RF Exposure

What kind of radiation this is

Radio signals are non-ionizing radiation. The distinction from ionizing radiation — X-rays, gamma rays, radioactive decay — is about energy per photon: RF photons do not carry enough energy to strip electrons from atoms and break chemical bonds.

That is a statement about mechanism, not a reassurance. RF deposits energy as heat, and enough of it burns tissue. Touching an antenna during transmission causes an RF burn to the skin — characteristically deep, slow to heal, and achievable at power levels that feel unremarkable.

Why the limits move with frequency

The maximum permissible exposure is not one number. It varies with frequency because the human body absorbs RF energy more efficiently at some frequencies than others.

Absorption peaks in the VHF range, roughly 30 to 300 MHz, where a human body is near resonant — a standing adult is on the order of a half wavelength at those frequencies. Limits are therefore strictest there.

Of the bands the Technician pool asks about, 50 MHz has the lowest maximum permissible exposure. Not the highest frequency, not the lowest: the one nearest the body’s absorption peak. Above and below that range the limits relax.

Duty cycle, and the arithmetic that follows

Exposure limits apply to average power density over an averaging period, not to instantaneous peaks. So the fraction of time you are actually transmitting matters directly.

Duty cycle is the percentage of time that a transmitter is transmitting at full power during the averaging time.

Because the limit is on the average:

Halving the duty cycle doubles the allowable power density. Going from 100% to 50% duty cycle increases the allowable power density by a factor of 2 — which is the pool’s wording, and the arithmetic is exactly that simple.

This is why mode matters as much as power. FM and most digital modes transmit a continuous carrier: duty cycle near 100%. SSB voice transmits only on syllables and only while you hold the key: closer to 20% in normal conversation. The same 100 W radio presents very different average exposure depending on which you use.

What actually affects exposure

The pool asks what factors affect RF exposure near an antenna and answers “all these choices”. They are worth having as a list, because they are also the list of things you can change:

• Frequency and power of the transmission
• Antenna height, gain, and directivity — where the energy goes
• Distance from the antenna to people
• Duty cycle of the mode in use

Reducing exposure

The single most effective action is the first one:

• Relocate antennas — distance and height do more than anything else.
• Reduce power, or reduce duty cycle.
• Choose an antenna that puts energy where people are not.
• Increase the time between transmissions.

Determining compliance

Acceptable methods — again “all these choices” — are:

• Calculation based on FCC OET Bulletin 65,
• Calculation based on computer modelling, or
• Measurement with calibrated field-strength equipment.

None requires professional help. For most amateur stations the calculation takes a few minutes with an online tool and the station’s own numbers.

Who is responsible, and when to redo it

The station licensee is responsible for ensuring that no person is exposed to RF energy above the FCC limits.

Not the manufacturer, not the club, not whoever put the antenna up. The licensee.

And an evaluation is not permanent:

Stay in compliance by re-evaluating the station whenever an item of equipment is changed.

Anything that touches the four factors invalidates the previous result — a new antenna, an amplifier, a move to a higher-duty-cycle digital mode, a change in where people spend time near the antenna. The evaluation describes a configuration, not a station.

Check yourself

1. Your duty cycle drops from 100% to 50%. What happens to the allowable power density?
2. Why is 50 MHz more demanding for exposure than 10 MHz or 1200 MHz?
3. You add a linear amplifier. What must happen?