Microstrip diplexer design for X-band RF/microwave front-end applications

This paper presents a microstrip diplexer operating at 9.79 and 11.47 GHz. The proposed diplexer consists of two parallel coupled line band-pass filters (BPFs) designed using a randomly defined image impedance technique. Thus, suitable coupled line gap distance can be obtained for conventional wet-etching fabrication. This allows easy design and fabrication of the parallel coupled line type BPFs and diplexers. To increase the isolation of the proposed diplexer, quarter wavelength impedance transformers are used between the two filters. Through EM simulation, insertion losses of less than 2.5 dB are achieved in both frequency pass-bands with a good isolation. The predicted results are compared with the EM simulated results and they show a good agreement.