Twisted Bessel Beams via Bull-Eye Antennas Excited by a Single Uniform Circular Array

The design of a microstrip bull-eye antenna operating at microwave frequencies and excited by a single uniform circular array of vertical coaxial probes is presented. The array feeder excites higher order cylindrical leaky waves with a fast, backward, radially propagating space harmonic capable of generating twisted Bessel beams in the near-field region. By varying the phasing of the source, beams with consistent axicon angles and different azimuthal orders can be synthesized, corresponding to different orbital-angular-momentum (OAM) states. A careful design of the feeder allows for obtaining Bessel-like field distributions in the radiative near field and stable farfield patterns as a function of the azimuth angle, as well as satisfactory matching at the input ports, both in terms of passive scattering coefficients and of active reflection coefficients, for five different OAM states. Numerical full-wave results are reported to validate the proposed design.