TABLE OF CONTENTS 'Little Sir Echo, how do you do?'
As reciprocal devices, scanners receive incoming signals from the same volume of space illuminated on transmission. The receive and transmit gains, radiation patterns, sidelobes and polarisation are identical. Gain benefits both the transmit and receive legs, so increased gain is doubly beneficial to detection of weak targets.
Most marine radars use linear polarisation in which the electric field exists in one plane, sometimes vertical but usually horizontal. Antennas can always efficiently receive the polarisation which they transmitted. Passage through the atmosphere does not significantly affect ray polarisation; most targets and sea clutter also more or less preserve the incident polarisation when they reflect. Any depolarisation to plane polarised signals is allowed for by a reduction of the nominal radar cross section (RCS) of the target or clutter. Targets and sea clutter are therefore received by plane polarised scanners without ostensible polarisation loss.
A circularly polarised (CP) electric vector proceeds corkscrew fashion. The nearly spherical shape of raindrops causes CP rays to be reflected at the opposite hand so, in principle, rain clutter is rejected by the scanner. In practice the drops wobble between oblate and prolate spheroidal as they fall and are imperfect spheres, so scanners have some residual ellipticity and CP does not entirely eliminate rain clutter. To CP, target depolarisation reduces RCS of many ship targets by a few decibels, so stronger echoes are received with linear polarisation. Overall, a good CP scanner can give very useful...
