A theoretical and experimental investigation into the use of the corrugated circular waveguide as a feeder and radiator for microwave antennas is described. Because corrugated waveguide feeders are ·overmoded, the effects on the propagation and radiation characteristics of mode conversion due to bends are investigated. New methods of providing higher order mode suppression are described, and substantiated by experimental results using resonant cavity and transmission line methods. The cross-polarisation properties of corrugated waveguides are shown to be intrinsically superior to those of smooth wall waveguides when the waveguide wall suffers elliptical deformation. As an alternative to the problems involved in overmoded operation a design of corrugated circular waveguide for HEll mono-mode operation, which still exhibits an attenuation advantage, is developed. The radiated co-polar and cross-polar performance of corrugated circular waveguides and narrow flare-angle corrugated concial horns are studied both experimentally and theoretically using a more accurate analysis than attempted by previous authors. Design curves for optimum cross-polar radiation performance are presented and a new method of improving the cross-polarisation performance of these structures is theoreti6ally predicted, and experimentally verifjed.