The objective of the work in this thesis is to design lightweight, low profile, circular polarization (CP) antenna technologies which have potential for use in satellite communication systems. A new class of CP Fabry-Perot (FP) cavity antenna which may be used as a replacement for bulky CP horn antennas is developed. A double-sided partially reflective surface (PRS) is employed in the design. The resulting class of antennas are appealing as they allow a specified gain in CP to be achieved while maintaining the simplicity of a single cavity structure and a linearly polarized (LP) primary feed. An example demonstrator antenna with a gain of approximately 21 dB at 15 GHz is designed using the full-wave electromagnetic simulation tool, CST Microwave Studio (MWS). A prototype is produced and measured in order to validate the simulation results. Frequency selective surfaces (FSS) are employed as quasi-optical diplexers in satellite communication systems to facilitate multiband reflector antenna operation therefore allowing multiple feed antennas to share the same aperture. In this thesis quasi-optical diplexers suitable for CP dual frequency band operation are considered. A new class of polarizing surface is developed which in a given frequency band reflects incident linearly polarized waves with circular polarization (CP) while at other frequencies it is transparent allowing incident waves to be transmitted. In addition, a new type of CP FSS which conserves a CP signal upon reflection and transmission is designed. The axial ratio in the reflection and transmission bands is reported for both structures. Numerical EM simulation supported as necessary with analytical analysis is used throughout.