ARRL Bulletin - RF Standards, Part 1
ARRL Bulletin - RF Standards, Part 2
National Council on Radiation Protection and Measurements (NCRP)
RF Safety Calculator
FCC Part 97, Rules and Regulations for Amateur Radio Operators were modified in August
1996 to add RF safety regulations to the Amateur Radio Service. Radio Amateurs are now
required to be knowledgable of the new regulations and under certain conditions may have
to change their station configurations or operating practices. As nore information becomes
available, I will include it here or will provide links to other web sites containing
from W5YI Report, December 15, 1996, Fred Maia, W5YI, Editor
According to Part 97.13(c) routine RF radiation evaluation is requred if the transmitter power of the station exceeds 50 watts peak envelope power (PEP). Otherwise the operation is categorically excluded from routine RF radiation evaluation.
Where routine evaluation of an Amateur station indicates that the RF radiation could be in excess of the FCC-specified limits, the licensee must take action to prevent such an occurrence. Such actions could be in the form of altering operating patterns, relocating antennas, revising a station's technical papameters ...such as frequency, power or emission type or combinations of these and other remedies.
In complying with the Commission's Report and Order, the FCC said Amateur operators should follow a policy of prudent avoidance of excessive RF exposure. The FCC said that it "...will continue to rely upon Amateur operators, in constructing and operating their stations, to take steps to ensure that their stations comply with the MPE (Maximum Permissable Exposure) limits for both Amateur operator (controlled) and general public (uncontrolled) situations."
In that regard, Amateur radio operators and members of their immediate household are considered to be in a "controlled environment" and are subject to the occupational/controlled MPE limits. Neighbors who are not members of an Amateur operator's household, are considered to be members of the general public, however, since they cannot reasonably be expected to exercise control over their exposure. In those cases, general population/uncontrolled exposure MPE limits will apply.
"In order to qualify for use of the occupational exposure criteria, appropriate restrictions on access to high RF areas must be maintained and educational instruction in RF safety must be provided to individuals who are members of the Amateur operator's household."
Persons who are not members of the Amateur operator's household but who are present temporarily on a Amateur operator's property may also be considered to fall under the occupational/controlled designation provided that appropriate information is provided them about RF exposure potential if transmitters are in operation.
Amateur radio facilities represent a special case for determining exposure, since there are almost an infinite number of possible antenna types that could be designed and used for Amateur stations.
The FCC said that "generic equations" can be used for analyzing fields due to almost all antennas, although the resulting estimates for power density may be overly conservative in some cases.
It was on this basis that the Question Pool Committee decided to utilize the RF exposure tables provided by Dr. Wayne Overbeck, N6NB. With the help of the ARRL Lab, these tables have been refined to come up with estimated distances to meet RF power density guidelenes for various antennas, frequency bands, and power limits in both controlled and uncontrolled environments. These sample exclusion distance tables now are part of the Element 2 and 3A question pools.
Here is a sample table based on a 14 MHz (20 meter) frequency. (Controlled limit: 4.59 mw per square centimeter, uncontrolled limit: 0.92 mw per square centimeter. These power densities were developed by applying a rather complex formula and take into consideration a ground reflection factor.)
|3-Element Triband Yagi Antenna - 6.5 dbi gain|
|100||4.6 ft||10.3 ft|
|500||10.3 ft||23.1 ft|
|1000||14.6 ft||32.7 ft|
|1500||17.9 ft||40.0 ft|
For example, this "worst type scenario" suggests that the above antenna running 1000 watts, (constant key-down transmitter) should be a minimum of 32.7 feet from their neighbors. The exclusion distance is shorter if the duty cycle (defined as the average ON vs. OFF time over 30 minutes in an uncontrolled environment; a 6 minute period in a controlled environment) is less. Amateur operators and their families should be at least 14.6 feet away from the antenna. The Overbeck formulas make it very easy for an Amateur to determine if he/she is in compliance with the new MPE guidelines. Amateurs that meet these worst scenario calculations will have performed the routine evaluation required by the new Part 97 RF safety rules. Overbeck will also shortly release his computer program that makes these calculations into the public domain.