Design of a Ultra Compact Filtering Crossover Based on a Circular SIW Cavity with Cross Transition and Air-Cavity Transition Instructions

This paper proposes a compact filtering crossover based on a circular substrate-integrated waveguide (SIW) cavity with integrated orthogonal cross transitions and an air cavity. By removing the central copper cladding and incorporating a cross transmission structure, the design achieves a passband with improved signal transmission and frequency selectivity within a single cavity. Two orthogonal feeding ports are introduced to exploit degenerate modes, providing simultaneous filtering and crossover functionality with high isolation. The device operates from 8.0 GHz to 9.1 GHz, corresponding to a fractional bandwidth of 12.9%, with a return loss better than 25 dB and an insertion loss below 2.0 dB. Furthermore, replacing the dielectric with an air cavity enhances thermal management and reduces footprint, making it suitable for RF front-end applications. Experimental results demonstrate the design's effectiveness, showing improved isolation and lower insertion loss compared to conventional multi-cavity architectures.