A Wideband, Scanning Array of Four-Arm Spiral Elements for Simultaneous Transmit and Receive (STAR)

Simultaneous transmit and receive (STAR) is attractive because they double spectral efficiency and alleviate radio frequency (RF) spectrum congestion. As expected, a major challenge in STAR is the high power leakage from the transmitter to the receiver, which dramatically reduces a receiver's dynamic range and then leads to desensitization. Therefore, it is paramount to ensure high transmit/receive (Tx/Rx) isolation. In this paper, we present a wideband STAR antenna array operating across 1.6-3.28 GHz (1 octave bandwidth), with a large transmit to receive isolation of 35 dB across the entire bandwidth. This is the first wideband and low profile (λ/16 at the lowest frequency) STAR array retaining strong isolation when scanning down to 30° from broadside. The array elements are four-arm spirals where two arms are for transmit and the other two for receive. To ensure high Tx/Rx isolation, symmetry is maintained everywhere in the structure, spiral arms, baluns, and feeds. The element spacing is kept at λ/2 at the highest frequency to avoid grating lobes. A 5 \times 5 array prototype was built and tested to validate the design. Measurements show excellent agreement with simulations.