REC Advisory Letter #8: Radio Frequency Exposure & Environmental Impact
On May 3, 2021, a Report and Order released by the FCC went into effect. This Order did not change the radio frequency (RF) exposure guidelines but it does require that stations verify that their antenna structures meet the RF guidelines. It also defines the format for signs in areas where exposure levels exceed the minimum exposure guidelines. Stations are not required to make any filings with the FCC after this verification is done. Stations are required to recertify that their installations meet the RF exposure guidelines any time when a construction permit, license or renewal application is filed.
The FCC uses a threshold of 200 microwatts per square centimeter (µW/cm2) for general population (uncontrolled) environments (where access is limited only to authorized persons) and 1,000 µW/cm2 for controlled environments (where there is access to other than persons working on the radio equipment or anything else within the immediate area of the antenna/tower). Broadcast stations that can’t meet the 200 µW/cm2 guideline must limit access to the area through fencing or locked doors that cover the area that receives this much exposure. Signage warning of RF exposure would need to be placed on the fencing or doors surrounding the area where the exposure guideline is exceeded.
If the LPFM antenna is the only one on the tower
For an LPFM station operating at the maximum effective radiated power (ERP) of 100 watts vertical and 100 watts horizontal, the FCC has simplified the RF Exposure guideline by requiring a minimum of 6.5 meters (21 feet) between the base of the tower or mast and the lowest part of the radiating antenna. It must also maintain a distance of 6 meters (20 feet) from the radiating part of the antenna to any person (such as a low level tower very close to an adjacent building). This means for a rooftop antenna, the antenna would need to be placed 21 feet off of the roofline to meet the FCC’s simplified guideline.
NOTE: For antennas located near airports where FCC structure registration is normally required, there is an exception to the registration requirement if the overall height of the antenna structure is less than 6 meters (20 feet) above the highest point of the building. LPFM stations seeking to use the “20 foot rule” would not meet the simplified guideline as mentioned on the FCC worksheet. In those cases, a simple exposure study using FM-MODEL could still demonstrate that despite the antenna being lower than 20 feet from the roofline, it would not reach any person in an occupied portion of the building. It can also show compliance in some cases where the LPFM station is operating at less than 100 watts ERP or is using an antenna with only vertical or only horizontal polarity (as opposed to circular polarized). REC can assist in those showings.
LPFM antennas on towers with other FM or TV broadcast stations
If the LPFM antenna shares a site with other FM or TV broadcast antennas, then the minimum distance is 22 meters (72 feet) per the simplified LPFM guideline.
If the height on a shared tower will be less than 22 meters (72 feet), then the RF exposure will need to be evaluated to determine if the addition of the LPFM antenna would require new fencing or expansion of the existing fencing. This is done through looking at each facility and using the FM MODEL tool to calculate the exposure. Remember, when looking at facilities that have both a vertical and horizontal ERP, both values must be considered when determining the ERP for the evaluated facility.
TIP: You can use FCCdata.org to find antennas on a tower by bringing up your station’s pending or granted construction permit or the call sign of any other FM broadcast station you know is on the tower and then click on the “Tower Payload” link to see the stations we show on the same tower. You can then view the technical parameters, including the horizontal and vertical ERP as well as the number of antenna bays for each of those facilities.
For more information on the RF exposure guidelines including the requirements for signage, see our LPFM Environmental Impact/Radio Frequency Exposure page.
Initial version released May 3, 2021. Minor correction made May 17, 2021.