A Compact Near-Field Horn-Fed 3-Bit Phase Gradient Metasurface Lens with Low F/D for Low-Cost Efficient Focusing at ISM-Band

Metasurface-based lenses offer strong potential for enhancing wireless power transfer (WPT) by concentrating electromagnetic energy at targeted focal regions. Conventional designs apply ideal phase compensation assuming planar wave-fronts, typically requiring far-field excitation or bulky beam-shaping elements, which limit compactness. In this work, we demonstrate a compact near-field WPT architecture by directly illuminating a passive metasurface lens with a standard horn antenna at a short standoff of 5A at 24 GHz, achieving a high normalized focal length and an ultra-low F/D ratio of 0.5. The lens comprises a 20 × 20 array of cross-shaped unit cells on a single-layer, double-sided FR-4 substrate, and employs a 3-bit (8-level) quantization scheme to realize the desired phase profile. Each unit cell occupies λ / 2 × λ/2 × λ / 8, enabling efficient subwavelength sampling. Full-wave simulations and experimental results confirm strong focal confinement, with FWHM-based analysis validating the lens's suitability for compact and efficient wireless power transfer.