Monday, July 5, 2010

Our club would like to put a 6-meter FM repeater atop a tower in town ...

Q Tim, K4TK, asks, “Our club would like to put a 6-meter FM repeater atop a tower in town. How do I determine if our installation would be in compliance with RF safety rules?”

A Although many amateurs must perform RF safety evaluationsof their stations, there are a number of “categorical” exemptions. The FCC exempted some stations from the need to perform evaluations because the power level, frequency and station type (and configuration) is such that it is very unlikely that the station could exceed the permitted exposure levels.

Two of the exemptions may apply to your repeater operation. First, stations that have 50 W PEP or less to the antenna are categorically exempt from the requirement to do an evaluation. If you have a 50 W repeater, and even a fraction of a dB of feed line loss, you have less than 50 W to the antenna. In most cases, the FCC does not require that you evaluate a station supplying less than 50 W to the antenna.

Many repeaters are also categorically exempt, regardless of power, as long as their antennas are not mounted on buildings. If your repeater is on a tower and it is located more than 10 meters above ground, the regulations do not require that the RF exposure be evaluated.

Now, there are exceptions to this evaluation. Even stations that are categorically exempt from the need to evaluate must not exceed the exposure limits. The FCC’s RF-exposure material clarifies that if a station is categorically exempt, but it has an unusual configuration that might result in excessive exposure, the amateur should evaluate the repeater anyway. A good example of this might be a 45-W repeater with an antenna located within a few feet of where people might be found.

If your repeater is not categorically exempt, by virtue of its power level or the height of the antenna on the tower, you will need to perform an evaluation. There are a number of different ways to do this. The easiest is a “worst-case” evaluation that runs your transmitter power, operating on/off times and antenna gain into a formula and determines the required distance in feet. To do this for an “uncontrolled” exposure environment (this means that it is possible that people who have no knowledge of RF exposure may be exposed by your repeater—ie, roof-top workers, etc), determine your average power by starting with your transmitter power, subtract the power lost in the feed line, multiply that result by the percentage of time the repeater could be transmitting in a 30-minute period. (For a repeater, this percentage is usually 100%.) Then, multiply this result by the numerical gain of your antenna (the gain expressed as a number, not in dB). For 30-300 MHz, you can use the formula:

Where D is the diagonal distance between the antenna and area of exposure, P is the power in watts and G is the numerical gain of the antenna (ie, 6 dB is G = 4).

This formula applies only to 30-300 MHz. For other frequencies, the permitted exposure level varies by frequency. Fortunately, there is an easier way. The Web site at the University of Texas,, can calculate the required distance from your average power and antenna gain. This simple formula approach gives a worst-case evaluation, for the distance if the person being exposed is in the main beam of the antenna. In most cases, repeater antennas direct energy to the horizon, not downward, so the exposure below the antenna is almost always less than the worst-case formula would predict. The ARRL book, RF Exposure and You (, contains additional charts and tables that give a more accurate analysis of typical repeater antennas. The FCC also permits you to choose other evaluation methods, such as antenna modeling, actual measurements and so on.

There is one other consideration that may apply to some amateur repeaters. The FCC rules do not cover just single-transmitter installations, but require that multiple transmitter sites also comply as a site, with all transmitter operators on the site being jointly responsible for overall site compliance in all areas where the RF fields from their own station exceed 5% of the level permitted for their own station. Fortunately, for most amateur repeaters, this “5%” area covers +/– 10 feet from the antenna on the tower. The FCC also clarifies that if your station is categorically exempt from evaluation, you generally do not share joint responsibility with others on the site. This joint responsibility is a complex issue that is discussed in great detail in RF Exposure and You. There are a number of useful Web sites that have more information. The ARRL info on the subject, at, contains links to the FCC material, downloadable reprints of several QST articles on the subject, and more.

From QST November 1999