Broadband Circularly Polarized Antenna Array via Metasurface and Partially Emptied Substrate

A metasurface including an empty region constituted by a square-shaped aperture is proposed to improve the performance of a circularly polarized patch antenna array based on sequential-phase feeding. The metasurface is applied/stacked to a conventional antenna consisting of a loop feeding structure, which enables sequential incremental phase to four patches. Surface waves propagating on the metasurface are excited, affecting the resonance features of the overall radiating structure, thereby improving both the input impedance-matching and the axial ratio bandwidths of the antenna. The square-shaped aperture in the top layer is introduced to extend the range of frequency over which the sequential phase feeding remains effective. The antenna prototype and the stacked metasurface are fabricated. The experimental results, in good agreement with simulation, show that the antenna with the compact size 2.16 λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 2.16 λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.12 λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub>, at the central frequency, provides a broad impedance relative bandwidth of 43.8% from 11.6 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GHz</i> to 18.1 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GHz</i> and a 3-dB axial ratio bandwidth of 36.6% (from 11.6 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GHz</i> to 16.8 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GHz</i>). In addition, the proposed antenna has a flat gain within the operating frequency band and a peak gain <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</i>=13.9 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dBic</i> at frequency <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i>=14 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GHz</i>.