In this paper, we present a compact tri-band bandpass filter (BPF) using two stub-loaded dual mode resonators (SLDMRs) combined with intra-coupled internal resonators. The designed filter operates at 1.575, 2.4, and 3.45 GHz, corresponding to the GNSS, WLAN, and WiMAX applications, respectively. The passbands of the filter are determined by odd- and even-mode frequencies created by the SLDMR and the internal open loop resonator inside of it. The corresponding even-mode frequency can be adequately tuned by adjusting the length of the stub while the odd-mode frequency is fixed. Two transmission zeros (TZs) are introduced on each side of the passband to improve the selectivity of the implemented filter. Five TZs around the edges of three passbands make the passbands highly isolated, and these transmission zeros can be placed according to the desired choice. The proposed tri-band BPF was designed, fabricated and measured, and the simulated and measured results corresponded very well.
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