A Novel Compact Triple-Mode Microstrip Bandstop Filter with Adjustable Reflection Zeros

In this letter, the effect of open-circuited stubs on the frequency response of a microstrip square loop resonator shaped as a meander form is investigated with a full-wave electromagnetic (EM) simulator and a triple-mode microstrip bandstop filter is designed by using this effect. Proposed filter configuration is consisted of a microstrip square loop resonator shaped meander and four open-circuited stubs vertically and horizontally located inside the resonator. These stubs not only can be used to adjust the reflection poles in the stopband, but also used to obtain the miniaturisation. Central reflection pole is related with the horizontally located stubs, whereas the other two reflection poles are obtained from the vertically located stubs. In addition, the proposed configuration allows the controlling of the reflection zeros by changing the lengths of the stubs. Designed filter is simulated, fabricated and tested. Measured results show a good agreement with the simulated results.