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
On Sun,12/25/2016 11:00 AM, Marv Shelton wrote:
Elevated radials.
I know all the science is good (I am an engineer after all) but what I found
easiest to do when I constructed my 160m inverted-L was to attach a 500' spool
of wire to the base and reel out the wire along previously installed insulated
posts, until my analyzer indicated resonance at the desired frequency. I then
cut off the wire from the spool and terminated it. I plotted the swr curve to
verify things.
I repeated the process for as many elevated radials as I had room for. The
result of adding radials was to widen the useable bandwidth of the antenna.
I think this 'practical' method is advantageous because it instantly accounts
for all the difficult to determine factors and variable that effect a
calculated system design (ground resistance/loss, height of radials above
ground, twists and bends in radial runs, type of wire, inductive affects of
nearby objects, etc)
Rather than trying to measure, estimate or calculate things, I just start
running thru the woods with my spool of wire. Much easier and faster,
especially in the middle of the winter contest season! smile
Marv - wa2bfw
Sent from my iPad
And YES it's cool!
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