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Simultaneous Transmit and Receive Spiral Antenna With Improved Isolation

A wideband, monostatic simultaneous transmit and receive cocircularly-polarized spiral antenna with improved isolation is discussed. The topology with four arms is configured such that the opposite pairs of arms are used for transmit (TX) and receive (RX). Focused design for robust fabrication that ensures geometrical symmetry with low component imbalances for applied excitations of the two spirals has led to a strong rejection of the coupled voltages and, therefore, high TX/RX isolation. More than 14 dB higher minimum isolation is achieved, when compared to previous spirals from the same STAR family. To ensure reasonable performance in terms of axial ratio and efficiency over 0.5–2.5 GHz, the antenna is restively terminated, cavity backed, and lens loaded. The fabricated prototype is stout and achieves > 50 dB self-interference cancelation over the bandwidth. The copolarized TX and RX modes have VSWR < 2, axial ratio < 4 dB, and symmetric patterns.