RF Safety: Exposure Limits and Antenna Placement

RF exposure is the safety topic that operators most often dismiss, usually on the grounds that radio waves are not the same as X-rays. That is true, and it is also beside the point. Radio frequency energy at high enough levels heats biological tissue, and the exposure limits exist to keep that heating below the level at which it causes harm.

What the limits describe

The limits are expressed in terms of specific absorption rate for the body, and in terms of field strength and power density for the environment. They are derived from the level at which heating becomes measurable, with a substantial safety margin applied.

Two sets of limits matter. Occupational limits apply where exposure is part of the job and the person is aware of it. General public limits are five times more restrictive, and they apply to anyone who might be near your antenna without knowing it — neighbours, family, someone walking a dog.

Evaluating your own installation

Most regulators require the operator to be able to show that their installation complies. In practice this means identifying the locations where a person could be present, calculating the expected field strength at those points for the frequencies and power you use, and comparing the result with the applicable limit.

For a typical HF station at 100 W with an antenna well away from occupied space, compliance is usually straightforward. The cases that need real care are high power, VHF and UHF with directional antennas, and any antenna mounted close to where people live or work.

Practical measures

• Mount antennas as high and as far from occupied space as the site allows.
• Restrict access to the area around an antenna base, particularly for high-angle radiators.
• Avoid operating at high power when anyone is working near the antenna.
• Keep indoor antennas — including attic installations — well away from living space.
• Reduce power rather than relying on distance when a person must be close.
• Remember that duty cycle matters: a digital mode at 50 W can exceed a voice mode at 100 W.