The FCC's RF exposure guidelines - a primer
Since January 1, 1998, all amateur radio stations have been subject to the FCC Maximum Permissible Exposure (MPE) limits. This requires amateurs to determine the safe distances that must be maintained from their antennas by any persons exposed to their RF fields.
The FCC has set exposure limits for two categories of persons:
· Controlled Environment: Persons exposed are aware of and can take steps to control their exposure - such as the amateur's family members and warned guests on the property. Exposure limits are higher for this category.
· Uncontrolled Environment: Persons exposed are unaware of their exposure - such as neighbors, unwarned guests on the property, and the general public. Exposure limits are lower for this category.
MPE limits vary according to frequency. Higher frequencies are more harmful, thus the MPE limits become more strict as the frequency increases.
MPE is calculated on the basis of RF power densities averaged over the body surface - expressed as milliwatts per cubic centimeter of body surface.
MPE can also be averaged over time, with the exposure for controlled environments averaged over a 6 minute period and uncontrolled over 30 minutes.
Multiple transmitter stations, such as multi-op contest sites, field day sites, and home stations running HF and VHF transmitters simultaneously, are special evaluation challenges. The combined RF exposure from all antennas in use (expressed as percent of MPE) must be added to determine if MPE limits are being exceeded. The Power Density Calculator option in the MPE Calculator software performs these calculations for you.
Amateur operators are responsible for evaluating their stations to insure that persons in both controlled and uncontrolled categories are not exposed to RF fields that exceed the MPE limits. The MPE Calculator was developed to perform this station evaluation for the amateur radio operator.