Hi All
>The work of Moxon, and others, has shown that elevating the radials just a
>few feet reduces ground coupling and ground losses considerably.... For 80
>meters, a 3 foot elevation of the radials reduces ground currents >90%...
The problem is one of geometry of the system. When the radial (or any wire)
is close to the earth in terms of wavelength or in terms of length the
coupling coefficient is close to unity.
If elevating a radial three feet is enogh to stop unwanted ground currents by
90%, why do we elevate dipoles and beam antenna 1/2 wl or more in the air?
After all, if the current in the ground is (as claimed) 90% gone at 3 feet,
what effect can the ground have if the antenna is more than three feet high??
> For my 160 array, the pair (as in 2) of elevated radials are about 7' at
>the low point.... attaching and then removing, etc. an 8' lead to a copper
>ground stake does not change the measured antenna impedence even 1 ohm...
A ground connection and removal test indicates nothing about efficiency,
since the earth below the antenna is already coupled very tightly to the
radials. In FS tests at BC stations and at my own home, adding radials
continues to improve efficiency even though there is no measurable change in
base impedance. It is a nearfield loss problem as much as a connection
resistance problem.
>(wet ground, rich pastoral soil, considered to have excellent
>conductivity)... this indicates, to me, that the pair of radials have
coupled
>to the vertical to an extent that they are screening it from ground with
>high efficiency....
>i.e. in the equation
>Effieciency = Ra / (Ra + Rg)
>the ground current due to Rg must be small....
The formula can NOT be applied as the resistance measured at one point in the
system. It applies only to effective loss resistances throughout the entire
system normalized to one common point. Let me give two examples of why it is
a useless formula when applied to the feedpoint of an antenna.
1.) A vertical dipole fed at various points.
Fed near the end, the feedpoint resistance is high. Fed in the center, the
feedpoint resistance is low. The efficiency of the antenna, including ground
losses, is the same regardless of feedpoint impedance.
2.) Elevated radials
For a fixed radiator if radials are resonant feedpoint impedance is low. If I
lengthen the radials base impedance increases dramatically while efficiency
remains nearly unchanged. If I shorten and inductively load the radials base
resistance decreases, but losses increase.
>Even after having reviewed the literature on this, I have to admit I was
both
>impressed and a bit surprised... Like most, I have the mental image of 120
>radials as being the ideal... OTOH, we don't fuss because a dipole antenna
>has only 2 elements!
Not when the dipole is a large fraction of a wavelength above ground we
don't, but it is VERY well known the efficiency of a low dipole is greatly
improved by an extensive counterpoise system.
Nearfield makes a smooth transition to farfield, but the distance accepted
for Fraunhofer (farfield) is the distance given by P (point) greater than
2D^2 over lambda where D is the diameter of an imaginary perfect sphere that
just completely encloses the antenna.
Even miltary tests, relayed by N7CL, agree with my FS tests on 80 meters and
those at WVNJ.
Going from four elevated radials to a full ground system results in about 5
dB improvement. Three A-B type tests in three isolated places by independent
people using FS meters (actually the miltary tested quite a few systems
according to N7CL) with the same results.
On the other hand NONE of the 100% efficient elevated radial systems (to my
knowlege) have been verified in an A-B test at one site.
73 Tom
>From patd@wolfenet.com (Patrick Dayshaw) Sat Dec 7 19:47:45 1996
From: patd@wolfenet.com (Patrick Dayshaw) (Patrick Dayshaw)
Subject: Tks for the help
Message-ID: <199612071952.LAA01308@wolfe.net>
Yes, Yes, Yes, Yes, YES!!!!
A man with integrity.
A man who recognizes the SPIRIT as well as the LETTER of the law.
A man who could have ended Diogenes' search.
Patrick
WA7VNI
patd@wolfenet.com
http://www.wolfenet.com/~patd/
In case you don't rememeber..... Diogenes was the Greek philosopher
who founded the Cynic school of philosophy, stressing self-control and the
pursuit of virtue. He is said to have wandered through the streets of
Athens in the light of day ,with a lantern, searching for a single truely
honest man.
----------
> From: K4RO - Kirk Pickering <k4ro@music-city.tdec.state.tn.us>
> To: jon.zaimes@dol.net
> Cc: contest@music-city.tdec.state.tn.us
> Subject: re: Tks for the help
>
> I was trying to work a weak K7 station last night in the ARRL 160 CW
test.
> I was CQing and he called me. The QRN level was fairly high at that
time,
> even on the Beverage. After a half-dozen or so tries, I couldn't copy
his
> section, although I had the call OK. Then this other loud station breaks
> in with "AZ" (or whatever the section was...) My reply to this was "tks
> but no tks no qso qrz de k4ro..." It was the first time I ever received
> any "assistance" during a contest. Never heard the K7 station again.
> Too bad, we would have probably made the QSO on the next try.
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