Compact Dual-Band Filtering Baluns Using Perturbed Substrate Integrated Waveguide Circular Cavities

This letter presents a novel class of dual-band filtering baluns using perturbed substrate integrated waveguide (SIW) circular cavities. By stacking the SIW cavities vertically, flexible filtering order can be realized. The stacked structure also permits the introduction of cross-couplings between non-adjacent cavities, which can be used to generate transmission zeros (TZs) for improving selectivity and band-to-band isolation. In addition, since the external
$Q$
-factors and internal coupling coefficients for both passbands are controllable, a flexible bandwidth ratio can be obtained. For demonstration, two prototypes for second- and third-order dual-band filtering baluns are implemented and measured, respectively. Both of them exhibit good filtering responses and balun characteristics in two passbands.