Microstrip interdigital filters

An approximate equivalent circuit for microstrip interdigital filters is derived, taking into account the effects of the inhomogeneous dielectric, causing the quasi-TEM-modes to differ in their phase constants. This equivalent circuit is represented by a ladder network being well suited for a filter synthesis. A realized filter confirms the theoretical results. It is assumed that the approach demonstrated here will be applicable to other microstrip coupled line structures as well, leading to comparably good results.