TABLE OF CONTENTS Most satellite communications systems use circular polarisation. The reason is that Faraday polarisation rotation in the ionosphere creates a changing orientation of the linear polarisations. Fixed, linearly polarised antennas cannot maintain good polarisation efficiency in such a situation, but for circular polarisations, the rotation does not affect this efficiency. Circularly polarised antennas are generally more complicated to design and build than linearly polarised antennas. For portable terminals, performance is traded for construction simplicity and so linearly polarised antennas are often used, despite a 3 dB polarisation mismatch. Similarly, the ideal pattern is traded off by what is practicable, and in doing so, the distributed directivity is further degraded from the ideal. In this section, the basic concepts of circular polarisation and circularly polarised mobile satellite antennas are reviewed.
Even in the multipath environment of land mobile communications, circular polarisation, in principle, offers some advantage, based on the polarisation purity of reflections from smooth surfaces. An incident right hand polarised wave reflects as a left handed wave for broadside reflection; see Chapter 3. For grazing angles along a conducting surface, the polarisation is linear, and for angles in between, the polarisation is left hand elliptic, as depicted in Figure 8.3.1. So for purely circular polarised antennas, the received power from first reflections is lower than for the case of linearly polarised antennas. For a line of sight situation where there is also some multipath, a circularly polarised link may perform better, since the...
