A coaxial feed system integrated within a Cassegrain subreflector

Describes a low-frequency feed fitted inside the subreflector of a Cassegrain antenna. One such facility has been added to a 13-metre Cassegrain (GB13C) to enable it to work simultaneously in L-band (1.525-1.6605 GHz) and C-band (3.60-4.20/5.85-6.454 GHz). The radiating element is an annular slot inset into the subreflector surface and fed by a coaxial matching network carrying the TE₁₁ mode. The design of the TE₁₁ mode coaxial matching network was carried out using mode-matching software written for the purpose and including an accurate model for the self-admittance of the coaxial aperture. Dielectric loading was used at the aperture and at one other point in the network for mechanical reasons. A coaxial low pass filter is placed within the network. This was also designed using the mode matching software and is necessary to ensure high rejection of the C-band transmit band power