Figure 1: Blind zone around a ship’s radar
Depending on the antenna height of a maritime navigation radar or a coastal defense radar, a blind zone appears in close range, where targets cannot be detected. In calm sea conditions, the antenna height h and the distance Dblind form a right triangle so that the blind zone can be calculated with a trigonometric function:
|Dblind = h· cot (θ/ 2)||(1)|
In the case of an antenna using a fan beam pattern with a vertical half-power beam width of θ = 20° … 25°, as formed by most navigation radars, this blind zone increases with increasing antenna height. At the same time, however, the maximum detection range on the sea surface up to the radar horizon also increases (behind which a blind zone also arises), so a compromise has to be found here. Large vessels can have up to several hundreds of meters of a blind zone. Often special short-range radars are installed additionally on ships for close range, which also have an essential function in combating pirate attacks.
Such a blind zone may also be arize by coastal surveillance radars with an elevated installation site (e.g., on a mountain). These mostly military radars often use more expensive antennas with an inverted cosecant squared pattern.