Compact circularly-polarised coaxial feed

Describes a circularly-polarised 4-port feed for use with f/0.4 parabolic reflectors. The horn is a dual choke circular waveguide radiator with low crosspolarisation over a wide band. Diplexing is performed immediately behind the horn aperture where the high and low frequency bands are separated into concentric waveguides, the outer one carrying the lower frequency via the coaxial TE₁₁ mode. A septum joining the inner and outer conductors of the coaxial guide acts as a circular polariser: left- and right-hand components in the low frequency band are extracted at pairs of 50 ohm coaxial connectors on opposite sides of the guide. The inner circular waveguide is filled with a dielectric material and passes through the centre of the low frequency section and is coupled to a conventional septum polariser in a square waveguide. The design is suitable for systems where the frequency ratio between the lower and upper bands is approximately 2:3, for example the 4/6 GHz or 20/30 GHz satellite communications bands