Lens antenna

Microwave lenses can be classified into two types by the propagation speed of the radio waves in the lens material: • Natural dielectric lens - A lens made of a piece of dielectric material. Due to the longer wavelength, microwave lenses have much larger surface shape tolerances than optical lenses. Soft thermoplastics such as polystyrene, polyethylene, and plexiglass are often used, which can be molded or turned to the required shape. Most dielectric materials have significant attenuation and dispersion at microwave frequencies. • Artificial dielectric lens - This simulates the properties of a dielectric at microwave wavelengths by a 3 dimensional array of small metal conductors, such as spheres, strips, discs or rings suspended in a nonconducting support medium made of an array of split rings, to refract microwaves • Constrained lens - a lens composed of metal leaves, ducts or other structures that control the direction of the microwaves. They are used with linearly polarized microwaves. :*E-plane metal plate lens - a lens made of closely spaced metal plates parallel to the plane of the electric or E field. This is a fast lens. :*H-plane metal plate lens - a lens made of closely spaced metal plates parallel to the plane of the magnetic or H field. This is a delay lens. :*Waveguide lens - A lens made of short sections of waveguide of different lengths • Fresnel zone lens - A flat Fresnel zone plate, consisting of concentric annular sheet metal rings blocking out alternate Fresnel zones. It can be easily fabricated with copper foil shapes on a printed circuit board. This lens works by diffraction, not refraction. The microwaves passing through the spaces between the plates interfere constructively at the focal plane. It has large chromatic aberration and so is frequency-specific. • Luneburg lens - A spherical dielectric lens with a stepped or graded index of refraction increasing toward the center. Luneburg lens antennas have several unique features: the focal point, and the feed antenna, is located at the surface of the lens, so it focuses all the radiation from the feed over a wide angle. It can be used with multiple feed antennas to create multiple beams. 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 (50–5 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,{{cite journal 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. Luneburg invented the Luneburg lens. == References ==