In this paper, new microstrip bandstop filters with single band, dual-band and tri-band by using U-shaped defected ground structures are presented without the assistance of coupled lines or certain resonators, and the application of DGS is developed. The proposed bandstop filters have good performances of low loss, multi-band operation, transmission zeros which improve the filter frequency selectivity, and miniaturization because of the cascade of DGS and minimum defected patterns which reduce the circuit size. The new designs are demonstrated by measurement.
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