Two dual-band second-order highly selective band pass filters operated at 3.5/5.5 GHz and 3.5/6 GHz for wireless local area network /worldwide interoperability for microwave access WLAN/WiMAX applications are introduced in this paper. The designed filters are inspired of utilizing two coupled open-loop resonators loaded with stub, spiral resonators and lumped capacitors. The filters are designed based on calculating the desired coupling matrix and the external quality factor. The first and the second filters are designed at the fundamental mode of 3.5 GHz then the first filter is loaded with two spiral resonators in the microstrip line to produce the desired band stop behaviour, which in turn achieves the second pass-band. However, the second band of the second filter is achieved by loading the stub with the lumped capacitors, which controls the second mode. The centre frequency of the second band is adjusted by varying the lumped capacitors values. The two designed filters have insertion loss less than 0.7 dB in the pass-band region, high selectivity with more than 4 transmission zeros and more than 20 dB attenuation level in the stop band region. The suggested filter has compact size and high selectivity with tunability behavior. The two filters are fabricated and measured to validate the simulated results.
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