Active Feed Tuning for Excitation Symmetry in Simultaneous Transmit and Receive Antennas

We present an actively tuned balun to enable simultaneous transmit and receive (STAR) antennas to achieve greater isolation. Specifically, for the first time, we propose placing a chip into the antenna feed to control the amplitude of the current density. Nearly perfect balance of the feed currents enables STAR antennas to maintain strong isolation despite unpredictable fabrication imperfections. Two identical prototypes were fabricated for comparison, one passive and the other with active tuning inserted. Measurements demonstrate a minimum isolation of 42 dB across a 250 MHz bandwidth, representing an 11 dB improvement over the passive case. This is the best reported transmit/receive isolation enhancement to date. Further, this novel tuning method is significant because it enables STAR antennas with low-cost substrates, such as FR-4, to achieve greater isolation than passive designs. Notably, the balun was tuned across 1.68-1.93 GHz, part of the Federal Communication Commission's AWS-3 spectrum auction. Measurements show good comparison with simulation.