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.
