In this paper, a new microstrip triplexer with flexible resonance frequencies is designed based on the properties
of coupled lines, steps, and spiral cells. It operates at 2.67 GHz for 4G LTE and at 3.1 GHz and 3.43 GHz for IEEE
802.16 WiMAX. The close resonance frequencies make it suitable for frequency division duplex applications. In order to
improve insertion loss, the LC equivalent circuit of the proposed resonator is analyzed. Moreover, careful alignment of
the coupled lines and step impedance structures is performed to improve the insertion and return losses so that they are
0.72/0.63/0.81 dB and 24.5/24/24.7 dB, respectively. The frequency response shows good selectivity for each channel.
Meanwhile, there are several transmission zeros above the first channel and below the last one that improve the stopband
properties. The proposed triplexer is fabricated and measured. The simulation and measurement results are in good
agreement, which validates the introduced methodology.