| Ultra-wideband feeds have extraordinary potential for utilization in electromagnetic reflector antenna applications. They can replace multiple clustered narrowband antennas or be used for pulse radiation. Nested concentric coaxial waveguide radiators are a convenient choice for implementing an ultra-wideband feed. Several coaxial cups can be nested in the inner conductor of the original coaxial waveguide to accommodate a wide frequency bandwidth. However, coaxial waveguide radiators are typically limited to narrowband applications due to their relatively large overall reflection coefficient and cross-polarized radiation levels. A simple method to reduce the reflection coefficient over a wide bandwidth is to extend the coaxial waveguide's center conductor past the outer conductor. In this dissertation a multiband nested coaxial waveguide feed and a four-cup 70:1 ultra-wideband nested coaxial waveguide feed with extended center conductors are designed and analyzed for use in feeding axially-symmetric paraboloidal reflector antennas. The analysis is performed using a rigorously derived method of moments solution to the integral equations describing the radiation and scattering of the reflector antenna system.; The multiband feed consists of two concentric coaxial cups with a center circular waveguide. The feed covers the L (1.5–1.7 GHz), S (1.7–2.5 GHz), X (7.2–8.4 GHz), Ku (8.4–14.5 GHz), and E (60–90 GHz) satellite communication bands. The center conductors of the coaxial cups are extended to reduce the TE11-mode reflection coefficient to typically less than −16 dB over the operating frequency bands. When illuminating a paraboloidal reflector, the multiband feed produces reflector efficiencies typically greater than 60%.; The ultra-wideband nested coaxial waveguide feed consists of four coaxial cups with extended center conductors covering a continuous 70:1 frequency bandwidth (0.6–42 GHz). The feed has less than −10 dB reflection coefficient over the operating frequency bandwidth and typically yields greater than 40% reflector efficiency over most of the band. However, due to mutual coupling with the inner nested coaxial cups, for frequencies near the TE11-mode cutoff of each inner coaxial cup the reflector efficiency dips to approximately 20%. |
