Two-Layered Microstrip Diplexer Based on High-Selectivity Wideband Bandpass Filters

This paper presents a microstrip diplexer based on two high-selectivity wideband bandpass filters (BPFs) in a two-layered structure. Both the lower and upper channels are designed with third-order microstrip-to-microstrip vertical transitions using open-circuit-ended slotline stepped-impedance resonators (SIRs). Specifically, open-circuit-ended microstrip lines on the top and bottom layers for the lower channel are employed, whereas the upper channel is equipped with short-circuit-ended ones. Thus, owing to the slotline SIRs, two close-to-passband transmission zeros (TZs) leading to sharp-rejection capabilities for both BPF channels are attained. The theoretical foundations and RF operating principles of the devised diplexer are discussed in detail. For practical-validation purposes, a two-layered microstrip wideband diplexer prototype is designed, simulated, and tested. It features two three-pole high-selectivity filtering responses for the measured lower and upper channels that are centered at 1.52 GHz and 2.76 GHz, and exhibit 3-dB fractional bandwidths of 60% and 24.93%, respectively.