A Dual-Polarized Ultrawideband Conformal Antenna for Microwave Brain Imaging

This letter presents a dual-polarized ultrawideband conformal antenna for microwave brain imaging systems. The proposed antenna fed from two individual ports features a square slot with rounded corners etched on the ground plane, which can operate in dual-linear-polarized radiation modes. To widen the operating bandwidth, a circular ring together with a cross-shaped patch is loaded at the slot center. Two open-ended microstrip networks with their geometries as same as those of the feeding structures, are utilized for guaranteeing symmetric current modes and thus considerably reducing cross-polarization levels. Conformally attached to the human head, the antenna utilizes the loading effect of brain tissue to direct its beam toward the brain. While maintaining a low profile of 0.0004λ0 without any reflector, it achieves unidirectional radiation with a front-to-back ratio over 11 dB. The antenna exhibits effective signal penetration into the human head and fidelity factors above 80% for stable pulse transmission with minimal distortion. The specific absorption rate analysis validates the electromagnetic safety for human exposure. The fabricated prototype exhibits an ultrawide overlapped bandwidth of approximately 0.54 GHz to 2.47 GHz and a fractional bandwidth of around 129% for both polarizations. These results validate the antenna design and its great potential for microwave brain imaging applications.