Compact tri-band bandpass filter based on quarter-wavelength resonators

In this work, a planar microstrip tri-band filter based on quarter-wavelength resonators is presented, which allows independent control of the positions of all three passbands. Furthermore, the topology of the filter provides five transmission zeros and thus results in a highly selective response in all passbands. A synthesis procedure of the filter is presented and used to design a tri-band filter which operates at WLAN frequencies of 2.4/3.5/5.2 GHz. The designed filter outperforms all other state-of-the-art tri-band filters in terms of compact size, high selectivity and transmission, and return-loss characteristics.

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