Focal cloud: Difference between revisions
Adding short description: "Aspect of a lens or reflector" |
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{{Short description|Aspect of a lens or reflector}} |
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''Focal cloud'' is a general definition referring to [[focal point]] of ''imperfect'' [[lens]],<br /> |
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{{unreferenced|date=May 2015}} |
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whether they are [[Lens (optics)|optic lenses]], [[Electrostatic lens]]es or electromagnetic lenses such as [[Parabolic antenna]]s or lens-type reflective antennas of all kinds. |
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[[Image:Focal cloud.jpg | right|thumb | Focal cloud of a reflector (red) compared to an ideal focal point (blue)]] |
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{{Antennas|sources/regions}} |
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A '''focal cloud''' is the collection of [[Focus (optics)|focal point]]s of an ''imperfect'' lens or parabolic reflector whether [[lens (optics)|optical]], [[Electrostatic lens|electrostatic]] or electromagnetic. This includes [[parabolic antenna]]s and lens-type reflective antennas of all kinds. The effect is analogous to the [[circle of confusion]] in photography. |
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<br /> |
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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 [[focus (optics)|focal point]]. 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 [[optical resolution|resolution]] of the optical system. Lens-reflector artifacts, geometry and other imperfections determine the actual diameter of the focal cloud. |
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[[Image:Focal cloud.jpg]] |
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<br /> |
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In this focal cloud illustration, a perfect reflector would focus all beams into blue spot. |
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But due to imperfections of lens construction, the actual signal beams follow the red path to several different focal points, which form the focal cloud. |
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The better the lens, the less wide is the focal cloud. |
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Lens - reflector artifacts, geometry and type of imperfections form the actual radius of the focal cloud. |
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== Satellite dish effects == |
== Satellite dish effects == |
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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]], the more attenuated they get coupled into the [[waveguide]]. On the other hand, the more the distance between antenna surface and feed horn varies, the bigger the [[phase shift]] gets - the closer the phase shift gets to 180 degrees (half a wavelength) 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.{{citation needed|date=January 2021}} |
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Apart for satellite dish performing worse than expected due to the effect, focal cloud can even render a satellite dish rather inefficient in relation to its size in specific frequencies if the imperfections are formed in a way that the [[phase shift]] of reflected waves at a large portion of the dish is near 180 degrees between them near the center of the cloud,where lies the [[feed horn]] thus resulting to self-cancellation of the specific frequency. |
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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). <!-- Why is tidal action relevant with regards to "Focal Cloud"? -- Tidal action tends to move a satellite in a figure-8 pattern. Ground-based antennas should be aligned when the satellite is at the center of this pattern, or when the "bird is in the box" (slang used by satellite operators). --> |
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Ground based dish antennas might benefit from minor deformation, producing a footprint encompassing satellite tidal motion.{{citation needed|date=January 2021}} |
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[http://www.mlesat.com/Article1.html |Satellite LNB's & polarizers] |
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{{reflist}} |
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[[Category: |
[[Category:Mirrors]] |
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[[Category:Physical optics]] |
[[Category:Physical optics]] |
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[[Category:Satellite broadcasting]] |
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[[Category:Television technology]] |
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[[Category:Consumer electronics]] |
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[[Category:Satellites]] |
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[[Category:Radio frequency antenna types]] |
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[[Category:Satellite television]] |
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Latest revision as of 11:32, 16 January 2024
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A focal cloud is the collection of focal points of an imperfect lens or parabolic reflector whether optical, electrostatic or electromagnetic. This includes parabolic antennas and lens-type reflective antennas of all kinds. The effect is analogous to the circle of confusion 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 focal point. 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
[edit]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, the more attenuated they get coupled into the waveguide. On the other hand, the more the distance between antenna surface and feed horn varies, the bigger the phase shift gets - the closer the phase shift gets to 180 degrees (half a wavelength) 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.[citation needed]
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.[citation needed]