This paper presents compact dual/tri-band bandpass filters (BPFs) with controllable frequency and high selectivity for WLAN applications. A stepped impedance resonator with a shorting stub and a uniform impedance resonator with an open stub are applied in the designs. Several techniques that can generate transmission zeros are combined, to improve the frequency selectivity. The resonators and the proposed filters are characterized by full-wave simulations. To validate the design strategies, a dual-band BPF centered at 2.4 GHz and 5.2 GHz was first designed. With a minor modification, a tri-band BPF centered at 2.4 GHz, 5.2 GHz and 5.8 GHz was then developed. Both simulations and measurements were carried out to demonstrate the effectiveness of the designs. Good agreements are achieved.
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