A New Family of Diplexers With Quarter-Wavelength Resonators and Frequency-Variant Couplings

In this article, we discuss a novel compact topology for waveguide diplexers, composed of inline rectangular waveguide filters based on λ g/4 resonators and frequency-variant couplings (FVCs). The FVCs also generate transmission zeros (TZs), thereby significantly enhancing the filters’ selectivity. The theory needed for the accurate synthesis and design of the waveguide structure is explained in detail. We also describe, step by step, the design of the two separate filters and the procedure to combine them into a diplexer. Finally, we discuss the measured results of a Ku-band diplexer prototype. The measured results are in very good agreement with theory, thereby validating both the new topology and the design procedure that we describe.