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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