This work presents a very compact microstrip lowpass-bandpass (LP-BP) triplexer, which is designed and analyzed based on a novel structure. Due to its complex design process, this type of triplexer is rarely designed. Compared to the previous LP-BP triplexers it has the most compact size of 0.006 λ2g , whereas an LP-BP triplexer with dimensions smaller than 0.01 λ2g has not been designed yet. This triplexer is designed based on a perfect mathematical method and optimization simultaneously. Its lowpass band has a cut-off frequency of 0.67 GHz, suitable for low-band 5G applications. The resonance frequencies of its bandpass channels are located at 2.15 GHz and 3.19 GHz. These bandpass channels make the proposed triplexer appropriate for 5G mid-band applications. This triplexer can suppress the harmonics from the first up to 8th harmonic. The bandpass channels are flat and wide with two fractional bandwidths (FBW) of 15% and 11.97%. To prove the designing process and its simulation results, the presented novel LP-BP triplexer is fabricated and experimentally measured. The comparison results show that the experimental measurement confirms the simulation results. The close alignment between the measurements and simulation results demonstrates a high level of accuracy of our designing method.
