A Novel Design of Broadband Waveguide Directional Couplers and 3-dB Hybrids

This paper presents a new approach to design waveguide-based directional couplers and 3 dB hybrids allowing broadband performance at high frequencies without limitations caused by the precise machining requirements used in conventional branch-line coupling. This novel design is based on the traditional layout with two parallel waveguides coupled through their broad-wall by employing a substrate-based coupling structure. The structure consists of two E-probes and its connecting microstrip line placed on a substrate, e.g., crystal quartz. The design facilitates the use of the split-block technique for the waveguide manufacturing and substrate-based coupling structures produced by using thin-film processing with ultimate-precision. This technology allows extending this design to very high frequencies. We demonstrate that this design provides a band up to 30% of the central frequency for directional couplers and up to 25% for 3 dB hybrid. We present experimental measurements of the sideband separation mixer that includes the -16 dB coupler at about 300 GHz frequency band