A multimode lens-loaded eight-arm spiral aperture is proposed for in-band full-duplex applications. Specifically, the single eight-arm spiral antenna is composed of a four-arm spiral for transmitting (TX) and a 45 degrees spatially rotated four-arm spiral for receiving (RX). The eight-arm spiral antenna is connected into two 4x4 Butler matrix beamforming networks (BFNs) to differentiate between the TX and RX arms/ports and distinguish their functionalities. In the absence of BFN imbalances and antenna geometry asymmetries, the isolation is theoretically infinite due to full cancellation at the RX antenna ports and RX beamformer ports. This approach also enables diverse co-polarized TX and RX radiation modes while maintaining theoretically infinite isolation. The considered spiral aperture modes are mode 1, with a broadside pattern, and modes 2 and 3, with conical patterns. The aperture is loaded with a hyperhemispherical dielectric lens to improve the far-field performance. The operational principles are discussed first under ideal conditions, followed by the computational and experimental results. Average measured isolation 38 dB is achieved over 3:1, 1.5:1, and 1.3:1 bandwidths for the radiating TX and RX modes 1-3, respectively. Once the TX and RX are driven with identical modes, similar radiation patterns with high envelope correlation coefficient are obtained.
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