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.