High isolation and miniaturized quad-channel diplexer using short stub-loaded resonators and distributed coupling feeding line

This paper presents a high isolation and miniaturized microstrip quad-channel diplexer. The quad-channel diplexer consists of a distributed coupling feeding line, two coupling feeding lines, and two pairs of short stub-loaded resonators combined by the pseudo-interdigital structure. Based on the even-odd mode method, the resonant modes of the short stub-loaded resonator are investigated. The mode splitting is created when a pair of short stub-loaded resonators are combined by a pseudo-interdigital structure. Each mode of the short stub-loaded resonator is split into two modes to form a two-order channel filter. The center frequencies of the quad-channel diplexer can be adjusted independently, while the bandwidths of channel filters can be tuned by the external quality factor and the coupling coefficient. Taking advantage of the distributed coupling feeding line, no extra matching network is needed and the loading effect is very small. For demonstration, a quad-channel diplexer working at 2.4, 3.15, 4.4, 5.8 GHz with better than 40 dB isolation is presented. The measured results agree well with the full-wave simulated results.