Geometry of Parabolic Reflectors Abdus Salam ICTP, February...
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Pietrosemoli, parab.-1 Geometry of Parabolic Reflectors Abdus Salam ICTP, February 2005 School on Digital Radio Communications for Research and Training in Developing Countries Ermanno Pietrosemoli Escuela Latinoamericana de Redes (Latin American Networking School) www.eslared.org.ve
Transcript of Geometry of Parabolic Reflectors Abdus Salam ICTP, February...
Pietrosemoli, parab. - 1
Geometry of Parabolic ReflectorsAbdus Salam ICTP, February 2005
School on Digital Radio
Communications for Research and Training in
Developing CountriesErmanno Pietrosemoli
Escuela Latinoamericana de Redes(Latin American Networking School)
www.eslared.org.ve
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Why parabolic reflectors?• Can provide high gain• Easily available as
surplus in most countries
• Can be added to any other antenna to increase the gain
• A complete transceiver can be located at the focus for a very cost effective long distancecommunication system
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Very good source for antenna information: www.w1GHz.org
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Geometry of parabolic reflectors
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The lower the f/D ratio, the lower the sidelobes, because the feed is more protected from stray raysSidelobes can also be reduced by means of additionalShielding on the rim of the parabolic reflector (skirt)
D
f
f/D=0.4 f/D=0.35 f/D=0.25
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Parabolic Dish
Truncated Parabolic Dish Parabolic
Cylinder
Examples of dishes
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Primary Source
Parabolic ReflectorTotal RadiationPatter
FeedRadiation Pattern
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Types of parabolic feeds
• Center Feed• Offset Feed• Cassegrain Feed
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Dish Illumination• The illumination of the dish• is crucial to the performance• We should illuminate as much• as the reflecting surface as• possible as well as avoiding spillover• The best results are obtained with a tapered
illuminator that will progressively diminish the illumination of the edges thus reducing sidelobes and spillover
• Another critical parameter is the position of the feed with respect to the reflector
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Radiation pattern with sidelobes
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Elevation Angle
Clinometer
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W1GHZ methods for determining elevation angle
• Look at the reflector from a distance, and have someone slowly change the elevation, when the reflector appears circular measure the angle.
• Elevation angle is arcsin(minor a./major a.)
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Paul Wade,W1GHZ
Location of thefocus of an offsetParabolic reflector
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W1GHZ method for determining focus
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Some commercial dishes dimensions
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What snow does to antennas..
