| Summary: | This article presents a compact ultrawideband (UWB) bandpass filter (BPF) with sharp selectivity, wide notched-band, wide out-of-band rejection, and approximately flat group delay. The proposed filter structure is composed of a U-shaped open-circuited line that is interdigitally coupled to the input/output feed-lines using high impedance lines. The structure generates multiple resonant modes across the UWB span between 3.1 and 10.6 GHz, however, high coupling between the feed-lines and the U-shaped structure transforms the discrete resonant modes into an UWB BPF. Coupling is enhanced by inserting dielectric slots in the ground-plane immediately under the coupling-lines. A wide notched-band is implemented by introducing asymmetry in the coupled lines. This was achieved by making one of the coupling lines longer by folding it and extending the line to partially couple with U-shaped structure. The length of the extended coupled line and the position of the folded section determine the center frequency of the notch-band and its 3-dB bandwidth. The notch was designed to be centered at 5.5 GHz with appropriate 3-dB bandwidth to blocking interference from WLAN signals. The selectivity of the filter was enhanced by loading the U-shaped structure with a T-shaped open-circuited stub to generate two additional resonant modes in the filter's passband and two transmission zeros that were strategically located at the upper and lower cut-off frequencies of the filter. The design was fabricated and its performance verified. The proposed filter has dimensions of 9.4 × 9.9 mm2.
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