This paper presents a lowpass-bandpass diplexer based on a novel microstrip structure. The lowpass channel is appropriate for wireless applications with a cut-off frequency at 2.4 GHz while the bandpass channel is located at 4.2 GHz for WiMAX applications. The lowpass channel is created by a novel structure, which is proposed for the first time in this work. A novel combination of thin transmission lines, rectangular and triangular patches is used to make the bandpass channel. The lowpass and bandpass filters are integrated by a compact simple transmission line. The diplexer structure is well miniaturized with an overall size of 0.036 λg2. The proposed diplexer has the most compact size in comparison with the previous lowpassbandpass diplexers. Moreover, the insertion losses at both channels are better than 0.18 dB, while the return losses at the lower and upper channels are 18.2 dB and 41.4 dB respectively. In comparison with the previous works, the designed diplexer has the lowest insertion and return losses. To confirm the simulation results, the introduced diplexer is fabricated and measured.
