In a message dated 2/26/01 10:28:03 AM Eastern Standard Time,
[email protected] writes:
<< ...my favorite theory for
the major cause of loss resistance at the moment is that it is
caused mainly by dielectric losses in the ground, where the electric
field of the antenna penetrates to some depth at LF. This is
contrary to the conventional view that the major losses are due to
the resistance of the ground system. I don't think there is really a
contradiction, just that amateur antennas have relatively high
dielectric losses because they are smaller than conventional LF
antennas. A bit of thought shows that a predominance of dielectric
loss would explain lower loss resistance at higher frequency, and
G3AQC's "footprint" effects, among other things. >>
For what it's worth, Jim's theory is entirely consistent with (pardon the
expression) commercial practice and theory. I agree that there is no real
contradiction. What we as amateurs lump together under the term "ground
loss" are actually a number of factors, and at LF the dielectric properties
of the ground certainly are a major component; perhaps _the_ major component
at 136kHz.
The soil conductivity charts we use to predict groundwave coverage at MF over
North America differ greatly from the ones used at LF. Skin depth is a major
reason usually cited for this discrepancy, along with different assumptions
about dielectric constant. The interaction between the ground system and
losses in the earth extend to a greater depth, where subsoil and rock
properties may diverge widely from those near the surface. Thus, not only is
the apparent conductivity different for MF and LF at any given location, but
the shapes of the conductivity contours don't bear much resemblance to each
other, either.
All this appears consistent with what amateurs are finding with practical
antenna systems at LF.
73,
John
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