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. Just as with an
off-center fed dipole, an EFHW of the right length would have low
reactance and high impedance that could be properly matched with the
right transformer, and you'd need common mode chokes for both to keep
currents off the shield of the feedline. The two antennas are different
purely in terms, not in physical reality or in the radiation patterns
they produce.
73,
Dave AB7E
On 1/12/2018 4:33 PM, Bill Johnson wrote:
Ron, love your comment. I use EFHW and it is amazing the confusion over how
they work. Got to have a great transformer and a chosen antenna length and
stick to it and perhaps a multiple that fits. My 160 EFHW is made for 1.900
and works well at 3.800 in certain directions on either band. I have do use a
remote tuner for slight deviations and also compare two different dipoles to
pick the best one to use. The 160 is a c shaped set in the trees. Insulated
wire. Now to get it isolated with insulators. Next spring... too much cold
weather and snow to deal with. :-)
73,
Bill
K9YEQ
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Ron D'Eau Claire
Sent: Friday, January 12, 2018 4:11 PM
To: [email protected]
Subject: Re: [Elecraft] Long wire antennas MORE
A mistaken idea that many Hams get is that a wire antenna has no radiation in the
"nulls". For example, a half-wave wire is often thought of as having no energy
radiated off of its ends. There is LESS off the ends, but a real-world wire has some
radiation in ALL directions as Dave notes. It's just stronger radiation in some
directions.
A real long wire (many wavelengths) is easy to match since the longer a wire is, the smaller the
impedance excursions across the RF spectrum. The hardest to match are wires a half wavelength (or
less) long. However, most compact ATUs are limited in matching range based on simple physics. Their
small size cannot tolerate the huge RF currents and voltages frequently encountered even at
moderate power levels. There's a good reason why the old time "antenna tuners" (matching
networks) were so huge. It's just a matter of basic physics. But today most of us use antennas that
offer an feed point impedance limited to the range our ultra-fast, super-smart "automatic
antenna tuners" can handle.
73, Ron AC7AC
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of David Gilbert
Sent: Thursday, January 11, 2018 11:41 PM
To: [email protected]
Subject: Re: [Elecraft] Long wire antennas MORE
There is only a fixed amount of total energy contained in all the lobes of an
antenna. You almost definitely did get lots of lobes ... but you also got lots
of nulls that exactly offset all those lobes. You just never heard the the
hams that were in those nulls and they never heard you. Whatever you gain in
one or more directions is sacrificed in one one or more other directions. This
is basic physics.
More lobes is not necessarily better. In fact, taken to the extreme it is self
defeating because a very large number of lobes (assuming they were somehow all
of equal strength as you stated) begins to approximate a unidirectional antenna
with no azimuth gain in any direction.
Just for grins I modeled your 700 foot antenna in EZNEC+ and on 20m it gave a
maximum gain of about 9 dbi in a fairly narrow lobe at 16 degree elevation in
both directions along the axis of the wire. It also gave a total of 36 other
sharply narrow lobes arrayed symmetrically in all other directions, each with a
gain of about 6 dbi. Between each lobe was a deep null of around minus 10 dbi.
This was all at the same 16 degree elevation angle ... there were literally too
many lobes to count on the 3D pattern, with lots of lobes and nulls at every
azimuth and elevation angle.
A simple dipole at the same 40 foot height would have given similar gain with a
much broader lobe (both azimuth and elevation) in the two main directions, but
of course without the multiple smaller side lobes. Three poles and two
perpendicular dipoles would have given better overall single band results ...
the only advantage of the long wire being that it gives a similar pattern along
with similarly ugly match on multiple bands.
Dave AB7E
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