A Wideband Circularly Polarized Omnidirectional Antenna Based on Excitation of Two Orthogonal Circular TE21 Modes

A novel wideband circularly polarized (CP) omnidirectional antenna operating at L-band is presented. The antenna operates based on excitation of a field distribution similar to circular waveguide TE21 mode over a wide bandwidth. It is shown that the circular TE21 mode generates a linearly polarized far-field electric field over the horizontal plane with its polarization tilt angle varying as a function of azimuth angle. It is also shown that the tilt angle varies by 90° for an azimuthal angle increase of 45°. To obtain circular polarization, two circular TE21 modes are excited such that one mode is spatially rotated by 45° with respect to the other mode. The phase difference between two modes is 90°. The modes are excited by an array of 16 monopole sectorial loop antennas to achieve a wide axial-ratio (AR) bandwidth. A right-handed CP antenna based on the proposed structure is fabricated and tested against simulation results. A feed network including Wilkinson power dividers and low-pass/high-pass phase shifters is also designed and fabricated to feed the antenna elements. The measurement results show 58% of 3-dB AR and 10-dB impedance bandwidth with the center frequency at 1.565 GHz. The total height of the antenna is 47 mm.