To: <topband@contesting.com>
> Date: Thu, 05 Mar 1998 08:15:23 -0600
> From: n4kg@juno.com (T A RUSSELL)
Hi Tom,
Gee, this sure spread to lot's of groups!
While that is a novel and simple feed method, and very creative,
users should be aware of some potential problems (just like many
antennas--such as slopers or zepps--have).
> I agree that my elevated GP antennas have losses due to the earth
> connection
> and low radials, but you make it sound like such antennas are not much
> better than a dummy load and on this point I must STRONGLY DISAGREE.
That's true. Even if efficiency is less than 10% it is still much
better than a dummy load, and the antenna will certainly produce many
DX contacts. People even work DX with Gaps on 160, and some Hams are
quite happy with even grossly inefficient antennas.
> (My GUESS at WORST CASE losses compared to a full size ground based
> vertical with lots of radials is 6 dB. My HUNCH is that it is actually
> much LESS.)
On the other hand WR4U picked up about 10 dB (groundwave
measurement) when he changed over from the system discussed to a
conventional shunt fed system with only about a dozen ground mounted
radials.
The main problems in establishing performance are unpredictable
values of earth-to-tower-base resistance at the bottom, as well as
not knowing exactly what's above the radial-to-tower feedpoint
connection or below and around the radials.
All of this has an awful lot to do with the efficiency of the overall
system.
There are three known but unpredictable losses at work.
One is earth loss because of the poor radial system. Measurements
show that loss is typically six dB or so with a full size radiator
(and MORE in a close spaced phased array or with a short radiator)
when using a small close to ground elevated system, when compared to
a conventional system. Adding more radials helps this problem by
reducing current flowing through the lossy soil.
The second is coupling from the radials to anything and everything
else around the radials. This is caused by the high induction
(electric and magnetic) fields around the radials. Contrary to rumor
this source of loss only greatly diminishes in the far field, and is
helped very little (if at all) by "balancing the current" in the
radials. Adding more and longer radials DOES help this problem,
however by reducing the field intensity around each radial.
The third source of loss is the end of the radiator is stuck down in
the mud, and excited by the feedpoint just as the rest of the tower
is.
To see interesting and more accurate modeling results, model the
antenna "correctly" by inserting a resistive load between the bottom
of the tower and mininec based ground. This resistance partially
corrects for mininec's incorrect assumption that anything connected
to the ground is a perfect zero ohm lossless connection.
There have been a lot of incorrect ideas presented that resonant and
properly tuned radials don't radiate in the near field. My post was
not intended so much to impugn your feed system as to simply point
out the user should be aware that his results may not be anywhere
like other people experience, because elevated radials (and even a
hot tower stuck in the mud) make for a very unpredictable system.
Predicting or even discussing the results of running a Beverage or
any other conductor near such a tangled unpredictable mess of
conducted and radiated fields is a waste of time and bandwidth. It's
all blind luck.
I hope this clarifies my point.
73, Tom W8JI
w8ji.tom@MCIONE.com
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