Wideband Multimode Monostatic Spiral Antenna STAR Subsystem

A wideband, multimode, and monostatic simultaneous transmit and receive (STAR) antenna subsystem is proposed. The configuration is composed of a single four-arm spiral and two analog circuit layers designed to maximize isolation between the transmitting (TX) and receiving (RX) channels. The first layer consists of two Butler matrix beamformer networks (BFNs); one for each TX and RX. The second layer integrates four, ideally phase-matched, circulators between the spiral arms and the BFNs. Theoretically, this configuration achieves infinite isolation irrespective of circulator's quality. However, the BFN imbalances and the dissimilarities between the four circulators degrade the overall isolation. The operational principles are discussed under the ideal conditions and in the presence of circuit imperfections. For a four-arm spiral, the utilized BFN provides orthogonal modes 1, -1, 2, and 3, allowing STAR capability with multimode and diverse radiation patterns. The subsystem isolation and far-field performances are characterized experimentally and computationally for different modes of operation. A simple approach to achieve a dual-polarized STAR operation over narrower bandwidth with the proposed subsystem is also discussed. The multimode subsystem is demonstrated over 4:1 bandwidth with high isolation, VSWR <; 2, consistent patterns, axial ratio <; 3 dB over a wide field of view for the broadside modes (1, -1) and <; 4.2 dB for other conical modes (2, 3).