Compact bandstop filter with high stopband attenuation

In this letter, a compact microstrip bandstop filter (BSF) is proposed. The topology of the proposed filter is formed by a pair of coupled line with open-ended stub connected by a T-shaped transmission line. Owing to the structure symmetry, the closed-form expressions of scattering parameters can be achieved by odd-and even-methods. Then transmission zeros can be adjusted by coupled line width with open-ended stub, and the centre frequency can be tuned by the length of coupled line and open-ended stub in the middle together. In order to verify the proposed topology, a bandstop filter with 20dB fractional bandwidth (FBW) of 28% at centre frequency f 0 =4 GHz has been fabricated, and stopband rejection depth is less than −35dB.