O. Quevedo-Teruel will explain the operation of lens antennas, their potential, and two innovative techniques that have become very important in recent years. The first technique is transformation optics, which can be employed to produce three-dimensional directive lenses. The second one is metasurfaces, which can be used to produce low-cost and planar two-dimensional lenses. Finally, he will introduce the concept of higher symmetries, that can be employed to enhance the bandwidth of conventional metasurfaces, or to increase their equivalent refractive indexes

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Lens antennas are commonly englobed in a more general type of antennas, named aperture antennas. As their name indicates, they make use of a lens to modify the field distribution at the aperture of the antenna, which is typically fed by a single source. The lens is employed to transform the waves arriving from the source into a desired radiation pattern. Commonly, the desired radiation pattern is a directive beam in a given direction. However, similar to arrays, reflectors or leaky wave antennas, the goal changes depending on the application. For example, other desired features may be to produce multiple beams, or a broad beam-width. Lenses were more commonly employed in optical applications. For this reason, most of the nomenclature comes from optics, and they are evaluated with rays theory. In this sense, the performance of the lens is conventionally described in terms of aberrations. An aberration is a failure of the rays to converge at the desired focus. This failure must be due to a defect or an improper design. Aberrations are classified as chromatic or monochromatic, depending on whether or not they have a frequency dependence. There are five monochromatic aberrations: spherical aberration, coma, astigmatism, Petzval field curvature, and distortion. However, this is not a common nomenclature for antenna designers in the radio-frequency and microwave regimes. In these regimes, the rays are substituted by electromagnetic fields, and the designers evaluate their antennas in terms of directivity, gain, efficiency, side lobe levels, cross polarization levels, etc. Therefore, there is a communication gap between both communities: optics and microwaves. In the THz regime, which is in between these two communities, researchers must understand both nomenclatures In these lectures, O. Quevedo-Teruel will explain the operation of lens antennas, their potential, and two innovative techniques that have become very important in recent years. The first technique is transformation optics, which can be employed to produce three-dimensional directive lenses. The second one is metasurfaces, which can be used to produce low-cost and planar two-dimensional lenses. In the case of metasurfaces, fully metallic solutions are possible, which is a clear advantage in terms of losses. However, with the available technology, metasurfaces are only able to scan in one single plane. Finally, we introduce the concept of higher symmetries, that can be employed to enhance the bandwidth of conventional metasurfaces, or to increase their equivalent refractive indexes

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P.D.1-1 - Novembre