An antenna reflector is a device that reflects electromagnetic waves. The reflection of sunlight on water Reflection is the abrupt change in direction of a wave front at an interface between two dissimilar media so that the wave front returns into the medium from which it originated. ...
Electromagnetic radiation is a propagating wave in space with electric and magnetic components. ...
It is often a part of an antenna assembly. A Yagi-Uda antenna An antenna or aerial is an electronic component designed to transmit or receive radio waves. ...
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.
Design criteria for reflector type antenna Buoy in San Diego Harbor. ...
A parabolic reflector (also known as a parabolic dish or a parabolic mirror) is a reflective device formed in the shape of a paraboloid of revolution. ...
Parameters to be taken into account include the following which have direct influence on 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 antennas 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 effects 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 edge absorber in addition to the circumference of the main paraboloid.