QRO HF Antenna RF Exposure: What a Distance Can Prove
QRO HF Antenna RF Exposure: What a Distance Can Prove
A separation distance is useful only when its frequency, power, antenna, field region, exposure category, averaging method and accessible geometry match the real installation.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
The chart in Mark’s video: In Avoid Frying Your Brains with RF, Mark K3ZD—Ham Florida Man—shows the QRO-distance chart from Joeri Van Dooren / ON6URE’s RF.Guru article and reads its band-and-power examples at about 02:32–04:13. Mark identifies 100% transmit duty, general-public exposure and assumed antenna gains; his video description links the article.
Do not use the video’s distances as a safe-distance rule. Its chart does not establish compliance for your installation. Even 100% duty does not make assumed gains, near-field geometry or accessible positions a worst case. Use the method accepted by the responsible authority with the actual station inputs, access controls and uncertainty. Mark’s opening account of an operator’s illness is not evidence that RF caused that illness.
I agree with Mark’s practical message: keep people out of hazardous RF fields and do not forget the feedline. But a chart cannot draw that boundary for every QRO HF station. A calculation can document that one defined installation passes a stated screening method; it cannot certify unmodelled near fields, accessible conductors, coupled metalwork, every operating mode or every legal framework. The useful question is not “which row is my antenna?” but “what reaches the places people can occupy?”
Safety and regulatory note: This article explains a bounded engineering screen; it is not a compliance certificate, medical advice or permission to use a particular transmitter power. Apply the current method required by the competent authority, keep energized conductors and matching components inaccessible, and obtain competent assessment where a simple screen does not fit the installation.
What a Screening Distance Means
A screening distance is the point at which a stated model reaches a stated exposure reference level. Its result is conditional on every input: frequency; the declared power quantity and reference plane; feedline loss; antenna gain and pattern in the direction being assessed; reflections; transmit duty and averaging interval; exposure category; source and person geometry; and every position that people can occupy.
Declare the power convention: distinguish amplifier or transmitter output, power delivered through the feeder, power accepted at the antenna terminals, and any ERP or EIRP value required by the selected method. Record whether the value is PEP, carrier, time-averaged power or another defined quantity, and include every transmitter that can contribute during the assessment interval.
The accepted conservative method remains below its limit for every relevant operating case and accessible location.
Compliance has not been shown by that shortcut; use a better model, measurement or stronger controls.
Contact current, RF burns, high voltage, lightning and mains hazards require separate controls.
Failing a conservative screen does not establish that an exposure limit has been exceeded or that injury will occur. It means the simplified route has not demonstrated compliance. Conversely, a numerical pass is credible only when the model covers the real source and all reasonably accessible positions.
Why HF Often Defeats a One-Line Formula
The familiar far-field calculation treats electric field, magnetic field and power density as linked by the plane-wave relationship. Close to a practical HF antenna, that relationship may not hold. The applicable field region depends on frequency, source dimensions and distance; ICNIRP cautions that no single near-field boundary is sufficient for every compliance assessment.
A person may stand near a high-voltage wire end, a high-current loop section, a loading coil, a matching capacitor, a counterpoise or feedline carrying common-mode current on the outside of its shield. Ground, buildings, railings, gutters, towers and other conductors can alter both the field and the current induced in accessible objects. A mast-centred circle does not represent all of that geometry.
HF screening rule: where the accepted method requires separate electric- and magnetic-field assessment, satisfy both. Do not convert one plane-wave-equivalent power-density value into a complete near-field result.
That is also why the loop, dipole and vertical gains read in the video cannot rank the near-field exposure of every antenna in those families. The shape, size and current distribution matter. A house inside a loop is a reason to assess occupied rooms and accessible conductors—not enough information, by itself, to declare an exposure limit exceeded.
At about 04:26–05:22, Mark brings the feedline into the discussion and recommends choking. That is an important part of the station: deliberately provide the antenna’s return path and use suitable choking to limit unintended current on the station-side coax. Do not treat a fitted choke as proof of exposure compliance, a universal placement recipe or permission to touch the feed system. Include the actual feedline and coupled metalwork in the assessment.
Average Power Must Match the Applicable Time Rule
Transmitter PEP, carrier power and exposure-averaged power are different quantities. SSB speech can have lower average power than PEP, but compression, speech statistics, transmit-time fraction and tune-up carriers change the result. Digital modes, AM, FM, RTTY and test carriers need their own operating cases.
Do not apply one convenient duty-cycle percentage to every limit. ICNIRP 2020 uses different temporal conditions for different quantities: whole-body average SAR is averaged over 30 minutes, local SAR and corresponding local field reference levels over 6 minutes, and additional peak-field provisions apply in the lower RF range. The legally applicable framework may use different categories, intervals or procedures.
Averaging is valid only when the assumed operating pattern is realistic, enforceable and permitted by the selected method. It must not conceal a continuous tune-up condition or a period during which someone can enter a controlled area.
Contact Current Is a Separate Hazard
At HF, touching a conductive object in an RF field can drive contact current through the body. At sufficient levels this can cause pain, burns or other tissue effects. An antenna conductor can also be directly energized at high RF voltage or current. Neither case is resolved by a whole-body distance calculation.
ICNIRP 2020 provides guidance rather than a generic contact-current reference level because the result depends strongly on the person, object, contact and coupling geometry. Its induced-limb-current reference levels are a different assessment quantity and apply when a person is not electrically isolated. The practical control is to prevent access to energized and coupled conductors, enclose matching components, use barriers or interlocks where appropriate, and inhibit transmission during maintenance.
Europe and Belgium: Identify the Controlling Rule First
EU Council Recommendation 1999/519/EC provides a general-public EMF framework, while Directive 2013/35/EU addresses workers. Neither makes an internet distance table the controlling assessment for every station. National and regional implementation, accepted methods and site conditions still govern.
For Belgium, the BIPT health and environment page states that control of transmitter-mast electromagnetic-field standards from 10 MHz to 10 GHz belongs exclusively to the regions and lists the contacts for Brussels, Flanders and Wallonia. Confirm the requirements for the station's exact location and frequencies with the competent regional or other responsible authority; do not extrapolate a 10 MHz statement to lower HF bands.
Exposure category is not a licence privilege. Under ICNIRP, occupational exposure assumes controlled occupational duties, RF-risk training and appropriate mitigation. A radio licence or ownership of the property does not automatically place family, neighbours, visitors or the operator in that category.
Treat Uncertainty as Part of the Result
A calculated or measured field is not exact. Antenna-pattern data, feeder loss, accepted power, source geometry, reflections, probe calibration, spatial sampling and repeatability can all contribute uncertainty. State the uncertainty method, confidence level and pass margin required by the controlling procedure rather than reporting a bare distance with false precision.
ITU-T K.91 provides guidance for RF-exposure assessment, evaluation and monitoring, including the treatment of uncertainty. It does not replace national or regional rules: apply the uncertainty treatment and decision rule accepted by the competent authority for the installation.
A Defensible QRO Assessment Workflow
- Establish the framework: jurisdiction, responsible authority, exposure category, accepted method and required records.
- Define maximum operation: every band, antenna, amplifier setting, mode, carrier condition, simultaneous transmitter and enforceable averaging assumption.
- Map accessible geometry: radiator, wire ends, coils, capacitors, counterpoises, feedline, tower, coupled metalwork, rooms, roofs, gardens and neighbouring property.
- Use the simplest accepted method that fits: a conservative calculation, a pre-assessed configuration, validated numerical modelling or competent measurement.
- Apply controls: more distance or height, less power, access restrictions, interlocks, operating limits and control of unintended feedline radiation.
- Record and reassess: document the power convention and reference plane, duty and averaging, antenna data, geometry, accessible locations, uncertainty, decision rule, pass margin and controls; repeat after any material change to power, antenna, feedline, mounting or accessible space.
My practical answer to the video is to make the exclusion area a feature of the station, not a guess from its antenna name. Establish the exposure and contact controls first; operate within that assessed envelope, and reassess it when the station changes.
Primary guidance
- ICNIRP 2020 RF EMF Guidelines, 100 kHz to 300 GHz
- ICNIRP explanation of the 2020 reference-level and contact-current changes
- EU Council Recommendation 1999/519/EC for the general public
- Directive 2013/35/EU for worker exposure
- BIPT: Belgian health, environment and regional contacts
- ITU-T K.91 (01/2024): RF-exposure assessment, monitoring and uncertainty guidance
Mini-FAQ
- Is there one safe distance for every QRO HF antenna? No. Separation depends on frequency, required average and peak conditions, antenna geometry, accessible points, environment, exposure category and the controlling method.
- Can I use a far-field formula close to an HF wire? Only as a documented screen when its field-region and geometry assumptions are valid. Otherwise use the accepted near-field method, modelling or measurement.
- Can duty cycle reduce the assessed distance? Only when the factor is realistic, documented, enforceable and permitted by the applicable averaging rules. Peak and local conditions can still control.
- Does passing a field screen mean the antenna is safe to touch? No. Contact current, RF burns and directly energized high-voltage or high-current components are separate hazards.
- Which RF-exposure rule applies in Belgium? Use the requirements and accepted method of the competent authority for the station's region, frequencies and circumstances; BIPT provides the regional contacts.