This paper proposes a Ka-band broadband circularly polarized (CP) antenna array with high cross-polarization discrimination (XPD). The design employs a four-layer stacked dielectric substrate structure, utilizing a 2 \(\times\) 2 sequentially rotated (SR) array of substrate integrated cavity (SIC) magnetoelectric dipole elements as the core radiator. A double-layer substrate integrated waveguide (SIW) SR feeding network achieves precise 90 \(^\circ\) phase delay at the center frequency, enabling the array to attain 30.33% impedance bandwidth over 23.45–32.02 GHz and 35.52% axial ratio (AR) bandwidth across 22.48–32.19 GHz. A \(4\times 4\) multi-layer square-loop array decoupling surface (ADS) is integrated on the topmost dielectric layer of the antenna array. By optimizing the geometry, dimensions of the square loops, and the substrate thickness, this structure generates reflected waves with specific amplitude and phase characteristics, effectively canceling out the coupling waves propagating between the antenna elements. This design significantly suppresses the mutual coupling among the radiating elements, resulting in a XPD better than \(-25\ \text {dB}\) across the operating band. It thereby substantially mitigates the mutual coupling issue commonly encountered in millimeter-wave antenna arrays. Furthermore, the ADS structure enhances gain performance, yielding a peak gain of 12.47 dBic with gain variations below 3 dB throughout 25.95–33.14 GHz. The fabricated array measures \(2.03 \times 2.03 \times 0.32\lambda ^{3}\) . The proposed CP antenna array demonstrates significant potential for application in 5G millimeter-wave communication systems.
