Microstrip diplexer constructed with new types of dual-mode ring filters

A microstrip diplexer consisting of two new types of ring filters is presented in this letter. The proposed ring filter is able to move one of transmission zeros closer to its passband corner by connecting the feeding capacitors at the different locations of the resonant ring. Thus, a diplexer implemented by connecting two kinds of ring filters with different feeding features can achieve the required isolation for the small channel separation. Then an open-stub is added at the common input port of the channel filters to achieve the required input matching at the two passbands of the diplexer. A diplexer prototype with two passbands at 1.75 GHz and 1.85 GHz with a channel separation 40 MHz was fabricated. The measured insertion losses in the pass bands are all less than 2.1 dB. The isolation between the two channels is greater than 20 dB and the input return losses of the three ports are about 20 dB.