A novel compact wideband bandstop filter (BSF) is proposed by using microstrip ring structure with stepped impedance lines. The equivalent circuit of the BSF is analyzed, and its transmission and reflection characteristics are obtained. With appropriate definition of the realizable reflection function of the BSF and an optimization function, the circuit parameters of the BSF are solved by using an optimization algorithm. As a result, the designed BSF can realize a general Chebyshev frequency response with three transmission zeros in its stopband and six reflection zeros in its passband. As an example, a BSF with a fractional bandwidth of 30% and a minimum stopband rejection of 30.0 dB is designed, fabricated, and measured, and the desired wide stopband, large stopband rejection, low passband insertion loss, and sharp skirt property are verified.
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