An Improved Narrowband Microwave Bandstop Filter

Capacitively coupled stub bandstop filters have excellent narrowband rejection performance at f0, but poor transmission characteristics in their upper passband, particularly around 2f0. An improved network, using capacitively loaded unit elements, gives good transmission characteristics in the upper passband, including around 2f0, up to the next resonance appearance. An example of a four resonator cellular band rejection filter with good passband performance over 2Ghz is presented, where all line impedances equal to 50 Ohms, including the resonant stubs. The improved network is useful for designs utilizing commercial ceramic coaxial resonators.