Lens antenna
A lens antenna is a microwave antenna that uses a shaped piece of microwave-transparent material to bend and focus the radio waves 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. They are used less than parabolic antennas due to chromatic aberration and absorption of microwave power by the lens material, their greater weight and bulk, and difficult fabrication and mounting. They are used as collimating elements in high gain microwave systems, such as satellite antennas, radio telescopes, and millimeter  wave radar and are mounted in the apertures of horn antennas to increase gain. Types. Microwave lenses can be classified into two types by the propagation speed of the radio waves in the lens material: The main types of lens construction are: "Zoned lens" - Microwave lenses, especially short wavelength designs, tend to be excessively thick. This increases weight, bulk, and power losses in dielectric lenses. To reduce thickness, lenses are often made with a "zoned" geometry, similar to a Fresnel lens. The lens is cut down to a uniform thickness in concentric annular (circular) steps, keeping the same surface angle . To keep the microwaves passing through different steps in phase, the height difference between steps must be an integral multiple of a wavelength. For this reason a zoned lens must be made for a specific frequency History. The first experiments using lenses to refract and focus radio waves occurred during the earliest research on radio waves in the 1890s. In 1873 mathematical physicist James Clerk Maxwell in his electromagnetic theory, now called Maxwell's equations, predicted the existence of electromagnetic waves and proposed that light consisted of electromagnetic waves of very short wavelength. In 1887 Heinrich Hertz discovered radio waves , electromagnetic waves of longer wavelength. Early scientists thought of radio waves as a form of "invisible light". To test Maxwell's theory that light was electromagnetic waves, these researchers concentrated on duplicating classic optics experiments with short wavelength radio waves, diffracting them with wire diffraction gratings and refracting them with dielectric prisms and lenses of paraffin, pitch and sulfur. Hertz first demonstrated refraction of 450 MHz (66 cm) radio waves in 1887 using a 6 foot prism of pitch. These experiments among others confirmed that light and radio waves both consisted of the electromagnetic waves predicted by Maxwell, differing only  in frequency. The possibility of concentrating radio waves by focusing them into a beam like light waves interested many researchers of the time. In 1889 Oliver Lodge and James L. Howard attempted to refract 300 MHz (1 meter) waves with cylindrical lenses made of pitch, but failed to find a focusing effect because the apparatus was smaller than the wavelength. In 1894 Lodge successfully focused 4 GHz (7.5 cm) microwaves with a 23 cm glass lens. Beginning the same year, Indian physicist Jagadish Chandra Bose in his landmark 6 - 60 GHz (25 to 5 mm) microwave experiments may have been  the first to construct lens antennas, using a 2.5 cm cylindrical sulfur lens in a waveguide to collimate the microwave beam from his spark oscillator, and patenting a receiving antenna consisting of a glass lens focusing microwaves on a galena crystal detector. Also in 1894 Augusto Righi in his microwave experiments at University of Bologna focused 12 GHz (3 cm) waves with 32 cm lenses of paraffin and sulfur. However, microwaves were limited to line-of-sight propagation and could not travel beyond the horizon, and the low power microwave spark transmitters used had very short range. So the practical  development of radio after 1897 used much lower frequencies, for which lens antennas were not suitable. The development of modern lens antennas occurred during a great expansion of research into microwave technology around World War 2 to develop military radar. In 1946 R. K. Luneberg invented the Luneberg lens.