A focal cloud is the collection of

focal point Focal point may refer to: * Focus (optics) * Focus (geometry) * Conjugate points, also called focal points * Focal point (game theory) * Unicom Focal Point UNICOM Focal Point is a portfolio management and decision analysis tool used by the p ...

s of an ''imperfect'' lens or parabolic reflector whether

optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...

electrostatic Electrostatics is a branch of physics that studies electric charges at rest ( static electricity). Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for ...

or electromagnetic. This includes

parabolic antenna A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or par ...

s and lens-type reflective antennas of all kinds. The effect is analogous to the

circle of confusion In optics, a circle of confusion (CoC) is an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source. It is also known as disk of confusion, circle of indistinctness, blur circle, or ...

in photography. In a perfect lens or parabolic reflector, rays parallel to the device's axis striking the lens or reflector all pass through a single point, the

. In an imperfectly constructed lens or reflector, rays passing through different parts of the element do not converge to a single point but have different focal points. The set of these focal points forms a region called the focal cloud. The diameter of the focal cloud determines the maximum resolution of the optical system. Lens-reflector artifacts, geometry and other imperfections determine the actual diameter of the focal cloud.

Satellite dish effects

A focal cloud can render a satellite dish less efficient, resulting in a reduced gain. The imperfections of the antenna lead to two problems: On the one hand the more the radio waves are directed away from the phase center of the

feed horn A feed horn (or feedhorn) is a small horn antenna used to couple a waveguide to e.g. a parabolic dish antenna or offset dish antenna for reception or transmission of microwave. A typical application is the use for satellite television ...

, the more attenuated they get coupled into the

waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...

. On the other hand the more the distance between antenna surface and feed horn varies, the bigger the

phase shift In physics and mathematics, the phase of a periodic function F of some real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is denoted \phi(t) and expressed in such a scale that it ...

gets - the closer the phase shift gets to 180 degrees (half a wave length) between different parts of the antenna surface, the more the signal gets extinguished (attenuated). Feed horn design might be able mitigate some of these losses. For countering the effect, there are several techniques, either in construction of the reflectors or lenses, or in the way signal beams are concentrated. Satellite-based dish antennas may be deformed intentionally to selectively distribute radiated power over a desired "footprint", in order to increase received power in the desired reception area (e.g. one selected nation), and reduce power outside of it (e.g. a neighboring nation). Ground based dish antennas might benefit from minor deformation, producing a footprint encompassing satellite tidal motion.

References

{{reflist Antennas (radio) Mirrors Physical optics Satellite broadcasting