A compact microstrip quad-channel diplexer with high-selectivity and high-isolation performances

A compact, high-selectivity, and high-isolation microstrip quad-channel diplexer has been presented in this paper. By introducing multiple coupling paths between input and output ports, the diplexer design is, therefore, more flexible and can be extended to higher-order multiplexers. In addition, no extra matching network is needed for this design, resulting in a smaller circuit size. For the demonstration, a quad-channel diplexer with third-order bandpass response has been designed and fabricated with microstrip technology. As a result, the diplexer occupies a compact size of 0.27 λ_g by 0.16 λ_g (0.18 λ₀ by 0.11 λ₀). The measured results are in good agreement with the simulated predictions, showing that an excellent isolation of better than 55 dB between channels is achieved.