Microstrip lowpass-highpass diplexer for a quadplexer application

A microstrip low-loss diplexer is designed based on a lowpass and highpass Chebyshev filter configuration and combined with two duplexers for a quadplexer application. A 50-Ω transmission line is added to the lowpass filter to have a high impedance at the highpass frequency band, resulting in low loss and high isolation. To minimize size of the diplexer, high impedance meander lines, interdigitated capacitors, and stepped impedance transmission lines are used. The proposed diplexer has the isolation higher than 41 dB with the insertion loss less than 1.6 dB at the designed frequencies. The designed quadplexer has a good transmission performances at all four frequency bands, and can be used for a dual-band frequency division multiplexing system with a dual-band/broadband antenna.