PARABOLIC ANTENNAS

Different types of parabolic antennas:

1.Offset Antenna

2.Prime Focus Antenna

3.Cassegrain Antenna

4.Gregorian Antenna

1.OFFSET ANTENNA: An offset dish antenna is a one type of parabolic antenna.It is the antenna feed is offset to the side of the reflector, In offset antenna, the feed is in front of the dish, on its axis.As in an axial-fed parabolic dish,the feed is located at the focal point of the reflector, but the reflector is an asymmetric segment of a parabolic,so the focus is located to the side.the purpose of this design is avoid  the feed blocking the beam.In an ordinary axial-fed dish antenna, the feed structure and its supports are located in the path of the beam of radio waves, partially, obstructing them.In the offset design, the feed is positioned outside the area of the beam.It is used foe small parabolic antennas or "mini-dishes", such as the Ku band direct-broadcast satellite dishes used for satellite television, where the feed structure is large enough in relation to the dish to block a significant proportion of the signal.this antenna consists of a dish and a feed.The feed is placed above the dish.The feed occupies 1/3 rd place of the dish.

It consists of different parts 

•Reflector
•Feed

•Gear system

2.PRIME FOCUS ANTENNA: The side of the reflector ,in contrast to a typical axial-fed parabolic antenna where the feed is in front of dish,on its axis.This antenna operation like offset antenna but prime focus antenna have fixed set up feed.As in an axial-fed parabolic dish,the feed is located at the focal point of the reflector, but the reflector is an asymmetric segment of a parabolic,so the focus is located to the side.the purpose of this design is avoid  the feed blocking the beam.In an ordinary axial-fed dish antenna, the feed structure and its supports are located in the path of the beam of radio waves, partially, obstructing them.In the offset design, the feed is positioned outside the area of the beam.It is used foe small parabolic antennas or "mini-dishes", such as the Ku band direct-broadcast satellite dishes used for satellite television, where the feed structure is large enough in relation to the dish to block a significant proportion of the signal. 

 

3.CASSEGRAIN ANTENNA:  This antenna consists of dish, feed and concave shaped sub reflector.The concave shaped sub reflector is placed above the dish.The feed is placed in the inside and also in the middle of the dish.When the signals are transmitted from satellite it reflects the surface of the dish and again reflects the concave shaped sub reflector and goes to the feed which is placed inside the dish.One advantage of of the cassegrain design is that the feed antennas and associated waveguides and "front end" electronics can be located behind the dish,rather than suspended in front of the dish at the focus, as in common front-fed parabolic antennas.therefore this design is used for antennas with bulky or complicated feeds, such as satellite communication ground antennas,radio telescopes, and the antennas on some communication satellites.another reason for using the cassegrain design is that modifying the shape of the secondary reflectors offers additional possibilities for shaping the beam pattern of the antenna over what is possible with a simple parabolic antenna.

4.GREGORIAN ANTENNA: This antenna consists of a dish, feed and convex shaped sub reflector.The convex shaped sub reflector is placed above the dish.The feed placed in the inside and also in the middle of the dish the parabolic antenna became a ubiquitous feature of urban, suburban, and even rural,landscapes.Extensive terrestrial microwave links,such as those between cell phone base stations, and wireless WAN/LAN applications have also proliferated this antenna type. Earlier applications included ground-based and airborne radar and radio astronomy.The largest "dish" antenna in the world is the Arecibo Observatory's radio telescopes at Arecibo, Puerto Rico,but, for beam steering reasons,it is actually a spherical, rather than parabolic, reflector.