In this work, a lowpass-bandpass diplexer is proposed based on a novel microstrip structure. This diplexer includes two lowpass and bandpass filters integrated by a simple transmission line. The lowpass channel has a cut-off frequency at 1.88 GHz which makes it suitable for GSM applications. The resonance frequency of the bandpass channel is 3.56 GHz for IEEE 802.16 WiMAX applications. In comparison with the previous reported lowpass-bandpass diplexers, the proposed design has the lowest insertion and return losses at both channels while it occupies the most compact implementation area of 0.03 λg2. The designed diplexer can suppress the harmonics up to 4.45FC with a maximum harmonic level of -20 dB, where FC is the cut-off frequency of the lowpass channel. The lowpass channel has a very sharp roll-off of ξ=220 while the bandpass channel has a high selectivity with a high fractional bandwidth of 23.8%. To verify the design method and simulation results, the designed diplexer is fabricated and measured. The simulated and measured results are in a good agreement.
