An Ultralow-Profile Folded Transmitarray Antenna Based on a Multifunctional Metasurface With Both-Sided Wavefront Control

In this article, a wideband, ultralow-profile circularly polarized (CP) folded transmitarray antenna (FTA) is proposed for millimeter wave (MMW) applications. The antenna comprises a receiver and transmitter (R/T) metasurface above a waveguide-integrated metal plate reflector. By controlling the orientations of the R/T elements, the metasurface can simultaneously and independently manipulate its reflection and transmission of electromagnetic (EM) wavefronts under opposite-handed CP incidences. The reflection mode of the metasurface, together with the bottom reflector, is used to confine the EM waves for multiple reflections, thereby reducing the antenna profile. In particular, the antenna profile-to-focal length ratio can be significantly reduced to less than the conventional FTA of one-third. Meanwhile, the transmission mode of the metasurface is devoted to compensating for the spatial phase delay to generate a high-gain beam. As a proof-of-concept, an FTA prototype with $19\times 19$ elements was modeled and fabricated with its profile reduced to one-fifth of the focal length. Measured results demonstrate that the designed CP FTA can achieve a peak gain of 24.2 dBic at 61.5 GHz with a wide 3-dB gain bandwidth from 51.4 to 71.2 GHz (32.3%) and 3-dB axial ratio (AR) bandwidth over 35.4%.