A compact, single-layer SIW cavity–based linear to-circular polarization converter is presented. The unit cell, realized on Rogers RT/duroid 5880 (εr = 2.2, tanδ = 0.0009, h = 1.57 mm) and sized for the TE₁₁₀ resonance mode (17.5 × 17.5 mm²), employs a front cross-shaped slot and a three-slot rectangular array on the backside. Floquet-mode simulations show that slot-length tuning and parametric optimization yield near-equal co- and cross-polar transmission with a  90 ° phase quadrature, producing a circular polarization (CP) wave at 9.8 GHz. The resulting design achieves a 7.1% relative axial-ratio bandwidth (B.W) with only 0.35 dB insertion loss (IL). A 9×9 array assembly fed by a standard horn provides an aperture gain of 13.7 dBi and good radiation characteristics. The proposed structure offers low IL, a suitable axial-ratio (AR) bandwidth, and a single-layer profile for a compact X-band CP.

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