Design of a novel compact microstrip diplexer with low insertion loss

In this article, a microstrip diplexer is implemented based on a novel combination of the coupled lines and spiral structures in order to reduce the circuit size as well as insertion losses. This diplexer with an elliptic function frequency response operates at 2.4/2.79 GHz for wireless applications. Several transmission zeros on the both sides of two passbands are realized, which improve the selectivity. A theory method is presented that helps to miniaturization, control the resonance frequency, improve the insertion loss, and external quality factor simultaneously. The proposed diplexer has a compact size of 0.3λg × 0.25λg with low insertion losses of 0.18/0.39 dB at 2.4/2.79 GHz. The isolation between the two channels (S23) is better than 20.5 dB from DC up to 6 GHz. The designed diplexer is fabricated and measured. There is a good agreement between the simulated and measured results.