My thought on this is that the very shallow angle grazing reflection
from the dielectric discontinuity at the distant ground surface is
pretty much 100%. The ground permittivity and conductivity variability
would have a bigger effect for higher radiation angles.
For fire protection and layout reasons, I have only two elevated radials
on my full-size quarter-wave vertical, and it seems competitive enough.
Because the SWR is low without any matching, I assume there's some
ground loss, but my 12° sloping foreground seems to make up for it. From
modeling, I expect a little gain to the northeast from the 140' tower
reflector behind it.
We used two full-size half-wave verticals above a sloping foreground in
our HC8 station, with very good results. One was always a little better,
but it was hard to predict which one by direction alone. "Person with
one watch knows what time it is; with two, never quite sure."
Dave
On 12/3/22 10:21 AM, Brian Beezley wrote:
Paul, W9AC: "Isn't the measured probe result only useful for near-field
system efficiency analysis? By near-field I mean to obtain system
efficiency within a wavelength or so of a vertical radiator.
But for skywave propagation field strength, don't we also need to know
more about the ground conductivity much further out to more than 1km on
160m?"
Paul, I don't know how distant ground affects 160m specifically. But if
you're on a hilltop, at low elevation angles your signal may reflect or
diffract from ground miles away. Its permittivity and conductivity might
be quite different than that of the ground beneath your antenna.
I think the main usefulness of knowing your ground characteristics is to
predict antenna efficiency. That could easily determine your choice
between a horizontal and vertical antenna on 80m or 40m.
If you have some idea of the ground quality far away, you could create a
separate model with that value to study low-angle effects. You could
take a drive and go measure distant ground. But you'd probably have to
take a number of measurements to satisfy yourself that you had a
representative sample. My ground probe calculator includes a utility
that will average probe measurements. I had in mind making multiple
measurements near the antenna, but you could use it to create an average
of far-away ground for a low-angle model.
Brian
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