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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