9.5 Antenna with Cosecant Squared Pattern
Antennas with constant height pattern or cosecant squared pattern (abbreviated as CSC² ) are specially designed for air-surveillance radar sets. These permit an adapted distribution of the radiation in the beam and cause a more ideal space scanning. This antenna pattern can get the required elevation coverage where the received power is independent of the radar range for a constant height target. It is a means of achieving a more uniform signal strength at the input of the receiver as a target moves with a constant height within the beam.
There are a couple of variation possibilities, to get a cosecant squared pattern in practice:
- deformation of a parabolic reflector
- a stacked beam by more horns feeding a parabolic reflector
- an antenna group with either different spacing or different feeding of elements
Deviating shape of the parabolic reflector
In practice, a cosecant squared pattern can be achieved by a deformation of a parabolic reflector. A radiator is in the focal point of a parabolic reflector and produces a relatively sharply bundled radiation lobe since the rays leave the reflector parallel in the ideal case. To get the cosecant squared pattern, a part of the radiated energy must be turned up. A possibility consists of a lower bending of the top of the reflector. The part of the rays which falls to the less bent area (in the top) is reflected up now. A possible method analogously for this one is to bend the lower part of the reflector more intensely.
The lobe of the radiator is weaker than the margin to; therefore the margins of the reflector are hit weaker than the center. Due to the fact that the rays turned up don’t have a large power density, the maximum range in the higher elevation is limited.
The ideal pattern as shown in figure 1 is a nearly rectangular shape with rounded corners. In reality, the diagram is superposed with any sidelobes mainly on the back at higher elevation angles. The shape of all these back sidelobes is merged into a nearly parabolic slope. It gives in near range an additional gain required for constant echo strength after the sensitivity time control.
A deviation from the ideal parabolic shape of the reflector curvature
a) convex reflector (“upper lip”)
b) concave reflector (“lower lip”)
Stacked Beam Cosecant Squared Pattern
A cosecant squared pattern is achieved by two or more horns feeding a parabolic reflector. Every feed horn already emits directionally. If one distributes the transmit power unevenly on a single radiating element, then the antenna pattern approaches a cosecant squared pattern. By using several receiving channels, a height allocation is also possible. The targets can be assigned to beams with defined elevation there.
The cosecant squared pattern isn’t restricted to parabolic reflectors. This can be realized also with other kinds of antennas. At an antenna array with Yagi antennas the pattern is achieved by the interference of the direct wave with this at the earth’s surface reflected quotas.