Transmitarray Antenna Generating Circularly Polarized Orbital Angular Momentum (OAM) Beams: Synthesis, Prototyping and Measurements

We present the synthesis, prototyping and measurement of a transmitarray antenna for the generation of circularly-polarized (CP) orbital angular momentum (OAM) beams. A novel "S-ring" transmitarray element is designed to sustain CP operation with only three metal patterned layers. The unit cell provides arbitrary CP phase compensation by merely changing the rotation angle of the element, while the transmission magnitude remains greater than 0.9. In previous work, transmitarrays that support circular polarization were designed based on the assumption of normal plane wave incidence; thus, the feed source of the transmitarray must be placed far from the transmitarray. In this work, an aperture phase synthesis methodology that accounts for the spherical phase of the feed is presented such that the feed can be placed near the aperture at about F/D = 1. A proof-of-concept prototype transmitarray antenna with a thickness of 0.3 cm operating at 19 GHz is constructed and measured for far-field performance. The measurements agree well with predictions obtained by full-wave simulations and demonstrate that the proposed transmitarray antenna can be a unique apparatus that generates OAM CP cone-shaped patterns.