Generalized interdigital filters made of multilayer striplines

In the first part of this paper is presented a relatively simple loss analysis method for many coupled line structures. The corresponding equivalent circuit consists of uncoupled lossy transmission lines whose parameters are determined relatively easily without a complex numerical field analysis by using fringing‐capacitance curves for rectangular strips. In the narrowband case further approximations are possible. The increase of the accuracy is most significant if the couplings between strips are tight. In the second part a realization of the generalized interdigital filter using a multilayer structure consisting of polyphenyleneoxide plates is discussed. The repeatability of the filters is improved with the aid of thin polyethylene films. The shorts are made by the copper coating that also effectively suppresses higher‐order modes. The fine tuning of the filter can be made by tuning screws. The improvement of the design accuracy without tuning elements using a computer simulation is also discussed. Finally, the effects of the losses are studied, connecting the loss analysis presented with ring resonator measurements.