Design of a compact broadband millimeter-wave folded lens antenna with high aperture efficiency based on gradient refractive index metamaterial

This paper introduces a compact, broadband folded lens antenna (FLA) with high aperture efficiency for millimeter-wave (MMW) communication systems. The design features a gradient refractive index (GRIN) metamaterial lens integrated with a folded structure containing an ultra-broadband polarization converter. The GRIN lens, operating in the non-resonant region with 0.133 λ 0 × 0.133 λ 0 size unit cells at the center frequency of 25 GHz, enables precise phase control across its aperture, thereby enhancing the aperture efficiency. The folded topology triples the electrical path length, reducing the focal length by a factor of three to maintain a low profile. To complement the broadband GRIN lens, a novel ultra-broadband polarization converter is realized by coating a substrate layer, achieving a -1 dB conversion bandwidth of 97.6%. Theoretical analysis validates the operating principles. The finalized FLA demonstrates a -3 dB bandwidth of 45.2%, a peak gain of 23.3 dBi, and a maximum aperture efficiency of 62.4%. With a low profile of 2.2 λ 0 , the compact design, broadband performance, and high efficiency of the FLA make it an ideal candidate for integration into modern MMW communication systems. • GRIN metamaterial enables a low-profile, broadband lens with low losses. • Ultra-broadband polarization converters decrease the focal length by 2/3, achieving a compact FLA. • Millimeter wave band FLA, with high directivity and aperture efficiency.