Satgen 486 Ground Reflections by GM4IHJ (BID SGEN486) 1998-07-18 You are waiting for a LEO VHF satellite which is about to appear on your station horizon. You hear the signal, and it gradually gets stronger. Then quite suddenly it fades for perhaps 15 or 20 seconds, ,just as you hear the DX, before recovering gradually and getting even stronger than before. What is happening ? Even if you see the satellite over a relatively unobstructed ground, you are getting the signal via at least two paths. One path is direct, the other is from a signal hitting the ground some way in front of your antenna and then being reflected or scattered up to your antenna. The ground path is longer than the direct line of sight path . So the reflected signal is not necessarily in phase with the direct signal, and, this phase difference is changing continuously as the satellite rises higher in your sky, thereby continuously altering the path geometry of the multiple arrivals. The first deep fade in your reception occurs when the indirect signal is one half wave length longer than the direct signal. There after you get a succession of fades and signal enhancement as the signal arrivals go out and in phase. When the difference is an odd multiple of half waves you get fades or, when an even multiple of half waves you get enhancement. Although as the satellite gets closer the later fades are obscured by faraday and other phenomena, blurring the effect of the multiple signals. In theory , if you have an antenna 10m above ground , you will get a maximum path length difference on an overhead pass of 20 metres, producing 10 fades and 10 peaks between horizon and direct overhead. In practice however the fade that matter most is the first one. A situation which has greatest effect, if you are looking for long range DX contacts when the satellite is skimming your horizon for just a few minutes. In this case the fade can be of quite long duration at just the wrong time. Indeed the only solution in this case if your antenna produces this problem , is to experiment with it at different heights , or use two antennas mounted at different heghts and switch between them, as necessary. You can test for this condition by recording either the beacon signal from the amsat of your choice , or if wefax is your target , you can check out the antenna on a wefax signal. This antenna multipath problem at VHF , is much worse if the signal path to you comes in over a good reflector such as the sea. Indeed, in the early days of VHF radar at sea, great care was taken to measure at what elevation and range approaching aircraft entered and left the various nodes of the antenna vertical beam pattern. Big ships with two masts often used two radars on different frequencies Eg 49 and 88 MHz , which resulted in antenna vertical polar diagrams which complemented one another ie where fades occurred on one the other set had peaks. A situation complemented by manual mark up of vertical plots on which the combat information centre operator could tell from the entry/ exit ranges of the target as it crossed each beam, just how high the aircraft was flying. All very agricultural. But it worked.