Compact microstrip diplexers based on a dual-passband filter

Novel microstrip diplexer structures composed of a dual-passband filter together with two matching circuits are proposed. By using only a single dual-passband filter, the proposed diplexer may be made compact when compared with the conventional diplexer configuration which has two single- passband filters. In this study, the dual-passband filter is realized using the parallel-coupled microstrip structures, and the matching circuits are developed based on the stepped- impedance transmission-line sections along with suitable open stubs. By suitably incorporating the open stubs into the matching circuits, a 3 rd-order Chebyshev microstrip diplexer with dual center frequencies at 2.36 GHz and 5.17 GHz and an acceptable isolation is implemented.