This work introduces a planar harmonic suppresser microstrip four-channel multiplexer based on an originally novel geometrical structure. It consists of coupled lines connected to engraved semicircular cells. The realized quadruplexer operates at 3.211, 3.276, 3.38, and 3.491 GHz for IEEE 802.16 WiMAX. Wide stopbands above all passbands are obtained up to about 20 GHz so that fifth harmonics are attenuated properly. The semicircular structures provide quite close channels, which make it appropriate for FDD scheme. Compared to the previous reported quadruplexer, the common port return loss is decreased significantly, where improving the common port impedance matching for very close passbands is an important challenge. To minimize the circuit size, a new matching network consisting of coupled line and step impedance cell is embedded. To verify the designing methodology, the proposed multiplexer is fabricated and measured, where the simulated and measured results are in a good agreement.
