Reflector (antenna)
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An antenna reflector is a device that reflects electromagnetic waves.
It is often a part of an antenna assembly.
The most common reflector types are
- corner reflector which reflects the incoming signal back to the direction it came from
- parabolic reflector which focuses the signal into one point
- flat reflector which just reflects the signal like a mirror.
[edit] Design criteria
Parameters to be taken into account include the following that directly influence efficiency:
- Spillover
- Aperture blockage
- Illumination taper
- 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 sidelobes have a two-fold effect, in that they contribute to system noise temperature 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 reflector, Cassegrainian system is very popular in practice because it allows minimum feeder length to the terminal equipment. The major disadvantage of this configuration is due to blockage from 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, 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.
