A Near-Field Probe Employing Absorber-Based Diffraction Regulation for Wide-Beam Radiation

In this communication, a spatial radiation field filter based on expanded polypropylene (EPP) absorbing materials is proposed to regulate the radiation field of the open-boundary quad-ridged horn (OQRH) antenna, enabling stable wide-beam performance across a broad frequency range for near-field probe applications. First, a rhombic cavity EPP is designed to dissipate up to 97% of the edge diffraction energy through exponential attenuation while preserving the main-lobe radiation, and this suppression effect reduces sidelobe levels and improves low-frequency beam equalization. Subsequently, the circular-aperture diffraction theory is applied to the design of absorbing materials for the first time. A novel EPP structure with a circular aperture (aperture radius r = 3λ /4 at 8 GHz) is proposed to address beam narrowing at high frequencies by regulating the aperture field distribution. In addition, a tapered cross-shaped slot is incorporated within the circular-aperture structure to improve field continuity, reduce low-frequency reflections, and enhance radiation efficiency by ∼20%. The final designed dual-polarized probe antenna achieves a stable half-power beamwidth (HPBW) ranging from 53° to 89° across the 2-8-GHz band. The measured reflection coefficient is below -10 dB, the port isolation is better than 30 dB, the sidelobe level is lower than -17 dB, and the difference between the HPBWs in the E-plane and H-plane remains smaller than 18°.