A Wideband, Unidirectional Circularly Polarized Antenna for Full-Duplex Applications

A wideband planar unidirectional circularly polarized (CP) antenna with simultaneous transmit and receive (STAR) capability is presented for full-duplex applications. The proposed STAR antenna consists of four radiating elements and is fed with a custom-designed feed network. The radiating elements are identical CP patch antennas, which are sequentially rotated along the center of the STAR antenna. The four radiating elements are symmetrically distributed along the perimeter of a circle. The feed network is composed of two identical 180° power dividers. Each pair of CP elements, placed opposite to each other on one of the axes of symmetry of the STAR antenna, is excited together using one of the 180° power dividers. This way, a null is created along the other axis of symmetry of the STAR antenna causing near-field cancellation at the location of the other two antennas. A prototype of the proposed STAR antenna operating in the 6.0-7.2 GHz frequency range is designed, fabricated, and experimentally characterized. Measurement results show that the antenna provides an isolation better than 40 dB, an axial ratio better than 3 dB, and a gain higher than 8.7 dBic over this entire band of operation.