An antenna reflector is a device that reflects

electromagnetic wave In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) ...

s. Antenna reflectors can exist as a standalone device for redirecting

radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the up ...

(RF) energy, or can be integrated as part of an

antenna Antenna ( antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two collid ...

assembly.

Standalone reflectors

The function of a standalone reflector is to redirect electro-magnetic (EM) energy, generally in the radio

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...

range of the

electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging fro ...

. Common standalone reflector types are *

corner reflector A corner reflector is a retroreflector consisting of three mutually perpendicular, intersecting flat surfaces, which reflects waves directly towards the source, but translated. The three intersecting surfaces often have square shapes. Radar co ...

, which reflects the incoming signal back to the direction from which it came, commonly used in radar. * ''flat reflector,'' which reflects the signal such as a mirror and is often used as a

passive repeater A passive repeater or passive radio link deflection, is a reflective or sometimes refractive panel or other object that assists in closing a radio or microwave link, in places where an obstacle in the signal path blocks any direct, line of sigh ...

Integrated reflectors

When integrated into an antenna assembly, the reflector serves to modify the radiation pattern of the antenna, increasing gain in a given direction. Common integrated reflector types are *

parabolic reflector A parabolic (or paraboloid or paraboloidal) reflector (or dish or mirror) is a reflective surface used to collect or project energy such as light, sound, or radio waves. Its shape is part of a circular paraboloid, that is, the surface genera ...

, which focuses a beam signal into one point or directs a radiating signal into a beam. * a passive element slightly longer than and located behind a radiating dipole element that absorbs and re-radiates the signal in a directional way as in a Yagi antenna array. * a ''flat reflector'' such as used in a Short backfire antenna or Sector antenna. * a ''corner reflector'' used in UHF television antennas. * a ''cylindrical reflector'' as used in Cantenna.

Design criteria

Parameters that can directly influence the performance of an antenna with integrated reflector: * Dimensions of the reflector ('' Big ugly dish'' versus small dish) * Spillover (part of the feed antenna radiation misses the reflector) *

Aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An ...

blockage (also known as feed blockage: part of the feed energy is reflected back into the feed antenna and does not contribute to the main beam) * Illumination taper (feed illumination reduced at the edges of the reflector) * Reflector surface deviation * Defocusing * Cross polarization * Feed losses * Antenna feed mismatch * Non-uniform amplitude/phase distributions The antenna efficiency is measured in terms of its effectiveness ratio. Any gain-degrading factors which raise

side lobe In antenna engineering, sidelobes are the lobes (local maxima) of the far field radiation pattern of an antenna or other radiation source, that are not the ''main lobe''. The radiation pattern of most antennas shows a pattern of "''lobes'' ...

s have a two-fold effect, in that they contribute to system

noise temperature In electronics, noise temperature is one way of expressing the level of available noise power introduced by a component or source. (This is to be distinguished from Temperature Noise in Thermodynamics or Principal Interferrometric Analysis Over C ...

in addition to reducing gain. Aperture blockage and deviation of reflector surface (from the designed "ideal") are two important cases. Aperture blockage is normally due to shadowing by feed, subreflector and/or support members. Deviations in reflector surfaces cause non-uniform aperture distributions, resulting in reduced gains. The standard symmetrical, parabolic,

Cassegrain reflector The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas, the main characteristic being that the optical path folds back onto itself, relative to t ...

system is very popular in practice because it allows minimum feeder length to the terminal equipment. The major disadvantage of this configuration is blockage by the hyperbolic sub-reflector and its supporting struts (usually 3–4 are used). The blockage becomes very significant when the size of the parabolic reflector is small compared to the diameter of the sub-reflector. To avoid blockage from the sub-reflector asymmetric designs such as the open Cassegrain can be employed. Note however that the asymmetry can have deleterious effects on some aspects of the antenna's performance - for example, inferior side-lobe levels, beam squint, poor cross-polar response, etc. To avoid spillover from the effects of over-illumination of the main reflector surface and

diffraction Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...

, a microwave absorber is sometimes employed. This lossy material helps prevent excessive side-lobe levels radiating from edge effects and over-illumination. Note that in the case of a front-fed Cassegrain the feed horn and feeder (usually waveguide) need to be covered with an edge absorber in addition to the circumference of the main paraboloid.

Measurements

Measurements Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. In other words, measurement is a process of determining how large or small a physical quantity is as compared ...

are made on reflector antennas to establish important performance indicators such as the gain and sidelobe levels. For this purpose the measurements must be made at a distance at which the beam is fully formed. A distance of four

Rayleigh distance Rayleigh distance in optics is the axial distance from a radiating aperture to a point at which the path difference between the axial ray and an edge ray is λ / 4. An approximation of the Rayleigh Distance is Z = \frac, in which Z is the Rayleig ...

s is commonly adopted as the minimum distance at which measurements can be made, unless specialized techniques are used (see '' Antenna measurement'').

References

{{reflist Radio frequency antenna types Satellite broadcasting

Standalone reflectors

Integrated reflectors

Design criteria

Measurements

See also

References