I read an article, many years ago that dealt with how to improve the current
sharing between the radials. The method was simple: You cut all your radials
short enough to make them about equal but capacitive. Then you add a single
inductor to tune out the total capacitance of the parallel connected radials.
The problem/issue was that when you deal with tuned radials, only a small
variation in length/tuning frequency will cause a large variation in current
through the radials. With shortened radials the length variation would not
cause such as large current difference between the radials and thereby
improving the systems efficiency.
What do you think? Shall we go out, cut the ends off the radials and then we
can use the wire we cut off to :-) make an inductor to tune out the "missing"
length?
I will try it for the next radial field I will put out (unless somebody convice
me it's a bad idea).
73 de,
Hans - N2JFS
-----Original Message-----
From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk <towertalk@contesting.com>
Sent: Sun, Dec 25, 2016 4:38 pm
Subject: Re: [TowerTalk] TowerTalk Digest, Vol 168, Issue 64 raised radials
Just to clarify -- the primary reason for adding more radials is to
reduce ground loss. N6LF has shown that 1) the more equally the current
divides between radials, the lower the total loss will be; 2) a greater
number of radials tends to improve that division; 3) making elevated
radials slightly shorter than resonant tends to improve that division;
and 4) as the number of radials increases, the current divides between
them, but because power is I squared R, the total power lost in the
radial system drops in proportion to the number of radials used. Rudy
also showed that we don't want radials longer than a quarter wave but
shorter than a half wave, because that range of lengths will produce a
current maximum on the radial at some distance from the feedpoint that
is actually greater than the current at the base.
An antenna like this can be seen as a simple series circuit, where the
radiation resistance, Rr, is in series with the wire resistance, Rw, and
the loss coupled from the earth, Rg. Rr is determined by the electrical
height of the antenna, and I squared Rr is the radiated power, while I
squared (Rw + Rg) is the loss. Rr is much lower than 50 ohms, so a lossy
ground (radial) system will look like a great match, while increasing
the number of radials will increase the SWR.
I like the suggestion, made by others, to tune the length of radials by
measuring pairs running in opposite directions (and, per N6LF, tuning
them a bit high in frequency). Some modeling I did years ago in NEC
showed that radials buried or laying on the ground typically have VF in
the range of 0.7 - 0.75, depending on soil, but that VF rises quickly as
radials are raised, so that by the time you're 3-4 ft off the ground VF
is getting pretty close to 1.
73, Jim K9YC
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