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.