Every station, every time
When must an RF exposure evaluation be performed on an amateur station operating on 80 meters?
An evaluation must always be performed.
This is the rule change that caught out a generation of operators. The old categorical exemption by power level is gone. Since 2021 every amateur station must be evaluated — the only question is whether the evaluation is trivial or demanding.
Which of the following types of equipment are exempt from RF exposure evaluations?
Hand-held transceivers sold before May 3, 2021.
Read the answer precisely: it is a grandfathering of specific equipment, not a category of low-power operation. Not “under 7 watts”, not “near-field only”. If you bought the handheld after that date, it needs evaluating like everything else.
Controlled and uncontrolled
When evaluating RF exposure levels from your station at a neighbor’s home, what must you do?
Ensure signals from your station are less than the uncontrolled maximum permissible exposure (MPE) limits.
The distinction from the General track, applied:
| Controlled | Uncontrolled | |
|---|---|---|
| Who | you, and people aware of the exposure | everyone else |
| Limit | higher | lower — more restrictive |
| Averaging time | 6 minutes | 30 minutes |
| Your neighbour’s house | this one |
Your neighbours have not consented to anything and cannot be expected to know when you transmit, so the uncontrolled limits apply to them — the tighter of the two.
Note the wording trap in the choices: maximum permissible exposure, not “emission”. Two of the four answers change one word.
Where the limits are tightest
Over what range of frequencies are the FCC human body RF exposure limits most restrictive?
30 - 300 MHz
VHF. The human body is roughly resonant at these wavelengths — a person is a significant fraction of a wavelength tall between 30 and 300 MHz — so energy couples into the body most efficiently there, and the limits are correspondingly lowest.
The practical consequence: a 2 metre station needs more careful evaluation than an HF station at the same power.
What does SAR measure?
The rate at which RF energy is absorbed by the body.
Specific Absorption Rate, in watts per kilogram. It is the quantity the MPE limits are derived from — the limits are field strengths chosen to keep SAR safely below the level where tissue heating matters.
Why are there separate electric (E) and magnetic (H) MPE limits at frequencies below 300 MHz?
All these choices are correct — the body reacts to both fields, ground reflections and scattering cause field strength to vary with location, and E field and H field intensity peaks can occur at different locations.
Below 300 MHz you are often in the near field, where E and H are not in the fixed relationship that free-space propagation assumes. They can peak in different places, so neither one alone describes the exposure and both must be evaluated separately.
Above 300 MHz, far-field conditions usually hold, the two fields track each other, and a single power-density limit suffices.
What hazard is created by operating at microwave frequencies?
The high gain antennas commonly used can result in high exposure levels.
Not ionisation — microwaves are firmly non-ionising, as the General track established. The hazard is antenna gain: a dish with 30 dBi gain multiplies power density by a thousand in the main lobe, so a modest transmitter can produce a genuinely dangerous field close in.
Shared sites
When evaluating a site with multiple transmitters operating at the same time, the operators and licensees of which transmitters are responsible for mitigating over-exposure situations?
Each transmitter that produces 5 percent or more of its MPE limit in areas where the total MPE limit is exceeded.
The 5 percent rule, and the number is worth memorising because the distractors are plausible.
The logic is proportionate responsibility. At a crowded repeater site nobody’s individual signal exceeds the limit, but the total might. Rather than blaming whoever happened to be measured, the rule makes every contributor of 5% or more jointly responsible for fixing it — and lets genuinely negligible contributors off.
Check yourself
- You run 100 W on 80 metres into a dipole and are certain you are nowhere near any limit. Must you evaluate?
- Your antenna is 20 feet from your neighbour’s bedroom window and 20 feet from your own operating desk. Which limits apply where?
- Why is a 10 W station on 146 MHz a bigger exposure question than a 100 W station on 3.5 MHz?
Answers
- Yes — an evaluation must always be performed. Being confident of the result does not exempt you from computing it; the categorical power exemptions were removed.
- Uncontrolled at the neighbour’s window, controlled at your own desk. The uncontrolled limits are the more restrictive of the two.
- The FCC limits are most restrictive from 30 to 300 MHz, where the human body couples RF energy most efficiently.