This from the FAA in 1980
There is a newer version of this that i cannot find just now!!
The following information can be used in establishing a minimum safe distance from the antenna for personnel near an operating airborne weather radar. An applicable graph is shown in figure 1.
1. NEAR FIELD/FAR FIELD INTERSECTION. The distance to the near field/far field intersection can be computed by:
Ri = Gl/8p
where Ri - Intersection distance from the antenna (in meters)
l - Wave length (in meters)
G - Antenna gain
2. DISTANCE TO 10 mw/cm" SAFE LIMIT. For a far field power density of10 mw/cm", the distance (in meters) from the antenna may be calculated by:
Rs = Sq Rt GP/400p
where Rs - The minimum safe distance in meters.
P - Transmitted average power in watts.
G - Antenna gain
3. PROCEDURES. The above formulas or the graphs of figures 1 and figure 2. may be used to determine the minimum safe distance. In either case the following procedures apply:
a. Determine the distance (Ri) to the near field/far field intersection (paragraph 1).
b. Determine the distance (Rs) to 10 mw/cm" power density (paragraph 2).
c. If the distance (Rs) determined in 3b above is less than (Ri) found in 3a above, use distance (Ri) as the minimum safe distance.
d. If the distance (Rs) determined in 3b above is greater than (Ri) found in 3a above, use distance (Rs) as the minimum safe distance.
4. EXAMPLE
a. Data. The following is typical data for an airborne weather radar.
Antenna Diameter : 22 inches = 56 cm
Transmitter Frequency : 9375 : 30 MHz
Wave Length : 3.2 cm
Pulse Length : 1.5 microseconds (search)
Pulse Repetition : 400 Hz
Peak Power : 40 kilowatts
Average Power : 24 watts (search)
Antenna Gain : 1000 (30db)
b. Calculations.
(1) Distance (Ri) to the near field/far field intersection.
Ri = Gl/8p
= 1000 x 0.032/8p
= 1.27 meters = 4.2 feet
(2) Distance (Rs) to 10 mw/cm" safe limit.
Rs = Sq Root GP/400p
= Sq Root 1000 x 24/400p
= 4.37 meters = 14.3 feet
The distance (Rs) is greater than (Ri), therefore, the minimum safe distance is 14.3 feet.
If you have any questions on this PLEASE DON'T ASK ME!!!!!!!!!!!