On 11/18/2022 12:45 PM, John Langdon wrote:
If you have really poor soil you could mount
the vertical on a copper disc with a radius of 1/4 or 1/4 wavelength and it
still wouldn't radiate as effectively as one with a few radials over
excellent soil.
Not quite. Soil conductivity affects signal strength two ways. First,
with return current in the earth surrounding the antenna, or the field
coupled to the earth by current in the radials. That's a direct
subtraction from TX power. The reason that more radials reduces that
loss is that power is I squared R, and as current divides between more
radials, I becomes smaller.
The second effect of lossy soil is in the far field, and in the strength
of the first reflection summing with the direct. We have no control over
that.
--
Elevated radials work because they "protect" the antenna
from the lossy ground.
YES.
The only reason I can think to put ground rods at the end of radials is to
spread out a lightning strike over a greater area in rocky soil,
That doesn't help because the radial is an inductance in series with the
rod. And it's a TERRIBLE idea, because for current distribution on the
radial to be ideal, the far end of the radial needs to be open. AND the
capacitive coupling between radials and earth will lower the impedance
to earth for a strike, which is NOT a DC event, but rather an RF event,
because it is a spike of fairly short duration. IEEE studies have shown
that the energy in lightning is broadly centered in a 2-decade range
above and below about 1 MHz.
or perhaps
to make sure they don't roll up and come in contact with a mower.
Ground staples are a better solution. Many OTs have observed that
on-ground radials will quickly be overgrown by grass and other growing
things. I don't have anything approaching a lawn, but I've got lots of
things coming out of the ground, and radials that have been there for a
while are pretty hard to find.
73, Jim K9YC
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