DIPLEXERS BASED ON MICROSTRIP LINE RESONATORS WITH LOADED ELEMENTS

In this paper, microstrip line resonators with loaded elements are proposed and studied to design microstrip diplexer. To demonstrate the design ideas, the equivalent circuits of the proposed resonators are built and studied. It is found that the different loads on different positions of the proposed half-wavelength resonator make the resonator have different features, which will easily control the characteristic of the diplexers. And here, resistor, open stub, and shorted stub are used as loaded elements. It is found the resistor loaded on the center of the microstrip line resonator can extremely reduce the unloaded quality factor of even-mode resonant frequency, which can be used to suppress the harmonics of the diplexer. The loaded open stub not only can reduce the size of the diplexer, but also can control the frequency ratio between the fundamental frequency and second harmonic of a resonator, which can increases the frequency ratio between the two passbands of the diplexer. As for the loaded shorted stub, it can enlarge the size of the diplexer. To demonstrate the design ideas, three diplexers are presented. The comparisons between the loaded and unloaded diplexers are given. The experimental results agree well to the theoretical predictions and simulations.