The term “RF exposure limit” defines being exposed to a particular amount of RF energy averaged over a particular time-frame. If you are exposed to a higher amount of energy (such as higher transmission power, or being closer to a transmitting antenna), then you can’t be exposed for as long of a period of time as you could be if it was a lower amount of power or you were further from the antenna. Since RF energy drops as you distance yourself from an antenna, putting greater distance from a transmitting antenna and reducing the transmitting power coming out of the antenna are both good ways of reducing RF exposure. This is one reason that people concerned about cell phone RF exposure can use headsets and keep their phone away from their body while in a call to reduce their exposure.
The first thing you need to be aware of is the difference between ionizing and non-ionizing radiation:
- Non-ionizing radiation is the type of RF energy emitted from Amateur radio operations, which is the same type using in broadcast AM and FM radio, WiFi, cordless phones, baby monitors, sunlight, low ultraviolet radiation, and microwaves. The primary hazard associated with non-ionizing radiation is the heating of body tissue and the resulting damage such as burns. (This isn’t the place to talk about sunburns and skin cancer. Leave that to another forum, please.) You can read more about non-ionizing radiation on Wikipedia.
- Ionizing radiation IS NOT produced during amateur radio operation. Sources of ionizing radiation include Gamma rays, X-rays, and higher ultraviolet radiation. The primary hazard from ionizing radiation is the destruction and mutation of DNA, which can directly lead to cancer, inheritable disease, or certain death. If you want, you can read more about this on Wikipedia.
For the sake of this article, I’m not entertaining any conversation on electromagnetic hypersensitivity. Discuss it somewhere else.
The next thing to be aware of is that the human body absorbs different frequencies in different amounts, so the safe exposure limit will vary based on the frequency in use. The human body is the most sensitive to frequencies around 50MHz (the 6-meter band), so the exposure limit at that frequency is the lowest; you have to maintain lower power and greater distance from the antenna around 50MHz than any other frequency.
The last thing you need to be aware of (before we get into actual numbers), is that RF exposure is measured over time, as was mentioned above. There are two different time-frames for this measurement: Controlled, and uncontrolled. Controlled exposure applies to when you are aware of the presence of RF energy, and can take steps to limit your exposure. Operating your station is considered controlled exposure because you are aware of your emissions and can take steps to lower your power or cease operations. A field day event is also considered controlled exposure because you are aware of radio operations and can leave (or walk away from the antennas) whenever you like. Your stations RF emissions extending into your neighbor’s house are considered uncontrolled exposure because they are not aware of the emissions and it’s not reasonable to expect them to take actions to prevent it. Emissions from your portable operation into a nearby picnic site are also considered uncontrolled exposure.
The FCC has defined Amateur Radio RF exposure limits in OET Bulletin 65. For the sake of public interest, I will sum up some of the important information. Bulletin 65 states: “A routine RF radiation evaluation is required if the transmitter power of the station exceeds the levels shown in Table 1 and specified in 47 CFR § 97.13(c)(1). Otherwise, the operation is categorically excluded from routine RF radiation evaluation…” Here is a PDF of Table 1 only:
If you are transmitting over the power levels established in Table 1, an RF exposure survey is required. For example, a station transmitting at 100 watts on the 10-meter band must perform an RF exposure survey and ensure that exposure levels are acceptable. A station transmitting at 100 watts on the 20-meter band, however, is exempt. It’s a good idea to consider RF exposure no matter what your setup, just to ensure that no unsafe conditions exist.
How does a ham perform an RF exposure survey without using expensive test equipment? Many online calculators exist, where all you have to do is plug in your frequency, antenna gain, transmission line length, and sometimes the distance from your antenna. The calculator will tell you if you are within the exposure limits, and it only takes a few minutes. Plus, if you are ever questioned about the RF safety of your station, you can refer to your RF exposure survey findings.
Here are links to a few RF exposure calculators: