Wideband Monostatic Co-Horizontally Polarized Simultaneous Transmit and Receive Antenna Subsystem With Integrated Beamforming Network

A wideband monostatic simultaneous transmit and receive (STAR) antenna subsystem with high isolation and Co-horizontally polarized (Co-HP) omnidirectional radiation is presented. The antenna subsystem consists of a STAR antenna and a beamforming network (BFN). The STAR antenna consists of a planar circular array having four printed dipole elements surrounded by two groups of parasitic strips. The parasitic strips are used to improve the impedance bandwidth (IMBW) and reduce the gain variation in transmit (TX) and receive (RX) modes. To achieve high isolation, the four ports of the STAR antenna are excited with two orthogonal phase modes for TX and RX using the BFN. The BFN is a six-port network consisting of two 180° hybrid couplers, one 90° hybrid coupler, and a Wilkinson power divider. The 180° hybrid coupler is elaborately designed with low imbalances since it is most key to achieve high TX/RX isolation. Measured results show that the proposed STAR subsystem achieves a TX/RX overlapping IMBW of 4.71–6.2 GHz, and a 40 dB TX/RX isolation bandwidth of 4.81–5.88 GHz, while maintaining horizontally polarized omnidirectional radiation patterns for TX and RX over the entire operating frequency band.