Microstrip bandstop filters using L- and T-shaped resonators

Bandstop filters in microstrip technology are introduced. The filters are based on a T-shaped resonator located between two identical L-shaped resonators; all of them are coupled to a main microstrip line, and separated by sections of transmission lines with appropriate length. According to ABCD matrix analysis, carried out for commensurate resonators, this structure produces two transmission zeros in the stopband as well as three reflection zeros in the passbands. X-band and Ku-band bandstop filters are designed based on the proposed method with bandwidths of 1.06 GHz and 0.97 GHz at mid-band frequencies of 9.72 GHz and 14.77 GHz, respectively. Both filters are prototyped and measured. Good agreement between simulated and measured results demonstrates the reliability and robustness of the design method.