Tapered Graded Index Lens Antenna With Enhanced Penetration for Near-Field Torso Imaging

A tapered graded index (GRIN) lens antenna that aims at enhancing electromagnetic penetration into human torso by generating focused planar near-field radiation is presented. The antenna is composed of a four-layer lens excited by a modified slot antenna. Each layer of the lens case is filled with fabricated mixtures of engineered materials that have specific relative permittivity values. The exciting antenna is made compact by utilizing meandered feed lines and substrate folded technique. The number of layers of the lens and the specification of each layer are determined by minimizing the generalized reflection coefficient of the antenna. A focused plan-wave radiation is achieved by tapering the lens based on the theory of total internal reflection. The study of $E$ -field quality inside the torso demonstrates that the proposed lens successfully converts spherical wave radiation into a local plane-wave radiation, resulting in 11 dB improvement in wave penetration inside the torso compared to the body-matched antenna (without lens). The measured results show that the antenna operates over a wide band from 0.43 to 1.85 GHz (125% fractional bandwidth), which is a merit for the torso imaging application. Furthermore, the antenna has the physical size of 110 $\times 110\times48$ mm3, corresponding to $0.15\times 0.15\times 0.06\lambda _{0}^{3}$ , where $\lambda _{0}$ is the wavelength at the lowest working frequency. The results indicate that the antenna accomplishes 4.9 dB (209% improvement) stronger wave penetration inside the torso compared to other existing GRIN lens antennas.