Design of a compact microstrip quadruplexer with closely spaced switchable and tunable channels based on asymmetrical dual-mode loop resonators

In this letter, a switchable microstrip quadruplexer based on asymmetrical dual-mode loop resonators (ADMLRs) is presented. Four ADMLRs at different electrical lengths for creating four channels are coupled to input/output (I/O) ports by means of thin film capacitors. Each resonator has two patch elements behaving as reference capacitors that are needed for switching process. In order to achieve the switching ability, varactor diodes are employed to provide variable capacitances at each resonator. Thus, all channels can be independently switched on/off by changing the capacitances of the varactor diodes. Moreover, all channels have fine bandwidth tuning mechanism depending on the capacitance changes. The designed quadruplexer was fabricated and the measurements were observed in a good agreement with the predicted results. Center frequencies of the proposed quadruplexer are adjusted to 1.8, 2.1, 2.4, and 2.7 GHz with the frequency ratios between the adjacent channels of 1.17, 1.14, and 1.13. Isolations between the outputs have been observed as better than 28 dB.