Earth station antennas must be highly directional. In the transmit mode, they radiate energy in a narrow beam; and in the receive mode, they extract the radiant energy that arrives within the angular boundaries of this beam. The antenna’s directivity is liable for its gain, both transmitting and receiving, and makes it possible for adjacent satellites, separated by only 2 ° (9 ° for DBS satellites) on the orbital arc, to operate on the same frequencies.
Since uplinks and downlinks operate on different frequencies, the same antenna can be used simultaneously for transmission and reception. This requires a waveguide filter in the coupling network to prevent leakage of transmitter power into the receiver.
There are three types of single beam antenna which is below:
a. Prime-focus-feed antenna
b. Dual-reflector antenna
c. Offset-feed antenna
In the most basic single-beam-antenna configuration, the reflector is a section of a parabolic; and the feed horn, located at its focus, illuminates it directly. It’s known as a prime focused antenna.
Figure – Prime-focus-feed antenna
The dual-reflector antenna is an important variation. The feed horn is aimed away from the main reflector; its beam is intercepted by a sub-reflector and reflected back to the main reflector.
Dual-reflector antennas come in a variety of configurations, depending on the shape of the two reflectors. If the sub-reflector is convex toward the main reflector. It is known as a Cassegrain antenna. If it is concave, it is known as a Gregorian. In still another variation known as the Gregux, the sub-reflector consists of a concave ring.
Figure – Dual-reflector antenna
A third variation, the offset-feed antenna, is based on the prime-focus-feed type where the dish is an of-center section of a parabolic. It has the advantage that the feed horn and its support are not in the way of the signal beam. This design is adaptable to low costs and it is widely used for DBS receivers.
Figure – Offset-feed antenna
Antenna designs are the result of a complex series of trade-offs between cost, directivity, and gain. Prime-focus-fed antennas are more inexpensive, suffer less loss of radiant energy by blockage, and, for lower gain antennas, have lower-level sidelobes. Dual-reflector antennas have greater design flexibility and offer a greater choice of trade-offs. The three most common configurations used in satellite systems are the prime-focus-feed, the Cassegrain, and the offset-feed. C-band receive-only stations frequently employ prime-focus feeds, while higher gain Cassegrain antennas are often used for C-band transmit-receive systems and for the Ku band.
If the feed horn is located on the axis of the main reflector, the antenna beam will be directed along this axis, but the beam can be pointed in other directions by offsetting the feed horn from the axis. Offset feeding degrades the performance of antennas somewhat, but it has almost become the standard for DBS receiving antennas. Offset-feeding’s other main application is in multiple-beam antennas.