Helical-Ring Antenna for Hemispherical Radiation in Circular Polarization

A compact helical-ring antenna is proposed to realize a Huygens source in circular polarization. The radiation pattern of this antenna is mainly directed toward an half-space without the use of any reflector. The principle relies on the combination of two sets of collocated electric and magnetic short dipoles. The helical ring is excited by means of in-quadrature capacitive probes. Solutions are proposed to adjust the resonant frequency and balance the contributions of the magnetic and electric dipoles. The direction of the radiation pattern can be electrically switched frontward or backward by modifying the in-quadrature probes. Besides, the size of the antenna can be reduced by increasing the number of turns, therefore, without high-permittivity materials. The properties of this antenna are analyzed by means of simulations and confirmed via measurements on a prototype.