A Broadband Waveguide Diplexer for D-Band Based on Singlet and Resonant Irises

This paper presents a novel design for a compact $\mathbf{D}$ band waveguide diplexer consisting of a bandpass filter and a pseudo-highpass filter. Both filters use a combination structure of a singlet resonator (TM<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">110</inf>) and two coupled resonant irises. By incorporating transmission zeros (TZs), a filter with three-order quasi-elliptical functional properties is constructed. The adoption of a compact geometry not only improves frequency selectivity, but also achieves an increase in filter order and a significant reduction in size by introducing coupled resonant irises instead of conventional resonant cavities. Simulation results demonstrate the performance characteristics of the diplexer at two passband center frequencies of 126.5 GHz and 156 GHz: the 3 dB fractional bandwidths (FBW) are $\mathbf{1 6. 6 \%}$ and $\mathbf{1 7. 9 5 \%}$, respectively, with the insertion losses in the passbands less than 0.23 dB and $\mathbf{0. 1 6 ~ d B}$. In addition, the return loss at the common port exceeds 13 dB and the isolation between the two channels is greater than 22 dB ∘