This study presents a compact wide stopband microstrip diplexer which is designed based on a novel resonator structure. This novel resonator consists of a main hairpin cell loaded by a smaller hairpin and T-shaped cells. The presented diplexer operates at 3.46 and 4.75 GHz for IEEE 802.16 worldwide interoperability for microwave access (WiMAX) and IEEE802.11a Wireless LAN applications, respectively. The main advantages of the proposed diplexer are its flat channels, two wide bandwidths, attenuated harmonics, low insertion losses better than 0.18 dB, several transmission zeros and sharp frequency response. From first up to fifth harmonics of the proposed diplexer are well attenuated with the maximum harmonic level −20 dB. Meanwhile, the simulation results show that the presented design can suppress harmonics up to 44 GHz. In other words, the harmonics from the 1st up to 12th are suppressed. The group delay at both channels is lower than 1.1 ns, which is good for a microstrip diplexer. This good performance is obtained, while the overall area of the designed diplexer is only 0.037λ2g. Based on the comparison results, the proposed diplexer has a compact size, the minimum group delay and a wide stopband rejection. The proposed diplexer is fabricated and measured. Both simulation and measurement results are in good agreement.
