RF Safety Evaluations

The FCC requires operators to perform an RF safety evaluation for every station. The technical details are intricate and beyond the scope of most amateur radio operators.

Fortunately, the ARRL has simplified this for us considerably by providing online tools that perform RF exposure calculations for us.

The first step is to determine the power level (Watts) that your station feeds to its antenna: in other words, the transmitter power level minus the feedline losses. At the frequencies we use, feedline losses can be very large. You can use this online calculator to estimate the power that reaches your antenna:

With that information, you can then use ARRL’s RF Exposure Calculator to determine the safe distances from your antenna.

• For “Power at antenna,” enter the resulting “Power Out” value from the coax line loss calculator, above.
• For “Mode duty cycle,” enter “FM (duty cycle=100%),” since virtually all ARES/RACES operations use FM.
• For “Transmit duty cycle,” you must estimate how much time you spend receiving (or idle) vs. transmitting. Use your best judgement based on the nature of your station and task. Some examples:
  • Typical field stations do a lot of listening (15 minutes) and very little transmitting (3 minutes).
  • Net control stations do a lot of both (3 minutes each).
  • Packet stations usually use a 10-minute BBS check interval, so 10 minutes “receive” (really idle) and 2 minutes transmit.
• For “Antenna gain,” enter the value from the antenna packaging or advertising, or use the “Need help with this?” link to see examples of antenna gain for specific antenna types. (Hint: the most commonly used ARES/RACES antenna is a J-pole, which is 2.15dBi.)
• For “Operating frequency,” enter 140, 222, or 445, depending on which band you’re using. If you’re using multiple bands, enter the highest of the frequencies.

When you press the “Calculate” button, the page will calculate the RF exposure for your system. The numbers that are of interest are the “Minimum Compliance Distance.” This is how far away from your antenna people need to be. There are two “Minimum Compliance Distances” listed: one in the “Results for a controlled environment” section and one in the “For an uncontrolled environment section.” You can use the (smaller) controlled environment distance if everyone who will be near the antenna is a ham and knows when the station is transmitting. If there are people nearby who aren’t hams or who won’t know that the station is transmitting, you need to use the uncontrolled distance.

Once you know the minimum compliance distance for your station, take steps to ensure that no one gets within that distance of your antenna for more that a brief period. This may require setting up barriers, caution tape, signage, etc.

Print out and retain a copy of your RF exposure calculation in case you need to show proof of compliance.