Friends in Radio Land -
The loss in signal intensity on the low-bands, like 160 meters,
is due to the fact that electromagnetic waves are elliptically
polarized in the ionosphere. Further, there is a limiting
polarization to waves as they go in and out of the lower
ionosphere and signal loss results when that polarization is
compared with that of the antenna on the ground. It's as "simple"
as that.
This is much like the optics of polarized light but the sort of
applications we are used to in optics involve linearly polarized
light. Two BBC engineers applied the idea to the elliptically
polarized broadcast band signals in Africa back in '65. I adapted
their ideas to 160 meters in an article in the March/April issue
of the DX Magazine in 1998 and gave a more extensive theoretical
discussion in the Spring 1999 issue of Communications Quarterly.
The most informative figures are found in the '98 DX Magazine
article, showing O-wave power coupling in North America (Omaha,
NE) and for Togo (7S, 1E) in Africa during the 5V7A DXpedition
during a recent DX contest. Interested parties should consult
that article or my recent book, "The Big Gun's Guide to Low-Band
Propagation", for a fuller, more elementary discussion.
The USA is a "happy home" for 160 meter polar signals to and from
vertical antennas but there's a 2 dB insertion loss to the south.
Horizontal antennas are a DISASTER if oriented E-W to get N-S
radiation. But Figure 2 in the DX Magazine article shows the
serious problems a vertical antenna has with E-W propagation at
low latitudes. Ten dB is not an insignificant insertion loss.
In spite of what ON4UN says, the geomagnetic field plays an
important role in low-band propagation.
73,
Bob, NM7M
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