Editing Cassegrain antenna (section)

==Advantages==
This design is an alternative to the most common parabolic antenna design, called "front feed" or "prime focus", in which the [[Antenna feed|feed antenna]] 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:
 
* The feed antennas and associated waveguides and "[[RF front end|front end]]" electronics can be located on or behind the dish, rather than suspended in front where they block part of the outgoing beam.<ref name="Chatterjee">{{cite book
  | last = Chatterjee
  | first = Rajeswari
  | title = Antenna theory and practice
  | edition = 2nd
  | publisher = New Age International
  | year = 2006
  | location = New Delhi
  | pages = 188
  | url = https://books.google.com/books?id=J4YcUA-rxJoC&q=parabolic+antenna+cylindrical&pg=PA188
  | isbn = 978-81-224-0881-2}}</ref><ref name="Welch">{{cite conference
  | first = W.J.
  | last = Welch
  | title = Types of Astronomical Antennas
  | book-title = Methods of Experimental Physics
  | volume= 12, Part B: ''Radio Telescopes''
  | pages = 13–14
  | publisher = Academic Press
  | date = 1976
  | location = New York
  | url = https://books.google.com/books?id=Acyhg3xq73AC&q=parabolic+antenna+ruze&pg=PA64
  | isbn = 0-12-475952-1
  | access-date = 2012-01-14}}</ref>  Therefore, this design is used for antennas with bulky or complicated feeds,<ref name="Chatterjee" /> such as [[satellite communication]] ground antennas, [[radio telescope]]s, and the antennas on some [[communication satellite]]s.
* Another advantage, important in satellite ground antennas and [[radio telescope]]s, is that because the feed antenna is directed forward, rather than backward toward the dish as in a front-fed antenna, the spillover [[sidelobe]]s 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.<ref name="Welch" />  In receiving antennas this reduces reception of [[ground noise]], resulting in a lower antenna [[noise temperature]].
* ''Dual reflector shaping'': 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.<ref>{{cite journal
 |title=Design of dual-reflector antennas with arbitrary phase and amplitude distributions
  |author=Galindo, V.
  |journal=IEEE Transactions on Antennas and Propagation
  |volume=12
  |issue=4
  |pages=403–408
  |year=1964
  |publisher=IEEE
  |doi=10.1109/TAP.1964.1138236 |bibcode=1964ITAP...12..403G
 }}</ref><ref>{{cite journal
 |title=RF Design and Predicted Performance for a Future 34-Meter Shaped Dual-Reflector Antenna System Using the Common Aperture XS Feedhorn
 |author=Willams, WF
 |journal=Telecommunications and Data Acquisition Progress Report
 |volume=73
 |pages=74–84
 |year=1983
 |bibcode=1983TDAPR..73...74W
 |url=http://tmo.jpl.nasa.gov/progress_report/42-73/73H.PDF |archive-url=https://ghostarchive.org/archive/20221009/http://tmo.jpl.nasa.gov/progress_report/42-73/73H.PDF |archive-date=2022-10-09 |url-status=live }}</ref>  Other dish illumination patterns can also be synthesized, such as patterns with high taper at the dish edge for ultra-low spillover [[sidelobe]]s, and patterns with a central "hole" to reduce feed shadowing.
* Another reason for using the Cassegrain design is to increase the [[focal length]] of the antenna, to reduce sidelobes, among other advantages.<ref name="Welch" /><ref>{{cite book
  | last = Cheng
  | first = Jingquan
  | title = The principles of astronomical telescope design
  | publisher = Springer
  | year = 2009
  | location = New York
  | pages = 359–360
  | url = https://books.google.com/books?id=z-mQWTu7zFoC&q=%22parabolic+antenna%22+%22focal+ratio%22&pg=PA358
  | isbn = 978-0-387-88790-6}}</ref>  Parabolic reflectors used in dish antennas have a large curvature and short [[focal length]]; the [[focus (optics)|focal point]] 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 [[focal ratio]] (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.
* The longer focal length also improves crosspolarization discrimination of off-axis feeds,<ref name="Welch" /> important in satellite antennas that use the two orthogonal [[polarization (antenna)|polarization]] modes to transmit separate channels of information.
 
[[Image:Picture bwg.gif|right|thumb|A beam waveguide antenna, a type of Cassegrain design, showing the complicated signal path.]]

A disadvantage of the Cassegrain is that the feed horn(s) must have a narrower [[beamwidth]] (higher [[antenna gain|gain]]) 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.