Wideband Absorbing Metasurface for Improving Axial Ratio of a Compact Archimedean Spiral Antenna

A wideband absorbing metasurface is proposed to improve the circularly polarized response of a 1–18 GHz low-profile Archimedean spiral antenna. The metasurface consisting of surface-mounted resistors between metallic patches in a circular arrangement is placed on the opposite side of the antenna substrate. After analyzing the unit cell behavior under an equivalent three-conductor transmission line, a concentric arrangement of the metasurface under the spiral is proposed. This metasurface is designed to absorb the edge-reflected waves to improve the circularly polarized response of the antenna below 6 GHz. The antenna substrate with the metasurface is placed above a flat conductor that makes an overall antenna height of 5 mm. After numerical investigations, a prototype is fabricated and characterized. The measured right-hand circularly polarized gain is positive above 2 GHz with a maximum gain of approximately 8.7 dBic at 10.6 GHz. An axial ratio (AR) better than 3 dB is observed above 1.125 GHz. The benefits of the metasurface compared to a commercially available absorbing material are highlighted, and its usefulness to design a compact spiral antenna is presented.