Antenna System for Full-Duplex Operation of Handheld Radios

A full-duplex handheld antenna system for ad hoc communications based on a circular array of four half-wavelength dipole antennas is discussed. The dipoles are fed in two orthogonal phase modes for the transmitting (TX) and receiving (RX), by means of a beamforming network (BFN). This mode multiplexing technique provides high isolation between TX and RX ports. Two different BFNs are researched, which differ only in the phase mode of the TX path. TX is fed in either mode 1 (M1) or a mixed mode (MM), and the RX is fed in mode 0 (M0). M1-M0 BFN achieves omnidirectional patterns for both TX and RX with a high system isolation (i.e., >30 dB) over the allocated frequency band (i.e., 1.75-1.85 GHz). When the TX is fed in MM, the system isolation increases to >43 dB. A prototype is built and measured, demonstrating high isolation level (i.e., >30 dB) over the operational bandwidth. For actual field implementation, the impact of imbalances in the commercial off-the-shelf BFN components, as well as a soldier's body and helmet on the antenna system isolation, is carefully studied.