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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