Wideband Diplexer Design and Optimization Based on Back-to-Back Structured Common Port

A “back-to-back” structured common port is proposed with high isolation obtained between the two channels. This port has the advantage of being able to easily achieve a low external quality factor (Qe). By introducing this type of port, a “back-to-back” structured diplexer with wide bandwidth is designed and fabricated. The bandwidths of each passband are 9.5% and 13.6%. Each channel filter is optimized by using the Cauchy method. Guided by a real-time Cauchy extracting system, the diplexer is tuned. The measured reflection coefficient in the passbands is below -23 dB, and the average insertion losses are 0.7 and 0.55 dB. The measured isolation between the two passbands is higher than 34 dB. An agreement is found between measured and simulated results.