Circularly Polarised Antenna Designs for a Semi Active Radar Target

This paper describes work done on the development of small aperture antennas for circular polarisation. In particular for the present application, which involved fitting antennas to an aerodynamic body, the antennas had to be conformal to minimise drag and also be of low weight and small size. A novel antenna design for the production of circular polarisation is reported. Improvements to the original design, which have resulted in extending the performance of this antenna are also reported Measured results, for axial ratio and return loss, show that this antenna operates very satisfactorily over a bandwidth of appraximately 1 GHz at X band. This paper also describes further work conducted on a second type of small antenna also for circulat polarisation. This type of antenna utilises a multisection quarter wave plate design to transform linear polarisation into circular polarisation. For the present application, which involved fitting the transmit and receive antennas in close proximity within the small nose assembly of a Semi Active Radar Target or S.A.R.T., a very high degree of isolation is required. This paper includes a description of techniques employed to achieve the required degree of isolation between antennas