On 12/26/16 1:14 AM, Hans Hammarquist via TowerTalk wrote:
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.
It's unclear why you'd want "improved current sharing" - in real life
the soil underneath (or surrounding) the radials is hardly uniform
properties. Nor are you typically in a situation where a radial is
seeing a current overload.
As K9YC points out, and shown by N6LF, functionally the radials
basically "improve the conductivity of the soil"
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.
But with short radials, you are basically in the "physically small
antenna" bucket - you now have a narrow band tuned antenna - The
Chu-Harrington equations say that you can make an efficient antenna
that's arbitrarily small, but the stored energy will go up, which in
real life means more current and voltage, which will increase copper and
other dissipative losses.
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?
Nope..
I think the basic principle is "more wire is better" because wire, in
what ever form, is a better conductor than dirt (unless, perhaps your
antenna is in the proverbial salt marsh).
The interesting questions come about from "how to arrange the wire",
where you can trade off things like number of wires, length, gauge, etc.
all played against the cost of copper and cost of installation time.
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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