HF RF-Exposure Screening Distances: What a Table Can—and Cannot—Prove

An RF-exposure table can answer a useful engineering question: does a deliberately conservative, simplified calculation clearly pass? It cannot certify that every point around a real HF antenna is “safe.” At HF, people are often in the antenna’s near field, where electric and magnetic fields do not behave as one plane wave and local hot spots can dominate.

First Correction: Call It a Screening Distance

A calculated separation distance is the distance at which a stated model reaches a stated exposure reference level. It inherits every assumption in that model: frequency, average power, antenna gain, ground reflection, exposure category, field region, accessible geometry and jurisdiction.

The “antenna” in this assessment may include more than the intended radiator. Common-mode current can make the outside of the feedline radiate. Loading coils, loop capacitors, counterpoise wires, towers, gutters, fences and other coupled conductors may create accessible field or contact-current hot spots.

Which Exposure Limit Applies?

Exposure categories are not selected by licence class or by who owns the house. Under current US FCC rules, the general-population or uncontrolled limits apply where people may be unaware of the exposure or unable to control it. Occupational or controlled limits require awareness, information and the ability to reduce or avoid exposure.

For US amateur stations, immediate household members may be evaluated using controlled limits only when the licensee and those household members have accessed appropriate training and information. Other nearby people must be assessed against the general-population limits. The FCC permits averaging over no more than 30 minutes for general-population exposure and no more than 6 minutes for controlled exposure.

Why HF Is Primarily a Near-Field Problem

In a far field, electric field strength E, magnetic field strength H and power density S have the plane-wave relationship:

Close to an HF antenna, that fixed relationship need not exist. Reactive energy storage, antenna current and voltage distribution, ground, feedline current, nearby conductors and the position of the person all change the local ratio of E to H. A high-impedance wire end or loading capacitor can be E-field dominant; a compact loop can be H-field dominant.

ICNIRP 2020 treats relevant personal exposure from 100 kHz to 30 MHz as near-field exposure and requires both E-field and H-field reference levels to be satisfied. It does not provide an incident-power-density reference level below 30 MHz for this purpose. It also gives separate local, peak and induced-limb-current provisions. This is the central reason that an FCC-style plane-wave-equivalent table must not be relabelled as an ICNIRP compliance table.

The FCC-Style Adjusted Free-Space Screen

The table below follows the quick-evaluation style used with FCC OET Bulletin 65 and its amateur Supplement B. The adjusted free-space expression is:

Here S is plane-wave-equivalent power density, P is average power delivered to the antenna, G is linear power gain in the direction being assessed and d is distance. The factor 2.56 represents the conservative maximum-reflection adjustment used by this screening approach.

The calculation below assumes:

• the current FCC general-population MPE limits;
• 3 dBi antenna gain, treated as a linear gain of approximately 2;
• 100% transmit duty and a continuous carrier;
• the 2.56 reflection factor;
• power delivered to the antenna after feedline loss;
• a frequency near the upper edge of each listed amateur band; and
• no credit for SSB statistics, receive periods or access control.

FCC general-population screening distances

The arithmetic is correct for those assumptions. The 2.5 kW and 5 kW columns are mathematical scaling examples, not permission or a recommendation to transmit at those powers. Operate only within the power authorised by your licence and jurisdiction.

Why the distance rises toward 10 metres

In the FCC general-population table, the plane-wave-equivalent MPE limit from 1.34 to 30 MHz is 180/f² mW/cm², where f is in MHz. The permitted value therefore falls as frequency rises through HF, and the screening distance increases for the same average power and gain.

Power, Duty Cycle and Distance Scaling

For the same frequency, gain and model, distance scales with the square root of average power:

That is why increasing from 100 W to 1 kW multiplies distance by about 3.16, not ten. The same relation can account for a defensible time-averaged power where the applicable rules permit averaging.

A better SSB example

Suppose a 100 W PEP SSB station has measured or conservatively justified average power equal to 20% of PEP while transmitting, and it transmits for half of the applicable averaging period. The time-averaged antenna power is then:

The corresponding distance multiplier is √(10/100) = 0.316. The 20 m entry becomes 2.16 m × 0.316 ≈ 0.68 m. This is an example of the calculation—not a universal “SSB factor.” Speech processing, compression, voice statistics, ALC, contest operation and tune-up carriers can change the result. FT8, RTTY, AM, FM and other high-duty modes need their own realistic values.

Gain Changes Direction, Not Just a Number

For the same model, distance scales with the square root of linear antenna gain. Relative to the 3 dBi table:

Use gain in the direction of the person, not a catalogue number detached from geometry. A rotating beam needs assessment over its swept directions. Far-field gain does not by itself predict local near fields around a loop, loaded radiator, wire end, matching network or feedline.

Where the Simple Screen Is Weakest

Verticals and inverted-L antennas

Ground-mounted vertical sections can put antenna current close to people, while wire ends and top-loading sections can be high-voltage regions. Keep radiators, radials, loading components and coupled metalwork away from paths, patios, gardens and play areas.

Dipoles and inverted-V antennas

A conventional, elevated dipole is often straightforward to screen, especially on upper HF. Low antennas on the longer bands need more care. The nearest point may be a sloping leg or low wire end rather than the feedpoint.

Full-wave loops around buildings

A loop around a house may pass close to upper floors, windows, balconies, gutters and roof access. Do not accept or reject it from far-field gain alone. Map the shortest three-dimensional distance to every occupied or accessible area.

End-fed and off-centre-fed systems

If the feedline, counterpoise or station wiring carries common-mode current, it participates in the radiating structure. An exposure assessment that models only the nominal antenna wire is incomplete.

Touch, Contact Current and Local Hot Spots

Whole-body exposure and contact hazards are related but not interchangeable. A person can satisfy a whole-body average screen and still receive an RF burn by touching an energized conductor or a coupled metal object. Matching capacitors, coils and antenna ends may carry high RF voltage even at moderate transmitter power.

Below 110 MHz, ICNIRP 2020 also provides induced-limb-current reference levels for people who are not electrically isolated from a ground plane. This is another reason not to translate a small calculated distance into “safe to touch.” Design out access: enclose components, raise conductors, secure barriers and prevent transmission during maintenance.

Turning a Distance into Real Geometry

For a straight horizontal conductor directly above level ground, a rough geometric check is:

Using 1.8 m as a conservative standing-person height, a 3 m separation would suggest about 4.8 m conductor height directly overhead. This is geometry only. It is not valid for sloping wires, vertical sections, balconies, ladders, accessible roofs, maintenance work or coupled structures.

What to Do When the Screen Does Not Pass

A failed screen means “evaluate or mitigate,” not “injury is certain.” Reasonable next steps are:

1. Use an accepted method for the jurisdiction. This may be a recognised calculator, pre-assessed configuration, validated numerical model or competent field measurement.
2. Improve the installation model. Include antenna geometry, height, direction, ground, nearby conductors, feedline common mode and simultaneous transmitters.
3. Reduce realistic average power. Set and document band- or mode-specific limits where the rules permit time averaging.
4. Increase separation. Raise or move the antenna and protect the complete accessible volume, not merely the mast base.
5. Control access. Use barriers, interlocks, signs, supervision or operating procedures appropriate to the actual exposure category.
6. Fix unintended radiation. A suitable common-mode choke can reduce feedline radiation, but it does not erase the intended antenna field.

A Defensible Station Record

Record the bands, maximum delivered power, feedline loss, modes, averaging assumptions, antenna gain and direction, accessible locations, chosen exposure category, assessment method, calculated or measured result and any operating restrictions. Reassess after changing the antenna, amplifier, feedline, choke, mounting height, building geometry or accessible areas.

Practical Takeaway

At 5–10 W, whole-body screening distances are often small, but close-body whips, compact loops, indoor wires and matching components still deserve deliberate placement. At 100 W, most conventional outdoor HF stations are manageable, while upper HF, continuous-duty operation and indoor antennas can require meaningful separation.

At 500 W–1.5 kW, exposure assessment is normal station engineering. At still higher powers, generic scaling remains mathematically useful but installation-specific evaluation, access control and legal transmitter limits become decisive.

The most honest conclusion is not “this table proves safety.” It is: this screen passes with stated margin, or this installation needs a better assessment or mitigation.

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