Compact dual-frequency feed for prime-focus paraboloid

A compact dual-frequency feed operating at 1420 and 408 MHz has been developed for a prime focus paraboloid. Efficient dual-mode performance at 1420 MHz results from the combination of modes in a circular waveguide of diameter 0.91 lambda ₁₄₂₀ and a surrounding cavity of diameter 2.61 lambda ₁₄₂₀. A single TE₁₁ mode is excited in the larger cavity at 408 MHz by four slots in its back wall, which acts as a dichroic structure, allowing the passage of 408 MHz energy while reflecting 1420 MHz fields. Satisfactory 408 MHz performance is obtained in a feed of depth only 0.55 lambda ₄₀₈, without significant effect on 1420 MHz performance.