Analysis of a perforated SIW structure with a rectangular air box and its application to the design of a step-impedance microwave filter

A new technique for reducing the substrate permittivity in a Substrate Integrated Waveguide (SIW) structure is presented, consisting of removing a centered rectangular box of substrate dielectric material, which increases the equivalent waveguide impedance. This technique can be applied to the implementation of high-impedance SIW sections (perforated regions) sandwitched between low-impedance sections (non perforated sections), the first ones acting as impedance inverters in a filter design. The dispersion properties of this perforated SIW structure have been analyzed and compared to that of a homogeneous equivalent waveguide with a defined effective permittivity. Finally, following the described technique for reducing the substrate permitivitty, a filter prototype has been designed and fabricated to demonstrate the feasibility of the proposed method.