Design method for wideband direct-coupled resonator filters with electric or magnetic couplings

The paper presents methodology for designing wideband and ultra-wideband filters with electric (capacitive) or magnetic (inductive) direct couplings based on closed-form formulas. For given filter specifications (required frequency characteristic, filter center frequency, bandwidth) the method gives filter parameters as a set of resonant frequencies, coupling factors, external quality factors and characteristic impedances of the filter resonators. The method is based on analysis of circuit models of filters with only-electric and only-magnetic direct couplings. Numerical experiments of the circuit models allowed to study properties of such filters and to build effective design method. In this paper method for designing 5th order filters is described in details. High accuracy of the presented method is also addressed.