# AC 20-68B: Recommended Radiation Safety Precautions for Ground Operation of Airborne Weather Radar - Type: Advisory circular (guidance, not mandatory unless a rule or AD requires it) - Status: Current - Approved: August 8, 1980 - Last reviewed: June 21, 2021 - Family: AC 20-68 - Cancels: AC 20-68A - 14 CFR parts: 1 - Pages: 6 - PDF: https://openairdocs.org/files/sha256/6a/9f/6a9fc67b593d4f2f31eebe6d5958c14203dab85cac90baf9abcb87ffd0e429ac - FAA DRS: https://drs.faa.gov/browse/excelExternalWindow/BF2F85702DC53ED9862569AE006D3437.0001 - DRS record last changed: November 21, 2025 - Page: https://openairdocs.org/ac/20-68 ## Versions - [AC 20-68B](https://openairdocs.org/ac/20-68) (Current), approved Aug 8, 1980, 6 pages ## Text Each page's text from the PDF; open the PDF at a page with #page=N. This is OCR from the scanned FAA advisory circular. OCR can misread letters and numbers. ### Page 1 (OCR) **AC 20-68B DATE 8/8/80** **ADVISORY CIRCULAR** DEPARTMENT OF TRANSPORTATION Federal Aviation Administration Washington, D.C. RECOMMENDED RADIATION SAFETY PRECAUTIONS FOR GROUND OPERATION Subject: OF AIRBORNE WEATHER RADAR 1. PURPOSE. This circular sets forth recommended radiation safety precautions to be taken by personnel when operating airborne weather radar on the ground. 2. CANCELLATION. AC 20-68A, dated Apr11 11, 1975, 18 canceled. 3. RELATED READING MATERIAL• a. Barnes and Taylor, Radiation Hazards and Protection (london: George Newnes Limited, 1963), p. 211. b. U.S. Department of Health, Education and Welfare, Public Health Service, Consumer Protection and Environmental Health Service, "Environmental health microwaves, ultraviolet radiation and radiation from lasers and television receivers - An Annotated Bibliography, FS 2.300: RH-35, Washington, U.S. Government Printing Office, Pp. 56-57. c. Mumford, W. W., "Some technical aspects of microwave radiation hazards," Proceedings of the IRE, Washington, U.S. Government Printing Office, February 1961, Pp. 427-447. 4. BACKGROUND. Dangers from ground operation of airborne weather radar include the possibility of human body damage and ignition of combustible materials by radiated energy. Low tolerance parts of the body include the eyes and testes. 5. PRECAUTIONS. Management and supervisory personnel should establish procedures for advising personnel of dangers from operating airborne weather radars on the ground. Precautionary signs should be displayed in affected areas to alert personnel of ground testing. a. General. (1) Airborne weather radar should be operated on the ground only by qualified personnel. Initiated by: AF0-512 ### Page 2 (OCR) (2) Installed airborne radar should not be operated while the aircraft is in a hangar or other enclosure unless the radar transmitter is not operating, or the energy is directed toward an absorption shield which dissipates the radio frequency energy. Otherwise, radiation within the enclosure can be reflected throughout the area. b. Body Damage. Io prevent possible human body damage, the following precautions should be taken: (1) Personnel should never stand nearby and in front of a radar antenna which is transmitting. When the antenna is not scanning, the danger increases. (2) A recommended safe distance fron operating airborne weather radars should be established. A safe distance can be determined by using the equations in Appendix 1 or the graphs of figures 1 and 2. This criterion is now accepted by many industrial organizations and is based on limiting exposure of humans to an average power density not greater than 10 milliwatts per square centimeter. (3) Personnel should be advised to avoid the end of an open waveguide unless the radar is turned off. (4) Personnel should be advised to avoid looking into a waveguide, or into the open end of a coaxial connector or line connector to a radar transmitter output, as severe eye damage may result. (5) Personnel should be advised that when high power radar transmitters are operated out of their protective cases, X-rays may be emitted. Stray X-rays may emanate from the glass envelope type pulser, oscillator, clipper, or rectifier tubes, as well as magnetrons. c. Combustible Materials. To prevent possible fuel ignition, an installed airborne weather radar should not be operated while an aircraft is being refueled or defueled. M. menang •. BEARD Director of Airworthiness Рат 5 ### Page 3 (OCR) 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. An applicable graph is shown in figure 1. NEAR FIELD/FAR FIELD INTERSECTION The distance to the near field/far fleld intersection can be computed by: R1 = GJ (1) 8 т where Ri = Intersection distance from the antenna (in meters) › = Wave length (in meters) G = Antenna gain DISTANCE IO 10 mw/cm2 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 T (2) where Rs * The minimum safe distance in meters. = Transmitted average power in watts. G = Antenna gain An applicable graph is shown in figure 2. 3. PROCEDURES. The above formules or the graphs of figures 1 and 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 (Rg) to 10 mw/cm2 power density (paragraph 2). If the distance (Rg) determined in 3b above 1s less than (R]) found in 3 a above, use distance (R¡) as the minimum safe distance. d. If the distance (Rs) determined in 3b above 1s greater than (R1) found in 3a above, use distance (Rg) as the minimum safe distance. ### Page 4 (OCR) Appendix 1 EXAMPLE ₴ • 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 (R;) to the near fleld/far field intersection. Ri = GA 8 T * 1000 x 0.032 * 1.27 meters = 4.2 feet (2) Distance (Rs) to 10 mw/cu safe limit. Rs = Y GP /400 m = ·1000 × 24/400 г = 4.37 meters = 14.3 feet The distance (Rs) is greater than (Ri), therefore, the minimum safe distance 1s 14.3 feet. ### Page 5 (OCR) Page 3 Appendix 1 AC 20-68 B 16 - 70 00 Go 65 do • 25 db 30 db 0 - 40 0b FREQUENCY IN MEGA HERTZ 8/8/80 Distance to Intersection of Near and Far Fields FIGURE 1 ### Page 6 (OCR) 8/8/80 AVERAGE POWER IN WATTS Appendix 1 AC 20-688 Page 4 Distance on Antenna Beam Axis for Power Density - 10mw/cm2 FIGURE 2 --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.