Computer-aided design of quadra-ridged waveguide transformers for full-band matching of half-height rectangular...

A quadra-ridged waveguide transformer (QRWT) is proposed for full-band matching between a standard waveguide and a half-height waveguide, which meets the need for compact waveguide components and the need for multiple sections of irregular transmission lines for better matching in wider operation bands. Unlike conventional matching circuits with monotonically changing impedances, QRWTs form abrupt cross-section changes between transmission line steps. The initial configuration of QRWT is built by CST MICROWAVE STUDIO, a system-level electromagnetic compatibility and general high frequency passive device simulation software, according to the basic electromagnetic (EM) concept and then is optimized by optimization functions from an advanced EM simulator, during the optimization, releasing restrictions as many as possible. To explain the concept, a sample QRWT is designed to match a standard rectangular waveguide WR34 to a half-height waveguide. The reflection coefficient is less than -24 dB in the full operation bandwidth, from 21.7 to 33 GHz. The optimized QRWT has a length of 3.088 mm, 0.218 times the waveguide wavelength at its center operation frequency of 27.35 GHz. For comparison, a two-stage quarter-wave transformer matching circuit is optimized. For similar reflection coefficients in the same bandwidth, the QRWT has a length 57% shorter than the conventional one, and it has an easier manufacturing method using wire-electrode cutting. To verify the design, a back-to-back matching circuit was fabricated using brass material without extra dielectric material. Its measured reflections are less than -16.6 dB, and the insertion loss is less than 0.2 dB.