Lens antenna

A lens antenna is a directional antenna that uses a shaped piece of microwave-transparent material to bend and focus microwaves by refraction, as an optical lens does for light. Typically it consists of a small feed antenna such as a patch antenna or horn antenna which radiates radio waves, with a piece of dielectric or composite material in front which functions as a converging lens to collimate the radio waves into a beam. Conversely, in a receiving antenna the lens focuses the incoming radio waves onto the feed antenna, which converts them to electric currents which are delivered to a radio receiver. They can also be fed by an array of feed antennas, called a focal plane array (FPA), to create more complicated radiation patterns. To generate narrow beams, the lens must be much larger than the wavelength of the radio waves, so lens antennas are mainly used at the high frequency end of the radio spectrum, with microwaves and millimeter waves, whose small wavelengths allow the antenna to be a manageable size. The lens can be made of a dielectric material like plastic, or a composite structure of metal plates or waveguides. Its principle of operation is the same as an optical lens: the microwaves have a different speed (phase velocity) within the lens material than in air, so that the varying lens thickness delays the microwaves passing through it by different amounts, changing the shape of the wavefront and the direction of the waves. Lens antennas can be classified into two types: delay lens antennas in which the microwaves travel slower in the lens material than in air, and fast lens antennas in which the microwaves travel faster in the lens material. As with optical lenses, geometric optics are used to design lens antennas, and the different shapes of lenses used in ordinary optics have analogues in microwave lenses. Lens antennas have similarities to parabolic antennas and are used in similar applications. In both, microwaves emitted by a small feed antenna are shaped by a large optical surface into the desired final beam shape.