Synthesis of microwave filters with “reactive” nodes

This paper introduces the synthesis of microwave inline filters of narrow or moderate bandwidth which include, in addition to the cavities, “reactive nodes”, which can be implemented either by lumped components or by non-resonating, open or short circuited stubs. The synthesis is approached by means of an equivalent circuit, directly defined in the bandpass frequency domain, where the cavities are represented by shunt or series resonators and the reactive nodes by capacitors and inductors (such network has no lowpass equivalent representation). The filter includes transmission zeros, realized by means of shunt-connected series resonators (or parallel resonator connected through an admittance inverter). The paper gives general indications about the evaluation of the polynomials associated to the equivalent circuit and the subsequent circuit synthesis. Some examples, which demonstrate the improvements which are obtained in comparison to the classical in-line synthesis based on non-resonating nodes, are also reported.