The investigation of coupling between matching circuit and input resonators of channels in a microstrip diplexer

This paper deals with investigation of the coupling coefficients between the matching circuit and the input resonators of channels in a microstrip diplexer. Diplexer channels are two-section filters, and matching circuit is regular non-resonant segment of a microstrip line placed between input resonators of channels. The common port is connected to outer end of the segment and its second end is opened. Research was carried out theoretically using a computer program developed on the basis of mathematical expressions obtained by means of one-dimensional models and quasi-static approximation. The results show that a value of coupling is quite large and the use of the matching circuit in a form of a non-resonant segment of a microstrip line allows to design diplexers with wide fractional bandwidth of the channels.