Information on Radio Frequency (RF) Station
Exposure Evaluation
RF exposure station evaluation became effective for Amateur Radio Operators by the Federal Communication Commission (FCC) May 3, 2021 with 2 years to perform and complete it. So, it is due May 3, 2023. RF Exposure (RFE) limits must be evaluated. Existing stations must be evaluated and must be re-evaluated if a station change occurs such as different antenna, antenna placement changes, or greater power is used. Previously, Amateur Radio Stations were excluded from the RFE evaluation. So, Amateur radio station operators must perform an RF evaluation. FCC RF-Exposure Regulations -- the Station Evaluation (arrl.org)They need to determine if their station complies with FCC limits on human safety exposure to RF electromagnetic fields.
The ARRL has a free download document to help with the RF evaluation called RF Exposure and You (316 pages) RF Exposure and You.pdf (arrl.org). The ARRL also has an RF Exposure web page RF Exposure (arrl.org).Also, the ARRL has a very helpful web page containing a useful RF Exposure Calculator (arrl.org). RF Exposure and You outlines ways to evaluate your station. You have to determine if you are in compliance with maximum permissible exposure (MPE) levels. You don’t have to do difficult measurements. You may be able to just use tables or use the ARRL RF Exposure Calculator to determine if your antenna is far enough away from people. MPE limits vary with frequency. The MPE levels represent the amount of energy that can be present where and when people are being exposed. There are two exposure environments. The uncontrolled group and the controlled group. The uncontrolled group applies to people who would not normally know they are being exposed. This is the general public or neighbors. The controlled group are people that are aware of their exposure and have the ability and knowledge to control it. Such as the station operator or their family members / guest who know you are a ham radio operator. The amateur station operator can perform the RF exposure evaluation. The MPE limits are not based on PEP but average power. To calculate average power, multiply PEP by the duty factor for the mode being used. (SSB w/ heavy compression is 50%). (FM is 100%). This is taken into account in the ARRL Exposure Calculator. The ARRL Exposure Calculator can be used to determine the distance people (controlled group & uncontrolled group) must be from any part of the antenna to avoid being exposed at levels exceeding the MPE limits. You will also need the antenna gain. A quarter-wave ground plane vertical or quarter-wave vertical has a gain dBi = 1. A half-wave dipole has a gain dBi = 2.15. For average power, it is recommended to use the highest power you have available (worst-case scenario).
Example: Using the ARRL RF Exposure Calculator : Power at Antenna: 25 watts, Mode duty cycle: FM (duty cycle=100%), Transmit duty cycle - You transmit for 3 minutes then receive for 3 minutes (and repeat), Antenna Gain (dBi) = 4 (5/8 wave vertical on mobile roof = 4 dBi), Operating Frequency (MHz) = 146.520 (2m simplex calling frequency). Controlled Group (Ham Station Operator/family) : Minimum Safe Distance (from antenna): 2.6 ft. Uncontrolled Group (General Public): 5.9 ft. So, it is recommended to measure the distance from the antenna to where person from the general public could stand when you are operating stationary mobile. You may need to reduce the power and re-calculate the uncontrolled group safe distance. After reducing the power to 15 watts, the results using the RF Exposure Calculator it is determined that the uncontrolled group (general public) minimum safe distance is now 4.5 ft. If the measurement from the distance from the antenna to where a person is standing was 5 ft then you would need to reduce the power from 25 watts to 15 watts so the uncontrolled group is not exposed to levels that exceed the MPE limits. This needs to be noted in your written station RF evaluation.
The RF Exposure Evaluation of your station must be documented and the results stored at your station location. This folder / notebook / computer file must be available to the FCC upon request. This evaluation does not need to be mailed into the FCC or filled-out on a special form or in a special format. The evaluation does need to be updated each time a change is made to your station i.e. increase of power (amplifier added), change of antenna (increase gain), movement of antenna (usually affects mobile, POTA operations, or ground mounted verticals). In general, antennas mounted on towers or wire antennas up in the air usually don’t present a problem with proximity to the general public. But, when operating from a mobile or POTA operations and when especially operating on higher HF bands, VHF & UHF, then you need to carefully evaluate those set-ups to insure the general public maintains a safe distance from your Amateur Radio Operations and to keep the uncontrolled group (general public) from being exposed at levels that exceed the MPE limits.
Dewey, K4CAB