Figure 1: Vertical projection of the radiation pattern of an antenna with cosecant squared characteristic. These permit an adapted distribution of the radiation in the beam and causing 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. In the 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.
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A cosecant squared antenna , sometimes known as a constant height pattern , is a modified form of parabolic reflector used in some radar systems. It is shaped to send more radio energy in certain directions in order to smooth out the reception pattern of objects as their range changes in relation to the radar.
The name refers to the fact that the amount of energy returned from a target drops off with the square of the cosecant of the angle between the radar and the target. The concept originated as part of the development of the H2S radar , which scanned the area under an aircraft to provide a radar map of the ground below. The ground directly below the aircraft is at a distance equal to the aircraft's altitude, which produces the strongest signal.
The terrain at further distances returns much less signal due to the radar equation. The slant range distance between the radar and the terrain is the cosecant of the angle between the fuselage and the target, and the energy falls off with the fourth root of that number.
Without correction, this produced a display where the ground under the aircraft was very bright on the cathode ray tube display, while the terrain at longer distances was almost invisible. To counteract this, the scanning antenna was re-aimed so that it was pointed almost directly forward, thereby sending most of the radar energy at low angles relative to the aircraft, thereby increasing the energy available at long range. This left the area directly under the aircraft receiving no energy at all, so the upper lip of the reflector was bent to reflect a small amount of energy in that direction.
This results in a more even display pattern. The same basic concept soon found many roles. For ground-based radars, the same modification could be used to provide scanning at high angles above the station while still sending most of the energy towards low angles to detect aircraft at long range as they rose above the radar horizon.
Cosecant squared antenna