Transequatorial Propagation
Transequatorial radio signal propagation (TEP) is an unusual mode of radio
propagation that was first noticed and studied by radio amateurs. Since then it
has much more has been learned about it.
This mode of propagation is supported by the F2 layer, and enables
frequencies of 100 MHz and more to be reflected in a north south direction when
the normal maximum useable frequency is considerably below this frequency. It is
found that the maximum useable frequency for TEP is normally about one a half
times the normal MUF for F2 layer propagation although greater enhancements in
frequency have been observed.
Openings via TEP are most noticeable at VHF when no long distance propagation
may be expected. It can be experienced on the 144 MHz amateur band and has even
been noticed at 432 MHz. The HF bands are also affected, and it is found that
there can be significant north south activity when normal activity is fading or
a path no longer exists.
Path lengths vary, but are generally between about 2500 and 5000 km, and both
stations should be approximately equidistant from the equator. Also the path
must cross the equator in a north south (or south north direction). Occasionally
angles up to 20 degrees from the north south direction have been known but the
closer to the north south direction the better.
It is found that TEP is generally a night-time mode, with openings generally
occurring in the late evening between about 8 and 11 p.m.
It is thought that TEP arises when there is an increased level of ionisation
in equatorial regions. This enables signals that enter the ionosphere at the
correct angle to be propagated across the equator. In view of the way in which
the signals are propagated they must enter the ionosphere virtually in a north
south direction, otherwise propagation does not occur. It is also found that
signals undergo two reflections by the ionosphere before they are returned to
earth.
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