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In this article, a novel band-notched ultra-wideband (UWB) bandpass fllter based on hybrid microstrip/slotline structure is proposed. A quad-mode stubs-loaded slotline resonator is fed by two orthogonal microstrip lines and ultra-wideband bandpass characteristic is excited. A stub-loaded dual- mode microstrip resonator is externally loaded to the quad-mode slotline resonator, and a notched band with sharp roll-ofi characteristic is achieved. The circuit model for the dual-mode microstrip resonator loaded slotline is given and analyzed. The proposed fllter is designed and fabricated. Simulated and measured return losses are i12dB/i18dB and i18dB/i10dB in lower and higher passband. The return loss in the notched band is greater than 15dB.

This paper presents a novel compact microstrip hybrid four-mode bandpass fllter (BPF) with good selectivity and multi- transmission zeros. By adding an external resonator to a triple- mode stub-loaded resonator, four modes are generated in the desired passband and a hybrid four-mode BPF is implemented. Odd- and even-mode theory is introduced to investigate the mode characteristics of triple-mode resonator in detail. Circuit model and coupling matrix are built to further explain the proposed methodology. A microstrip BPF with central frequency of 3.4GHz for wireless communication was designed and fabricated. Three transmission zeros are obtained at 3.2GHz, 3.55GHz and 3.73GHz, which improve selectivity and out- of-band rejection of the fllter. The measured results of the fabricated fllter represent good in-band and sharp sideband characteristic, which match well with simulated results.

A novel dual-mode bandpass fllter (BPF) using stub- loaded defected ground open-loop resonator is proposed in this article. Defected arrow-shaped stub is loaded to a defected ground open- loop resonator, and two non-degenerate modes are excited for dual- mode characteristics. Based on even- and odd-mode theory, dual- mode characteristics of the resonator is analyzed. Design equations for the defected-ground resonator are investigated. A two-pole dual- mode bandpass fllter operating at 2.4GHz with fractional bandwidth of 7.97% is designed, fabricated, and measured. Good agreement between simulated and measured results verifles the validity of this design methodology.