the EFHW with a 3.5 foot "counterpoise." The
feedpoint impedance changed to 3317 - j3115 ohms. Which do you think would be easier to
match (or easier to understand)? The 3D radiation pattern was indistinguishable from
each other in all three cases, of course.
It doesn't matter what you call it, but if you take a piece of wire and feed it
some distance from the end you now have an off-center fed dipole ... an extreme
version possibly, but an off-center fed dipole nonetheless.
In my opinion, the term "counterpoise" is mostly a silly obfuscation.
Dave AB7E
On 1/13/2018 6:44 AM, Don Wilhelm wrote:
Dave,
All information I have seen says that the counterpoise needed for an
EFHW is 0.05 wavelength - at 40 meters, that is about 3.5 feet.
If you make it longer than that, it becomes an offset center fed
antenna, longer than a halfwave, in other words, it is a random length
wire. Both the half wavelength wire and the counterpoise wire will
radiate.
73,
Don W3FPR
On 1/12/2018 7:43 PM, David Gilbert wrote:
Well, since you brought up EFHW there is a relevant comment I've
wanted to make for a while.
An EFHW with a counterpoise wire (which everyone seems to claim is
important to have) is basically just an extreme version of an
off-center fed dipole. A half wave dipole has its lowest impedance
at the center, where the current is high and the voltage is low. As
you move out away from the center the current decreases and the
voltage increases, which is equivalent to saying that the impedance
increases. As you get to the end of the wire the current obviously
goes to near zero except for capacitive currents while the voltage
goes very high ... meaning high impedance. The "counterpoise" for an
EFHW is merely an extension that puts the feedpoint back toward the
center where the impedance isn't quite as high. And as with any
dipole, it isn't critical how that "counterpoise" is physically
arrayed because the current there is small so it doesn't affect the
pattern much ... just as is the case with a dipole with drooping ends.
I think if everyone viewed EFHW antennas as off-center-fed dipoles
there would be a lot less confusion about how they work.
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