A Highly Efficient Spherical Luneburg Lens for Low Microwave Frequencies Realized With a Metal-Based Artificial Medium

This article describes a novel spherical lens antenna constructed of planar layers of lightweight foam with equally spaced conducting inclusions of varying sizes on an orthogonal grid. This construction largely overcomes the problems of weight and cost that have tended to make larger low-frequency Luneburg lenses impractical. A penalty for this type of design is that some anisotropy exists in the lens's dielectric. This effect is examined using both ray-tracing techniques and full-wave simulation, and it is found that the principal consequence is that the focal length of the lens varies in different directions. Methods for mitigating the effect are proposed. A prototype lens antenna intended for cellular use in the band 3.3-3.8 GHz with dual-linear slant polarized feeds was designed and constructed to confirm the findings. The measured results show a peak gain of 23 dBi, which is less than 1 dB lower than the maximum possible directivity from the lens's cross-sectional area. Scanning loss is less than 0.8 dB over the whole sphere. The simulated and measured performances show excellent agreement over the whole sphere. The overall performance of the prototype lens antenna demonstrates that this type of lens should be very suitable for use in high-gain multibeam antennas at lower microwave frequencies.