Section 6.1 · Problems · 9 of 11
Problem 6.1.9: Unsafe zone around a radar antenna
Unsafe zone around a radar antenna
#A radar antenna radiates a -GHz electromagnetic wave with the amplitude of the electric field intensity approximately given by the following function of the distance from the radar:
What is the radius of the unsafe zone around the antenna, according to the IEEE (the standard of Problem 6.1.8)?
Source: B. M. Notaroš, Electromagnetics, Problem 9.17.
At distances of metres the spherical wave of the antenna () is locally a plane wave with , so the limit on is a limit on . The unsafe zone ends where the field meets that limit.
At rules (ii) and (iii) of Problem 6.1.8 both give , so .
The zone is unsafe where :
. With rms values, against , the same .
Inside a sphere of radius the time-average exposure exceeds the limit. Since , the radius grows as the square root of the transmitted power: four times the power doubles it. Real radars rotate and pulse, which lowers the time-average exposure (Exercise 6.1.14), and fences are set with the peak beam direction in mind. Antenna sites are fenced, the masts of base stations carry warning signs at the height of the main beam, and calculations like this one are part of the site licence.