So again my question - if this low-angle ground-wave (aka surface-wave)
energy dies off so quickly (e.g. down 20dB at just 20 miles), how does any
of it get to the ionosphere where it can be useful for topband DX?
Is the disagreement about how useful the really low angles are, or is the
disagreement about if a low angle measurement (groundwave) is meaningful in
determining changes in radiation at useful higher angles?
Groundwave has no value at all for working long distances, and under nearly
all conditions extremely low angles have no value on 160 meters for DX.
On the other hand, I don't think many would dispute a groundwave measurement
of FS changes between various vertically polarized radiators would be
closely tied to FS at usable higher angles. The exception would be those
cases where high angle horizontal propagation is a dominant mode.
I have about ten pages of ABC tests from here to VK/ZL and I'm pretty
comfortable that angles at or below 20 - 30 degrees dominate almost all of
the time, with the most common exceptions only at sunrise or during
geomagnetic disturbances. This even compared a dipole at about 280 feet
effective height above ground, so there was "lowish" angle horizontal
polarization in the test.
Groundwave is a very good way to evaluate vertical antenna efficiency, but
certainly not a horizontally polarized mode. I know someone who measured a
horizontal antenna at a modest distance and claimed he improved efficiency
10-20 dB by removing his balun and altering feedline length. :-)
We never want to measure in a null or on the slope of a null. That's why we
can't measure low horizontally polarized antennas at very much distance, but
why verticals are fine.
73 Tom
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