A study in small scale antenna diversity as a means of reducing effects of satellite motion induced multipath fading for handheld satellite communications systems

Multipath fading will cause serious link reliability problems for future L-band handheld satellite communication systems using omnidirectional antennas. Over the past two years a study has been in progress by Virginia Tech and COMSAT Labs on the effects of multipath fading at L-band. A 1.537 GHz pilot tone from the INMARSAT II F-4 geosynchronous satellite at 55 degrees W was used as a source for long term signal level measurements with a fixed position receiver. The receive antenna was right-hand circularly polarized with hemispherical coverage. Multipath fading in excess of 10 dB was observed on numerous occasions. Since margins for the proposed handheld satcom systems are much less than 10 dB fading to this level will cause severe link degradation. The most practical solution to the problem is replacement of the conventional omnidirectional antenna with an adaptive array. Experiments at Virginia Tech have been undertaken to determine the feasibility of a multielement small scale diversity antenna for use in combating multipath fading. These studies have shown that a simple four element bi-planar patch array can produce a significant improvement in link reliability over an omnidirectional antenna. Not only was the average signal level improved by 3 to 4 dB, but the maximum fade depth was also reduced from 10 dB or more to under 3 dB. This paper describes the VA Tech experiments and presents some of the preliminary results.