Friends in Radio Land -
Given the recent discussion of power coupling loss (PCL) and
160 meter propagation, let me give a brief summary of the essential
points.
1. The coupling or transfer of RF energy in and out of the lower
ionosphere is the greatest when the E-field of the RF waves is
parallel to the local geomagnetic field. It is the least when the
E-field is perpendicular to the field.
2. A simple example of favorable coupling is for propagation in
all directions from a vertical antenna at the geomagnetic pole.
Least coupling is for the broadside direction from a dipole at the
geomagnetic pole.
3. At the geomagnetic dip equator, where the field is horizontal,
the most favorable situation is for broadside (E-W) propagation
from a dipole oriented N-S in the field. The least favorable
direction for propagation from a vertical antenna is in the E-W
direction relative to the field.
4. In the USA, magnetic field lines dip down into the earth from
the south at 60 degrees to about 70 degrees or so with respect to
the horizontal direction. That means power coupling for vertical
antennas is the greatest to magnetic north at radiation angles
ranging from 30 degrees along the southern border to about 20
degrees along the northern tier of states. Power coupling for
horizontal antennas is least to the north for those angles.
5. Power coupling, as discussed recently, dealt with the first and
last transit of RF at one's QTH. But it can be important for
DXpeditions where antennas are often selected or mounted for
convenience, with some directions suffering large coupling loss.
6. Power coupling loss is geometrical in nature and for a given
direction, it varies inversely with frequency, i.e. it decreases
linearly with increasing frequency . But power coupling loss shows a
strong dependence on wave direction relative to the local magnetic field,
being greatest when propagation is closest to perpendicular to the field
and and least when closest to being parallel.
7. For details with regard to your situation, consult the
International Geomagnetic Reference Field (NGDC, 1990).
73,
Bob, NM7M
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