Ultrasharp-Roll-Off Ku-Band Diplexer With Contiguous Channels in Two-Level Groove Gap Waveguide Using Input/Output Extracted Cavities

A novel class of highly selective ${\textbf{Ku}}$ -band diplexer with ultrasharp-rejection spectrally contiguous channels realized in groove gap waveguide (GGW) technology is presented. Each channel is implemented using a quadruplet-based bandpass filter (BPF) with input $\boldsymbol {/}$ output extracted-cavity sections. It enables the generation of two out-of-band transmission zeros (TZs) at each passband side, that is, two double TZs and two single TZs in total, and up to six reflection zeros (RZs) per channel, leading to high-order filtering responses with extremely abrupt cut-off slopes. Additional TZs also appear due to the interchannel interaction. To reduce the circuit size, a two-level vertically stacked scheme is adopted in the GGW implementation. Each channel is integrated into a different layer, and both channels are interconnected at the input via a coupling window. The RF operational principles of the proposed diplexer and its building channel BPF unit are detailed through their coupling-routing diagrams (CRDs). Furthermore, a proof-of-concept prototype is designed, fabricated, and tested. It features roll-off rates of 824.95–813.06 dB $\boldsymbol {/}$ GHz for the lower and upper channels, respectively, along with closely spaced transmission bands centered at 14.45–15.11 GHz.