Recommended Radiation Safety Precautions for Ground Operation of Airborne Weather Radar

AC 20-68B · FAA

Public domain · FAAAdvisory Circulars

Overview

The Recommended Radiation Safety Precautions for Ground Operation of Airborne Weather Radar (AC 20-68B) is a public-domain FAA advisory circular, republished here as a free chaptered HTML edition with a linked table of contents and the official PDF.

Publisher: FAA
Document: AC 20-68B
Pages: 6
Chapters: 2

Key points

This advisory circular provides recommended radiation safety precautions for operating airborne weather radar on the ground.
Personnel should never stand nearby and in front of a transmitting radar antenna to prevent potential body damage.
Airborne weather radar should not be operated in a hangar unless the radar transmitter is off or directed toward an absorption shield.
A minimum safe distance from the radar antenna should be established based on specific calculations to limit exposure to 10 milliwatts per square centimeter.
Airborne weather radar should not be operated while an aircraft is being refueled or defueled to prevent ignition of combustible materials.

Frequently asked questions

What is the purpose of AC 20-68B?

The purpose of AC 20-68B is to set forth recommended radiation safety precautions for personnel operating airborne weather radar on the ground.

What precautions should be taken when operating airborne weather radar?

Precautions include ensuring that only qualified personnel operate the radar, maintaining a safe distance from the antenna, and avoiding standing in front of a transmitting antenna.

How is the minimum safe distance from the radar antenna determined?

The minimum safe distance can be calculated using specific equations provided in the document or by referring to the applicable graphs.

What should personnel avoid doing near a radar antenna?

Personnel should avoid standing nearby and in front of a transmitting radar antenna, looking into a waveguide, and being near the open end of a coaxial connector when the radar is on.

Why should airborne weather radar not be operated during refueling?

Operating airborne weather radar while an aircraft is being refueled or defueled can lead to the ignition of combustible materials.

Appendix 1

8/8/80 AC 20-68B Appendix 1 APPENDIX 1. SAFE DISTANCE DETERMINATION The following information can be used in establishing a minimum safe distance from the antenna for personnel near an operating airborne weather radar. l,,zJ.

applicable g;aph is shown in figure 1.

1. NEAR FIELD/FAR FIELD INTERSECTION. The distance to the near field/far field intersection can be computed by: where R ~ Intersection distance from the antenna (in meters) ). ·"' Wave length (in meters) G • Antenna gain 2. DISTANCE TO 10 mw/cm SAFE LIMIT. For a far field power density of 10 mw/cm , the distance (in meters) from the antenna may be calculated by: Rs == / GP/400 n (2) where Rs~ The minimum safe distance in meters.

P = Transmitted average power in watts.

G = Antenna gain An applicable graph is sho~~ in figure 2.

3. PiWCE::-~·F2:s-. The abovE: formulas or the graphs of figures l and 2 ma y bE used to determine the minimum safe distance. In either case the following procedures apply: a. Determine the distance (R ) to the near field/far field interse c tion {paragraph 1).

b. Determine the distan~e (R ) to 10 mw/cm power density ( paragraph 2).

c. If the distance (R ) determined in 3b above is less than (R ) found in 3 a above, use distance (Ri) as the minimum safe distance.

d. If the distance (R ) determined in 3b above is greater than (R ) founc ir, 3.;. above., use diztance (R ) as the minimum safe distance.

Next: Appendix 1 →

Appendix 1

AC 20-68B 8/8/80 Appendix 1 4. EXAMPLE a. Data. The folloYing 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 (R;) to the near field/far field intersection.

Ri • G >.

• 1000 X 0.032 Sr.

• 1.27 meters• 4.2 feet . .

(2) Distance (Rs) to 10 mw/cm safe limit.

Rs .. l cp /400 11

"' llooo x 24/400"

4.37 meters = 14.3 feet The distance (Rs) is greater than (Ri), therefore, the minimum safe distance is 14.3 feet.

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