Inverse Cosecant Square Antenna Pattern
Figure 1: Coastal surveillance radar antenna
© 2005 EASAT
Figure 2: Coverage of an inverse cosecant squared diagram (upper) vs. the simple “Fan-beam” (lower diagram)
ASDE and VTS radar systems use antennas designed to provide inverse Cosec² coverage and direct energy preferentially towards the surface giving constant gain for targets on the surface.
The coverage diagram shows the antenna pattern of a vessel radar with an inverse cosecant squared antenna patter in the upper diagram. The antenna is designed to preferentially radiate below 0° (the horizon line) to provide constant detection for targets approaching on sea surface. The simple Fan-beam in the lower diagram using the same power leads to losses in the near and far region. A big amount of power is radiated in unwanted directions (e.g. into the air-space).