This paper presents a novel design of lowpass-bandpass microstrip diplexer using patch and interdigital cells. In comparison with the previously reported lowpass-bandpass diplexers, the proposed design has the most compact size with the overall area of 0.018 λg2 (351.7 mm2 ) fabricated on 0.787 mm dielectric substrate height. The lowpass channel has a cut-off frequency (fc) at 1.57 GHz. The resonance frequency of the bandpass channel is located at 3.35 GHz, which is suitable for WiMAX applications. Having low insertion losses at the lowpass and bandpass channels is another feature of this work. For the frequencies above the bandpass channel, the harmonics are attenuated up to 2.8 fc. A theory method is presented to show the behavior of the proposed structure. Finally, the designed diplexer is fabricated and measured, where the simulation results match well with the measurements.
