**Subject :**Radar and sonar engineering

**Unit :**Introduction to RADAR

## Cosecant-squared Antenna

**Introduction: **

Search radar with an antenna pattern proportional to cosec^{2} Θ, where Θ is the elevation angle, produces a constant echo-signal power for target flying at constant altitude. Fan-beam search radars generally employ this type of pattern. A cosecant squared antenna is one forming a cosecant squared beam in elevation. This beam can be generated with an array or shaped reflector antenna.

Figure 9.23 Cosecant-squared antenna produced by displacing the reflector surface from the original parabolic shape. |

**Design of Antenna**:

· A common method of producing the cosecant-squared pattern is shown in Fig. 9.23.

· The upper half of the reflector is a parabola and reflects energy from the feed in a direction parallel to the axis, as in any other parabolic antenna.

· The lower half, however, is distorted from the parabolic contour so as to direct a portion of the energy in the upward direction.

· A spoiler plate is sometimes used to give a cosecant squared pattern.

· A cosecant-squared antenna pattern can also be produced by feeding the parabolic reflector with two or more horns or with a linear array.

· If the horns are spaced and fed properly the combination of the secondary beams will give a smooth cosecant-squared pattern over some range of angle.