Design of a Microstrip Low-Pass-Bandpass Diplexer Using Direct-Feed Coupled-Resonator Filter

Previous systematic low-pass-bandpass diplexer was using coupled-feed line to design its bandpass channel response. However, a coupled-feed line coupled-resonator bandpass filter typically leads to the realization problem in the external quality factor because of the line space limitation between the coupled-feed line and the resonator. Based on our investigation, systematic low-pass-bandpass diplexer design using direct-feed coupled-resonator bandpass filter to relax this problem has never been published in the literature. Therefore, a low-pass-bandpass diplexer using direct-feed coupled-resonator bandpass filter is presented and a systematic matching design is also included in this letter.