Implementation of Distributed-Element Foster Section and Its Applications to Bandpass Filters

An accurate method to implement a bandpass-domain coupled-line Foster section is presented. Two types of practical coupled-line structures are presented and closed-form equations for their design parameters are given in terms of lowpass prototype parameters, reference impedance, and fractional bandwidth so that one can readily use them in various filter designs. The design process of a bandpass filter using Foster sections is described via an example.