Study guide
7.7 RF Exposure: EMR Hazards and Safe Distance
ACMA Amateur Radio Standard Recognition Certificate - Theory Exam
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1 Chapter 1: Electrical Principles and Components
1.1 Mathematics, Units and Metric Prefixes 1.2 DC Circuits, Voltage, Current, and Ohm's Law 1.3 Power, Energy, and Resistance Calculations 1.4 Resistor Colour Code and Component Symbol Identification 1.5 Capacitors: Construction, Ratings and Safety 1.6 Inductors: Construction, Factors Affecting Inductance and Combinations 1.7 Capacitor and Inductor Combination Calculations 1.8 AC Circuits, Frequency, Wavelength, and RMS Values 1.9 Reactance and Impedance in AC Circuits 1.10 Resonance, Q Factor and Tuned Circuits 1.11 Transformers: Turns Ratio, Impedance Matching and Eddy Currents 1.12 Rectifiers, PIV and Power Supply Smoothing 1.13 Active Components: Semiconductors, Diodes, and Transistors 1.14 Transistor and FET Symbols, Configurations and External Behaviour
2 Chapter 2: Transmitters and Receivers
2.1 Superheterodyne Receiver Architecture and Block Diagrams 2.2 Frequency Converters, IF Amplifiers and Transceiver Sharing 2.3 Receiver Performance: Sensitivity, Selectivity, and AGC 2.4 Signal-to-Noise Ratio and Receiver Noise Performance 2.5 Transmitter Architecture, Oscillators and Frequency Synthesisers 2.6 Transmission Quality: Frequency Drift, Parasitic Oscillation and Over-Modulation 2.7 Modulation Techniques: AM, FM, and SSB 2.8 Amplifier Classes and Power Amplifiers (Class A, B, C, and AB) 2.9 Modulation, Duty Cycle and Rated Transmitter Power 2.10 Transceiver Control: ALC, Squelch, and Digital Modes 2.11 Digital Transmission Modes and Bandwidth
3 Chapter 3: Transmission Lines and Antennas
3.1 Transmission Lines: Coaxial, Twin-Lead, and Characteristic Impedance 3.2 Standing Wave Ratio (SWR) and Reflection Loss 3.3 Impedance Matching, Baluns, and Antenna Tuning Units (ATUs) 3.4 Resonant Antennas: Dipoles, Verticals, and End-Fed Antennas 3.5 Angle of Radiation, Antenna Height and Current Distribution 3.6 Directional Antennas: Yagis, Gain and Radiation Patterns 3.7 Effective Radiated Power: System Gains and Losses
4 Chapter 4: Radio Wave Propagation
4.1 Electromagnetic Waves: Velocity, Wavelength and Polarisation 4.2 Ground Wave, Direct Wave and Line-of-Sight Propagation 4.3 Ionospheric Layers (D, E, F1, F2) and Skywave Propagation 4.4 Critical Frequency, Maximum Usable Frequency (MUF), and Fading 4.5 Solar Activity, Sunspot Cycles, and VHF Anomalous Propagation 4.6 Seasonal and Diurnal Effects on Band Selection
5 Chapter 5: Electrical and RF Measurements
5.1 Multimeter Operation: Voltage, Current, and Resistance Measurements 5.2 RF Power and SWR Measurement (PEP vs. Average Power) 5.3 Frequency Counters, Dummy Loads, and Test Equipment 5.4 Frequency Standards, Crystal Calibrators and Standard-Frequency Transmissions
6 Chapter 6: Interference and Electromagnetic Compatibility (EMC)
6.1 Harmonic Emissions, Spurious Signals, and Transmitter Filtering 6.2 How Interference Enters Television and Broadcast Receivers 6.3 Masthead Amplifier Overload and Audio-Stage Demodulation 6.4 Electromagnetic Interference (EMI), Breakthrough, Ferrites, and EMC 6.5 Filter Types, Response Curves and EMC Filter Selection 6.6 EMC Good Practice: Minimum Power, Balanced Antennas and Feedline Routing 6.7 Vehicle EMC and Resolving Community Interference Complaints
7 Chapter 7: Safety and Environmental Hazards
7.1 Mains Voltage Safety, Earthing, and High-Voltage Capacitors 7.2 Equipment Approval, Mains Wiring and Fuse Practice 7.3 Station Layout, Cabling and Emergency Shut-Off 7.4 Lightning Protection and Station Security 7.5 Electric Shock: Prevention, Rescue and Emergency Response 7.6 Battery, Chemical and Hearing Safety in the Amateur Station 7.7 RF Exposure: EMR Hazards and Safe Distance 7.8 Antenna Erection, Siting and Power Line Safety
7.7 RF Exposure: EMR Hazards and Safe Distance
Key Takeaways Explain This Topic AI
The danger from electromagnetic radiation varies with frequency, power and proximity — the three variables the syllabus names, and the only three you can actually control at a station.
Australia's exposure standard is ARPANSA RPS S-1 (100 kHz to 300 GHz), made enforceable for radiocommunications through the ACMA's electromagnetic energy arrangements; amateurs are not exempt.
General public reference levels are about five times lower in power density than occupational levels because the public has no awareness of the field and no control over exposure; family and neighbours are always general public.
The human body absorbs RF most strongly near whole-body resonance, roughly 30-300 MHz, so 6 m and 2 m operation deserves more caution than the same power on 80 m.
Safe distance depends on effective radiated power (transmitter power x duty cycle x antenna gain), operating frequency, antenna type and antenna orientation — and in the far field power density falls with the square of distance.
Last updated: July 2026
7.7 RF Exposure: EMR Hazards and Safe Distance
ACMA Exam Focus: Syllabus items 10.15, 10.16 and 10.17 — recall that people and animals must be kept a safe distance from antennas; that electromagnetic radiation (EMR) can be dangerous and the level of danger varies with frequency, power and proximity ; and that the safe distance depends on effective radiated power, operating frequency, antenna type and antenna orientation.
Non-Ionising Radiation and What It Does to Tissue
Radio waves between 100 kHz and 300 GHz are non-ionising radiation . Individual photons carry far too little energy to strip electrons from atoms or break chemical bonds, which is what makes X-rays and gamma rays so damaging. That does not make radiofrequency (RF) energy harmless — it simply changes the mechanism of harm.
The established hazard is thermal . RF energy absorbed by the body is converted to heat, and if the body's cooling system cannot shed it, tissue temperature rises. Organs with poor blood flow cool badly and are the classic concern: the lens of the eye and the testes. Two secondary effects matter to amateurs as well. Below roughly 10 MHz, induced body currents can stimulate nerve and muscle tissue. And at any frequency, direct contact with a radiating conductor produces an RF burn — a deep, slow-healing injury. A hundred watts into an end-fed wire can put several thousand volts at the high-impedance end of that wire, so a fence-height antenna within reach of a hand is a genuine hazard even at Standard-licence power.
The quantity that describes dose is the Specific Absorption Rate (SAR) — the rate at which RF power is deposited in tissue, in watts per kilogram (W/kg). SAR cannot be measured in a living person, so exposure standards publish reference levels that you can measure or calculate instead: electric field strength in volts per metre (V/m), magnetic field strength in amperes per metre (A/m), and incident power density in watts per square metre (W/m2). Stay under the reference level and you are deemed to comply with the underlying SAR limit.
Frequency, Power and Proximity (10.16)
The syllabus wording is precise, and the exam tests it: the level of danger varies with frequency, power and proximity.
Variable Why it changes the hazard What it means at your station
Frequency Absorption peaks near whole-body resonance, so the exposure limits themselves are at their lowest through the VHF region 50 W on 2 m can be a bigger exposure problem than 50 W on 80 m
Power Power density is directly proportional to the average power actually radiated Halving power halves the field; going from 10 W to 100 W needs about 3.2 times the distance
Proximity In the far field, power density falls with the square of the distance Doubling your distance quarters the exposure — distance is the cheapest control you own
A standing adult behaves electrically like a resonant antenna roughly a quarter-wavelength tall, which places the strongest whole-body absorption in the low VHF region. That is why exposure standards set their most restrictive whole-body reference levels across roughly 30 MHz to 400 MHz — squarely covering the 6 m and 2 m amateur bands.
The Australian Framework: ARPANSA and the ACMA
Australia's exposure standard is ARPANSA RPS S-1 , the Standard for Limiting Exposure to Radiofrequency Fields — 100 kHz to 300 GHz , published by the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) . It is aligned with the international ICNIRP guidelines and replaced the earlier RPS 3.
RPS S-1 by itself is a health standard. It becomes enforceable for radio transmitters through the Australian Communications and Media Authority (ACMA) and its electromagnetic energy (EME) arrangements, made under the Radiocommunications Act. Amateur stations are covered. Very low powered devices are exempt from the arrangements, but a typical 100 W amateur station is not, and the responsibility for compliance sits with the operator — not the equipment manufacturer and not the antenna supplier.
Two Tiers of Limit
Tier Who it applies to Effect
General public (unrestricted) Neighbours, family, visitors, passers-by, anyone outside your control Roughly five times lower power density than the occupational tier
Occupational (restricted) Trained people who know RF is present and can control their own exposure Higher limits, but only for the informed operator inside their own station
In the most restrictive region for whole-body exposure — about 30 MHz to 400 MHz — the general public incident power density reference level is 2 W/m2 , and the occupational level is 10 W/m2 . Note carefully who counts as public: your partner in the next room, the children in the yard and the neighbour on their balcony are all general public , no matter how well you understand RF.
Both tiers are time averaged . Whole-body exposure is averaged over tens of minutes (30 minutes in the current standard) and localised exposure over 6 minutes. Short transmissions with genuine listening breaks really do reduce assessed exposure.
Average Power, Not PEP
Exposure is assessed on average power , never on peak envelope power. Multiply the peak power by the mode's duty cycle:
- Mode Typical duty cycle Average power from 100 W PEP
- FM, RTTY, unmodulated carrier 100% 100 W
- FT8 and similar digital modes about 50% (15 s transmit in each 30 s cycle) 50 W
- CW at normal keying speed about 40-50% 40-50 W
- SSB voice, no speech processing about 20-25% 20-25 W
- SSB voice, heavy compression about 30-40% 30-40 W
A second reduction applies on top: over the averaging window you are also receiving. A rag-chew that is half transmit and half receive halves the assessed average again. A beacon, a repeater or an unattended digital station transmitting continuously gets no such credit.
What Sets the Safe Distance (10.17)
Syllabus 10.17 lists four determinants, and you should be able to recite them:
Effective radiated power (ERP). ERP is transmitter power, reduced by feeder and matching losses, multiplied by antenna gain relative to a half-wave dipole. EIRP (equivalent isotropically radiated power) is the same idea referenced to an isotropic radiator, and is 1.64 times ERP (2.15 dB higher). A 10 dBi beam multiplies your power tenfold along the main lobe.
Operating frequency. The applicable limit itself changes with frequency, tightening through VHF.
Antenna type. A ground-mounted vertical or a low wire radiates in every direction at head height; a Yagi at 15 m concentrates energy in one direction, well above people, with only modest side and rear lobes.
Orientation. The same beam aimed at the neighbour's bedroom and aimed at the horizon are two completely different exposure situations. Height and pointing are free mitigation.
Estimating a Separation Distance
In the far field, power density spreads over the surface of a sphere:
S = EIRP / (4 * pi * d^2)
Rearranged for the minimum distance at which the field falls below the applicable limit:
d = sqrt( EIRP / (4 * pi * S_limit) )
Worked example. A 2 m station runs 100 W of FM (100% duty cycle) into an antenna with 6 dBi gain, a linear gain factor of about 4. EIRP is 100 x 1.0 x 4 = 400 W. Using the general public limit of 2 W/m2:
d = sqrt( 400 / (4 x 3.1416 x 2) ) = sqrt(15.9) = about 4.0 metres
So nobody outside the station should be able to get within about four metres of that radiator while it is transmitting.
Treat such a figure as a floor, not a guarantee. The formula assumes far-field conditions, so results closer than a couple of wavelengths from the antenna are unreliable, and ground reflection can add to the field at some points. ARPANSA and the Wireless Institute of Australia (WIA) publish guidance and calculators that apply conservative assumptions; use them for a real installation.
Keeping People and Animals Clear (10.15)
Mount antennas high and remote. Roof level or up a mast at the back of the block puts distance between the radiator and the living areas, for free and permanently.
Never transmit into an antenna anyone can touch. Ground-mounted verticals, low end-fed wires along a fence and mobile whips beside a footpath are the common offenders. Fence, screen or relocate them.
Handhelds: hold the antenna away from your face and eyes, use the low power setting, and use a speaker-microphone so the radiator is off your body.
Tune into a dummy load , not into the antenna, whenever you can.
Animals matter too. A dog chained under an HF wire, an aviary beside a vertical or livestock in a paddock crossed by a beam cannot read a warning sign and cannot walk away. The syllabus names animals alongside people for exactly this reason.
Mark the exclusion zone. Rope, fence or sign a calculated boundary at a club or field-day site, and brief helpers before they set up nearby.
Use the minimum power necessary for the contact, and rotate the beam off occupied buildings before any long, high-power transmission.