Microstrip Eight-Channel Diplexer With Wide Stopband

A microstrip eight-channel diplexer with a compact size and a wide stopband is presented. There are sixteen stepped-impedance resonators controlling the passband characteristics. Every two resonators are used for one passband resulting in a high design freedom. There are four passbands filtered out at each of the two output ports. The circuit size is only 0.1 λg² at the lowest channel frequency. The isolation is more than 29 dB, and the 20 dB upper stopband extends up to 8.22 × the lowest operation frequency.