A lensantenna 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, 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 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; 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:
Delay lens : in this type the radio waves travel slower in the lens medium than in free space; the index of refraction is greater than one, so the path length is increased by passing through the lens medium. This is similar to the action of an ordinary optical lens on light. Since thicker parts of the lens increase the path length, a convex lens is a converging lens which focuses radio waves, and a concave lens is a diverging lens which disperses radio waves, as in ordinary lenses. Delay lenses are constructed of
Fast lens : in this type the radio waves travel faster in the lens medium than in free space, so the index of refraction is less than one, so the optical path length is decreased by passing through the lens medium. This type has no analog in ordinary optical materials, it occurs because the phase velocity of radio waves in waveguides can be greater thanthe speed of light. Since thicker parts of the lens decrease path length, a concave lens is a converging lens which focuses radio waves, and a convex lens is a diverging lens, the opposite of ordinary optical lenses. Fast lenses are constructed of
The main types of lens construction are:
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 structures that control the direction of the microwaves. They are used with linearly polarized microwaves.
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 circular steps. 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
