Reflector (antenna)
Reflects or redirects electromagnetic waves in antenna systems.
An antenna reflector is a surface that bounces electromagnetic waves. It may be used on its own to redirect radio frequency (RF) energy, or it can be built into an antenna assembly to shape the radiation pattern and boost gain in a specific direction.
**Standalone reflectors** A standalone reflector simply redirects electromagnetic energy, usually in the radio part of the spectrum. Common types include the corner reflector, which sends an incoming signal straight back where it came from (often used in radar), and the flat reflector, which works like a mirror and is frequently employed as a passive repeater.
**Integrated reflectors** When part of an antenna assembly, the reflector alters the antenna’s radiation pattern to increase gain in one direction. Common integrated types are the parabolic reflector, which focuses a beam to a point or turns a radiating signal into a beam; a passive element slightly longer than and placed behind a radiating dipole, which absorbs and re-radiates the signal directionally (as in a Yagi array); a flat reflector used in short backfire or sector antennas; a corner reflector found in UHF television antennas; and a cylindrical reflector used in a Cantenna.
**Design criteria** Several parameters directly affect the performance of an antenna with an integrated reflector: the reflector’s dimensions, spillover (when part of the feed antenna’s radiation misses the reflector), aperture blockage (when some feed energy bounces back into the feed antenna instead of contributing to the main beam), illumination taper (reduced feed illumination at the reflector’s edges), reflector surface deviation, defocusing, cross-polarization, feed losses, antenna feed mismatch, and non-uniform amplitude or phase distributions. Antenna efficiency is measured by its effectiveness ratio. Any gain-degrading factor that raises side lobes has a double effect: it increases system noise temperature and reduces gain. Aperture blockage and surface deviation are two important cases. Blockage usually comes from shadowing by the feed, subreflector, or support members. Surface deviations cause non-uniform aperture distributions, which lower gain.
The standard symmetrical parabolic Cassegrain reflector system is popular because it minimizes feeder length to the terminal equipment.
- Standalone types
- corner reflector, flat reflector
- Integrated types
- parabolic reflector, passive element (Yagi), flat reflector (Short backfire, Sector), corner reflector (UHF TV), cylindrical reflector (Cantenna)
- Key design parameters
- dimensions, spillover, aperture blockage, illumination taper, surface deviation, defocusing, cross polarization, feed losses, antenna feed mismatch, non-uniform amplitude/phase distributions
- Measurement distance
- four Rayleigh distances minimum
Lore & Background
Standalone reflectors redirect EM energy, generally in the radio wavelength range. A corner reflector returns the incoming signal to its source, commonly used in radar. A flat reflector acts like a mirror and is often used as a passive repeater. Integrated reflectors modify an antenna's radiation pattern. The parabolic reflector focuses a beam into one point or directs a radiating signal into a beam. A passive element slightly longer than and behind a radiating dipole absorbs and re-radiates directionally, as in a Yagi array. Flat reflectors appear in Short backfire and Sector antennas; corner reflectors in UHF television antennas; cylindrical reflectors in Cantennas.
Reader's Guide
The performance of an antenna with an integrated reflector is influenced by several parameters: dimensions, spillover (feed radiation missing the reflector), aperture blockage (feed energy reflected back into the feed), illumination taper (reduced feed at reflector edges), surface deviation, defocusing, cross polarization, feed losses, antenna feed mismatch, and non-uniform amplitude/phase distributions. Antenna efficiency is measured as an effectiveness ratio. Gain-degrading factors that raise side lobes also contribute to system noise temperature. Aperture blockage, often from feed, subreflector, or support members, and surface deviations are important cases. The standard symmetrical parabolic Cassegrain reflector system minimizes feeder length but suffers blockage from the hyperbolic sub-reflector and struts, especially when the main reflector is small. Asymmetric designs like the open Cassegrain avoid blockage but may introduce inferior side-lobe levels, beam squint, or poor cross-polar response. Microwave absorbers are sometimes used to reduce spillover and edge effects. Measurements of gain and sidelobe levels require a distance of at least four Rayleigh distances.
Did You Know?
- A corner reflector reflects an incoming signal back to the direction from which it came.
- Aperture blockage is normally due to shadowing by feed, subreflector, or support members.
- The standard symmetrical parabolic Cassegrain reflector system allows minimum feeder length to terminal equipment.
- A microwave absorber is sometimes employed to avoid spillover from over-illumination of the main reflector surface.
More in Antenna Types, Part 2 1-24
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