Small scale antenna diversity as a means of reducing the effects of multipath fading for handheld satellite communications systems

This document discusses the measurements conducted at Virginia Tech, in Blacksburg, VA, and at COMSAT Labs, in Clarksburg MD, to determine the impact of multipath fading upon a circularly polarized L-Band satellite-to-handheld communications system using an omni-directional antenna. A significant finding was that motion of the satellite could induce fades of up to 10dB, well in excess of the small link margins available to satellite-to-handheld systems (typically ≤3dB). A method for ameliorating multipath fading was then sought.; It was also found that the multipath fading effect was significantly different at as small a distance as half a wavelength from the original antenna position. Multipath signals could actually combine at the new position to form a small increase in signal to noise ratio (S/N), rather than a fade of as much as 16dB. Therefore it was determined that a small-scale antenna diversity system might offer a solution to the problem of multipath fading.; A series of experiments was set up with several different antenna types spaced roughly λ/2 apart to determine which orientation would provide the best S/N improvement. It was concluded that two vertically polarized patch antennas spaced vertically in the same plane by ≈λ/2 offered the best antenna space diversity method. Decorrelation between array elements was high, with multipath induced fades of up to 16dB experienced at one element being counteracted by a multipath induced peak signal in the other. A method was needed to determine the best way of using the signal improvements: either switching between antennas, or optimally combining the signals from both to get the best overall S/N.; It was determined that optimally combining the signals from each antenna, so that they matched perfectly in phase, yielded the best diversity gain, between 2 and 3 dB better than switching between antennas.