160:: The presence of a second reflecting surface in the signal path allows additional opportunities for tailoring the radiation pattern for maximum performance. For example, the gain of ordinary parabolic antennas is reduced because the radiation of the feed antenna falls off toward the outer parts of the dish, resulting in lower "illumination" of those parts. In "dual reflector shaping" the shape of the secondary reflector is altered to direct more signal power to outer areas of the dish, resulting in more uniform illumination of the primary, to maximize the gain. However, this results in a secondary that is no longer precisely hyperbolic (though it is still very close), so the constant phase property is lost. This phase error, however, can be compensated for by slightly tweaking the shape of the primary mirror. The result is a higher gain, or gain/spillover ratio, at the cost of surfaces that are trickier to fabricate and test. Other dish illumination patterns can also be synthesized, such as patterns with high taper at the dish edge for ultra-low spillover
183:(f-number, the ratio of the focal length to the dish diameter) of typical parabolic antennas is 0.25–0.8, compared to 3–8 for parabolic mirrors used in optical systems such as telescopes. In a front-fed antenna, a "flatter" parabolic dish with a long focal length would require an impractically elaborate support structure to hold the feed rigid with respect to the dish. However, the drawback of this small focal ratio is that the antenna is sensitive to small deviations from the focal point: the angular width that it can effectively focus is small. Modern parabolic antennas in radio telescopes and communications satellites often use arrays of feedhorns clustered around the focal point, to create a particular beam pattern. These require the good off-axis focusing characteristics of a large focal ratio, and because the convex secondary reflector of the Cassegrain antenna increases it significantly, these antennas typically use a Cassegrain design.
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210:) to focus its radiation on the smaller secondary reflector, instead of the wider primary reflector as in front-fed dishes. The angular width the secondary reflector subtends at the feed horn is typically 10–15°, as opposed to 120–180° the main reflector subtends in a front-fed dish. Therefore, the feed horn must be longer for a given wavelength.
222:
is a type of complicated
Cassegrain antenna with a long radio wave path to allow the feed electronics to be located at ground level. It is used in very large steerable radio telescopes and satellite ground antennas, where the feed electronics are too complicated and bulky, or requires too much
223:
maintenance and alterations, to locate on the dish; for example those using cryogenically cooled amplifiers. The beam of incoming radio waves from the secondary reflector is reflected by additional mirrors in a long twisting path through the axes of the
55:
secondary reflector suspended in front of the primary reflector. The beam of radio waves from the feed illuminates the secondary reflector, which reflects it back to the main reflector dish, which reflects it forward again to form the desired
114:
itself is mounted suspended in front of the dish at the focus, pointed back toward the dish. The
Cassegrain is a more complex design, but in certain applications it has advantages over front feed that can justify its increased complexity:
247:. The first Cassegrain antenna was invented and patented by Cochrane and Whitehead at Elliot Bros in Borehamwood, England, in 1952. The patent, British Patent Number 700868, was subsequently challenged in court, but prevailed. The
96:
of the hyperboloid, while the focus of the primary reflector coincides with the near focus of the hyperboloid. Usually the secondary reflector and the feed antenna are located on the central axis of the dish. However, in
123:" electronics can be located on or behind the dish, rather than suspended in front where they block part of the outgoing beam. Therefore, this design is used for antennas with bulky or complicated feeds, such as
101:
configurations, the primary dish reflector is asymmetric, and its focus, and the secondary reflector, are located to one side of the dish, so that the secondary reflector does not partially obstruct the beam.
146:
caused by portions of the beam that miss the secondary reflector are directed upwards toward the cold sky rather than downwards towards the warm earth. In receiving antennas this reduces reception of
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The longer focal length also improves crosspolarization discrimination of off-axis feeds, important in satellite antennas that use the two orthogonal
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is located near the mouth of the dish, to reduce the length of the supports required to hold the feed structure or secondary reflector. The
227:, so the antenna can be steered without interrupting the beam, and then down through the antenna tower to a feed building at ground level.
171:
of the antenna, to reduce sidelobes, among other advantages. Parabolic reflectors used in dish antennas have a large curvature and short
251:
launched in 1977 is, as of
September 2024, 24.6 billion kilometers from Earth, the furthest manmade object in space, and it's 3.7 meter
484:"RF Design and Predicted Performance for a Future 34-Meter Shaped Dual-Reflector Antenna System Using the Common Aperture XS Feedhorn"
142:, is that because the feed antenna is directed forward, rather than backward toward the dish as in a front-fed antenna, the spillover
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Cassegrain satellite communication antenna in Sweden. The convex secondary reflector can be seen suspended above the dish, and the
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This design is an alternative to the most common parabolic antenna design, called "front feed" or "prime focus", in which the
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Closeup of the convex secondary reflector in a large satellite communications antenna in
Pleumeur-Bodou, France
60:. The Cassegrain design is widely used in parabolic antennas, particularly in large antennas such as those in
694:
447:
Galindo, V. (1964). "Design of dual-reflector antennas with arbitrary phase and amplitude distributions".
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A beam waveguide antenna, a type of
Cassegrain design, showing the complicated signal path.
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Moving
Targets Elliott-Automation and the Dawn of the Computer Age in Britain, 1947 – 67
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312:. The advantage of the Cassegrain design is that the heavy complicated feed structure
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88:. The geometrical condition for radiating a collimated, plane wave beam is that the
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A disadvantage of the
Cassegrain is that the feed horn(s) must have a narrower
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Cassegrain spacecraft communication antenna in
Goldstone, California, part of
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developed around 1672 and attributed to French
Province England priest
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Another reason for using the
Cassegrain design is to increase the
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554:(1 ed.). London: Springer Verlag London Ltd. p. 376.
380:(2nd ed.). New Delhi: New Age International. p. 188.
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164:, and patterns with a central "hole" to reduce feed shadowing.
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Another advantage, important in satellite ground antennas and
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Type of parabolic antenna with a convex secondary reflector
84:, while the shape of the convex secondary reflector is a
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is mounted at or behind the surface of the concave main
491:
Telecommunications and Data Acquisition Progress Report
263:is still able to communicate with ground stations.
235:The Cassegrain antenna design was adapted from the
190:
modes to transmit separate channels of information.
281:is visible projecting from the center of the dish.
119:The feed antennas and associated waveguides and "
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526:The principles of astronomical telescope design
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449:IEEE Transactions on Antennas and Propagation
420:. New York: Academic Press. pp. 13–14.
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316:doesn't have to be suspended over the dish.
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529:. New York: Springer. pp. 359–360.
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18:
576:"How Far Away is Voyager 1 from Earth?"
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599:Cassegrain subreflector design article
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13:
14:
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1055:Circularly disposed antenna array
873:Folded inverted conformal antenna
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512:from the original on 2022-10-09.
410:"Types of Astronomical Antennas"
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550:Lavington, Simon (2011-05-19).
414:Methods of Experimental Physics
150:, resulting in a lower antenna
51:dish and is aimed at a smaller
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374:Chatterjee, Rajeswari (2006).
1:
1081:Radio frequency antenna types
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105:
695:Dielectric resonator antenna
7:
377:Antenna theory and practice
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131:, and the antennas on some
80:The primary reflector is a
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10:
1102:
328:Cassegrain antenna on the
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23:Types of parabolic antenna
1006:
958:Regenerative loop antenna
808:
660:
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62:satellite ground stations
953:Reflective array antenna
863:Corner reflector antenna
523:Cheng, Jingquan (2009).
469:10.1109/TAP.1964.1138236
416:. Vol. 12, Part B:
133:communication satellites
70:communication satellites
853:Collinear antenna array
125:satellite communication
1035:Reconfigurable antenna
998:Yagi–Uda antenna
973:Short backfire antenna
710:Folded unipole antenna
220:beam waveguide antenna
214:Beam waveguide antenna
199:
158:Dual reflector shaping
92:is located at the far
24:
690:Crossed field antenna
197:
22:
1007:Application-specific
898:Log-periodic antenna
770:Rubber ducky antenna
745:Inverted vee antenna
720:Ground-plane antenna
482:Willams, WF (1983).
455:(4). IEEE: 403–408.
408:Welch, W.J. (1976).
345:Cassegrain reflector
249:Voyager 1 spacecraft
241:reflecting telescope
237:Cassegrain telescope
918:Offset dish antenna
765:Random wire antenna
503:1983TDAPR..73...74W
461:1964ITAP...12..403G
259:Cassegrain antenna
49:parabolic reflector
1060:Television antenna
908:Microstrip antenna
848:Choke ring antenna
843:Cassegrain antenna
740:Inverted-F antenna
652:Isotropic radiator
330:Voyager spacecraft
310:Deep Space Network
245:Laurent Cassegrain
200:
37:Cassegrain antenna
29:telecommunications
25:
1068:
1067:
1045:Reference antenna
938:Parabolic antenna
858:Conformal antenna
780:Turnstile antenna
675:Biconical antenna
561:978-1-84882-933-6
536:978-0-387-88790-6
387:978-81-224-0881-2
350:Nasmyth telescope
152:noise temperature
127:ground antennas,
99:offset Cassegrain
41:parabolic antenna
1093:
1086:Antennas (radio)
1019:Corner reflector
833:Beverage antenna
795:Umbrella antenna
760:Monopole antenna
715:Franklin antenna
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418:Radio Telescopes
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225:altazimuth mount
140:radio telescopes
129:radio telescopes
66:radio telescopes
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1025:Evolved antenna
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988:Vivaldi antenna
963:Rhombic antenna
888:Helical antenna
878:Fractal antenna
823:AS-2259 Antenna
804:
735:Helical antenna
705:Discone antenna
685:Coaxial antenna
670:Batwing antenna
662:Omnidirectional
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978:Slot antenna
948:Quad antenna
933:Planar array
928:Phased array
903:Loop antenna
893:Horn antenna
842:
800:Whip antenna
785:T2FD antenna
730:Halo antenna
725:G5RV antenna
579:. Retrieved
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431:. Retrieved
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260:
239:, a type of
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217:
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188:polarization
173:focal length
169:focal length
157:
148:ground noise
112:feed antenna
109:
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90:feed antenna
79:
45:feed antenna
36:
26:
828:AWX antenna
810:Directional
680:Cage aerial
181:focal ratio
177:focal point
86:hyperboloid
1075:Categories
581:2024-09-13
433:2012-01-14
356:References
106:Advantages
82:paraboloid
1021:(passive)
883:Gizmotchy
790:T-antenna
644:Isotropic
497:: 74–84.
279:feed horn
204:beamwidth
162:sidelobes
144:sidelobes
121:front end
1040:Rectenna
838:Cantenna
507:Archived
339:See also
314:(bottom)
206:(higher
76:Geometry
635:Antenna
499:Bibcode
457:Bibcode
231:History
1014:ALLISS
558:
533:
424:
384:
307:NASA's
257:X-band
175:; the
68:, and
53:convex
993:WokFi
637:types
510:(PDF)
487:(PDF)
94:focus
39:is a
33:radar
556:ISBN
531:ISBN
422:ISBN
382:ISBN
255:and
208:gain
58:beam
35:, a
31:and
465:doi
27:In
1077::
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495:73
493:.
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453:12
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396:^
364:^
218:A
72:.
64:,
627:e
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253:S
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135:.
Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
