T1 · Published 18 October 2026 · By The Ham Desk

Station Setup, Grounding and RF Exposure Limits

This is the unit where the answers are not merely exam answers. Radio frequency exposure limits are enforceable, grounding practice decides whether your equipment survives a nearby strike, and every one of these questions describes a decision you will make on a real bench.

Band 2 m
Study time 18
Level Core
questions 10

Waveform and antenna diagram for Station Setup and RF Safety — Station Setup and RF Safety — waveform overlay and radiation pattern

Pre-flight briefing

Exposure limits, and the strictest number in the table

The FCC sets maximum permissible exposure limits in two environments: controlled, where the person knows the field is present and can move away, and uncontrolled, where the general public may be exposed without knowing. Controlled limits are averaged over six minutes and uncontrolled over thirty, and the uncontrolled limits are the stricter of the two.

The lowest number in the whole table appears between 30 and 300 MHz, where the uncontrolled limit is a power density of 0.2 milliwatts per square centimetre - the same limit that covers 6 metres and 2 metres. In the controlled environment that band allows 1 milliwatt per square centimetre, and above 1500 MHz controlled exposure rises to 5 milliwatts per square centimetre. The reason VHF is restricted is that the human body resonates electrically in that range and absorbs energy more efficiently than at HF or microwave frequencies.

An evaluation is required whenever the station might exceed these limits. You do not have to measure anything: you can use the formulas in the FCC's own bulletin, an online calculator, or a calibrated field strength meter, and the evaluation must be repeated whenever you change the station in a way that could increase exposure - more power, a higher-gain antenna, a lower antenna, or a new band.

Reducing exposure, and the practical tools

When an evaluation shows a problem, the fixes are matters of geometry and power. Move the antenna further away, raise it so the main lobe is above head height, replace a high-gain antenna with a lower-gain one and reduce power, restrict access to the roof or the tower base with a locked gate and warning signs, or simply operate less on the band that causes the problem. Distance is by far the most effective: power density falls with the square of the distance, so doubling the separation quarters the exposure.

The working tools matter too. A ground-fault circuit interrupter protects people from electric shock by cutting the circuit when current leaks to ground, and it belongs on every outlet in a basement or garage shack. A single-point ground system - a common copper bus behind the operating desk, bonded to a driven ground rod outside with short, heavy strap - keeps all equipment at the same potential so that a strike or a nearby surge does not travel through your transceiver on its way between two grounds.

Antennas, towers and the power line rule

The single most important safety rule in amateur radio concerns power lines. An antenna, mast or guy wire that can come into contact with a power line is a lethal hazard, and more amateurs are killed this way than by any other mechanism. Antennas and their supports must be placed so that, if they fall, they cannot touch a power line - and if any part of an installation ever does contact one, stay clear and call the utility.

For tower work, the rules are just as concrete: never climb alone, de-energise and lock out any rotator or amplifier circuits, and never climb in wind, ice or a threat of lightning. Before working on equipment, disconnect the feed line at the shack end and short the centre conductor to the shield - a long wire antenna can hold a static charge that will bite even with the radio switched off.

Terms you must be able to define
Term	What the examiner wants to hear
MPE	Maximum permissible exposure: the FCC's limits on RF field strength and power density.
Controlled environment	One where the exposed person knows about the field and can leave; limits are less strict than for the public.
Power density	RF power per unit area, in milliwatts per square centimetre.
GFCI	Ground fault circuit interrupter: opens the circuit when current leaks to ground.
Single-point ground	A common bus bonding all station equipment so no potential difference can develop between them.

Question deck 10 questions

T5A01 Recall

In which frequency range is the FCC's maximum permissible exposure limit for an uncontrolled environment the strictest?

T5A02 Core

What is the maximum permissible exposure limit for a controlled environment in the 30 to 300 MHz range?

T5A03 Recall

What do the FCC's RF exposure limits describe?

T5A04 Core

When must a station's RF exposure evaluation be repeated?

T5A05 Core

Which action reduces RF exposure near an antenna most effectively?

T5A06 Core

What does a ground-fault circuit interrupter protect against?

T5A07 Core

Why is a single-point ground system used in an amateur station?

T5A08 Core

What is the most important safety consideration when choosing a location for an antenna?

T5A09 Stretch

Why should the feed line be disconnected and the centre conductor shorted to the shield before working on an antenna system?

T5A10 Stretch

Under what condition is a station exempt from a routine RF exposure evaluation?

Reference sheet

The explanations for all ten questions sit on this sheet. Pick an answer on the left and the matching card lights up.

T5A01 Explanation 1

Key B — 30 to 300 MHz

Between 30 and 300 MHz the uncontrolled limit is 0.2 milliwatts per square centimetre, the lowest value in the whole table. Human bodies resonate electrically in this range and absorb RF more efficiently, which is why 6 metres and 2 metres carry the strictest public limit.

Rule 47 CFR §97.13(c) and §1.1310 - RF exposure limits

T5A02 Explanation 2

Key A — 1 milliwatt per square centimetre

In a controlled environment, where the person is aware of the field and can leave, 30 to 300 MHz allows 1 mW/cm2 averaged over six minutes. The uncontrolled limit for the same band is five times lower at 0.2 mW/cm2 averaged over thirty minutes.

Rule 47 CFR §1.1310 Table 1 - controlled exposure

T5A03 Explanation 3

Key D — The maximum electric and magnetic field strength or power density a person may be exposed to

The limits constrain the field a person is exposed to, not the transmitter's output. That is why the same 100 watt transmitter can be entirely legal on one antenna installation and non-compliant on another with more gain or a lower mounting height.

Rule 47 CFR §1.1310 - radiofrequency radiation exposure limits

T5A04 Explanation 4

Key C — Whenever the station is changed in a way that could increase exposure, such as more power, more antenna gain or a lower antenna

The evaluation tracks the installation, not the calendar. Adding an amplifier, swapping a dipole for a beam, moving the antenna closer to an accessible area or starting to operate on a new band all change the field, so the evaluation has to be repeated before operating.

Rule 47 CFR §97.13(c)(1) - routine evaluation

T5A05 Explanation 5

Key B — Increasing the distance between people and the antenna

Power density falls with the square of distance, so moving from two metres away to four metres away reduces exposure by a factor of four - far more than a small power reduction, and without sacrificing coverage the way backing off the power does.

Rule 47 CFR §97.13(c)(2) - exposure mitigation

T5A06 Explanation 6

Key A — Electric shock caused by current leaking from the circuit to ground

A GFCI compares the current leaving and returning on the two conductors and trips within milliseconds when they differ by a few milliamperes, which is the signature of current flowing through a person to earth. It is essential in any damp location where a shack is likely to be.

Rule 47 CFR §97.13(a) - station safety

T5A07 Explanation 7

Key D — It keeps all equipment at the same potential so surges do not travel through the equipment between two grounds

If two pieces of equipment are grounded at different points, a strike or surge raises one above the other and the difference drives current through the interconnect cables - and through the transceiver. Bonding everything to one bus removes that path.

Rule 47 CFR §97.13(a) - good engineering practice

T5A08 Explanation 8

Key C — That no part of the antenna or its support can contact a power line, even if it falls

Contact with a power line is the leading cause of fatal accidents in amateur radio. The requirement is a fall zone: if the mast or a guy can come down across a power line, the installation is unsafe no matter how good the SWR is.

Rule 47 CFR §97.13(a) - antenna installation safety

T5A09 Explanation 9

Key B — A long antenna can hold a dangerous static charge even with the radio switched off

Wind and nearby weather build up static on an antenna and its feed line, and the charge has nowhere to go until something - possibly you - provides a path to earth. Shorting the line at the shack end bleeds it off safely before you touch anything.

Rule 47 CFR §97.13(a) - working safely on antenna systems

T5A10 Explanation 10

Key B — Only if it falls within the FCC's categorical exemption, based on frequency, power and antenna gain

The FCC exempts stations that plainly cannot exceed the limits - low power on the higher bands, for example - and publishes a table of the power and gain combinations that qualify. Everything outside the exemption needs an evaluation, which may be done by calculation rather than measurement.

Rule 47 CFR §97.13(c)(1) - categorical exemption

How to use this unit

Work the ten questions in one sitting with the reference sheet covered, then uncover it and read every explanation - including the ones you got right. The wording of the real exam is different, so learn the reasoning rather than the answer letters.