Design of a Substrate Integrated Waveguide Balun Filter Based on Three-Port Coupled-Resonator Circuit Model

A novel balun filter is proposed using the substrate integrated waveguide (SIW) technique, which is designed with a three-port coupled-resonator circuit model. Considering the loading effect of other resonators, a modified balanced resonant mode in a higher-order SIW cavity is utilized to provide both a negative cross-coupling and the conversion from an unbalanced signal to a balanced one. Our developed balun filter has good frequency selectivity, and the balance of amplitude and phase between two outputs is also achieved, which is demonstrated by good agreement between its simulated and measured results.