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Transequatorial propagation, TEP, is an unusual form of radio signal propagation.
TEP was first noticed in the late 1940s by military operators and radio amateurs who were making contacts from one side of the equator to the other. It appeared that propagation paths were available, well after traditional theory and calculations predicted there would not be any paths open. Often the frequencies used were into the VHF region, typically with a maximum of 60MHz or so.
Since its initial discovery, transequatorial propagation, TEP has been studied much more and there is a much better understanding of it.
What is transequatorial propagation, TEP?
Transequatorial propagation supports communications over distances between 2500 and 5000 km. These paths occur across the equator in a north south or south north direction.
Occasionally angles up to 20° from the north south direction have been known but the closer to the north south direction the better.
Also transequatorial propagation generally occurs in the late afternoon or early evening. Signals propagated using this mode tended to suffer some distortion as a result of multipath effects., but despite this, modes like single sideband, SSB are usable.
Transequatorial propagation & ionosphere
TEP arises from several factors. But before any explanations it is worth distinguishing between the geographic and geomagnetic equators.
- Geographic equator: The geographic equator relates to the position of spin of the earth and the nearness to the sun. The geographic equator receives the maximum solar radiation, and this varies little over the course of the year.
- Geomagnetic equator: This equates to the position between the geomagnetic north and south poles. At the magnetic poles it is found that the magnetic lines of force are almost vertical with respect to the Earth’s surface. At the geomagnetic equator they are parallel to the Earth’s surface.
It would be expected that the maximum levels of ionisation would be expected at the equator because it receives the maximum radiation from the Sun – in the same way that the equator is hotter than more temperature climes. However it is found that a high electron peak is noted either side of the geomagnetic equator at magnetic latitudes of around 10° to 20° .
These peaks in the level of ionisation give rise to higher critical frequencies than those that exist at the geomagnetic equator.
The reason for this results from a combination of the electric and magnetic fields that exist.
As expected, the Sun creates higher levels of ionisation at the geographic equator as a result of the higher level of radiation received here. However the charges start to move under the action of the magnetic and electric fields outwards from the equator.
The ionisation levels start to build up after sunrise, when the ionosphere starts to receive radiation. The level of ionisation rises and by mid afternoon the ionisation levels are noticeably higher in the regions 10° to 20° out from the geomagnetic equator.
After sunset when no more radiation is received from the sun, the levels of ionisation start to fall as ions and electrons recombine.
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.
VHF / UHF TEP
There also appears to be a slightly different form of transequatorial propagation. This form of TEP normally occurs in the evening often between about 1900 and 2300 local time, and it supports communication on frequencies up to about 450MHz on occasions.
This form of TEP is not well understood. But it is believed to arise from some form of equatorial spread F phemonmenon. When spread F occurs, this region of the ionosphere appears to break up into a number of bubbles of ionisation which support propagation via some form of field guided mode.
When using this mode of transequatorial propagation, TEP, the signals are subject to rapid fading , sizable Doppler shifts and significant distortion.
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