RF “Safe Distance” Guide for Common HF Antennas

HF Amateur Stations from QRP to QRO: RF-Exposure Screening Distances

Running power on HF is not only a question of antenna efficiency, coax loss, amplifier rating, or whether the tuner can make the transmitter happy. Human RF exposure is also part of the engineering problem. At QRP levels, the required separation distances are usually modest, but close-body antennas, indoor wires, magnetic loops, loading coils, and feedlines carrying common-mode current still deserve attention. At 100 W, most conventional outdoor stations are easy to manage, but upper-HF bands and continuous-duty modes can already require meaningful distance. At QRO levels, RF-exposure safety becomes a real station-design constraint.

This guide gives practical screening distances for HF amateur stations from 5 W to 5 kW. The values are intended as conservative first-pass engineering guidance. They are not a replacement for your legal national exposure assessment, NEC modeling, measurement, or recognized calculator tools such as ARRL, FCC, RSGB, or Ofcom guidance.

What an RF-exposure distance means

The separation distance is the minimum distance between accessible human space and the transmitting antenna system so that exposure stays below the applicable Maximum Permissible Exposure, or MPE, limits.

That “antenna system” may include more than the visible radiator. In a practical station it may also include feedlines with common-mode current, loading coils, loop capacitors, counterpoise wires, conductive support structures, gutters, fences, towers, and nearby metalwork coupled into the RF field.

Use the shortest three-dimensional distance from any accessible place where a person can stand, sit, lean, climb, or work. Do not reduce the problem to a simple horizontal distance from the mast.

General-public and controlled exposure

Most domestic amateur stations must consider two exposure categories:

• General-public or uncontrolled exposure applies to neighbors, visitors, family members, children, public paths, gardens, rooftops, balconies, and any location where people are not trained and cannot control their exposure.
• Controlled or occupational exposure applies only where exposed people are aware, informed, and able to avoid or reduce exposure. This requires real access control and awareness, not just the fact that the station owner is a radio amateur.

In FCC terminology, general-public exposure may be averaged over a period not exceeding 30 minutes, while controlled exposure may be averaged over a period not exceeding 6 minutes. Other jurisdictions may use ICNIRP-based rules and may treat low-HF near-field exposure differently.

For a domestic station, use the general-public values anywhere a neighbor, child, visitor, delivery person, maintenance worker, or family member could reasonably be present.

Why HF near-field exposure is difficult

HF antennas are physically large, and the near field around them can be large too. Around low HF antennas, the electric and magnetic fields do not necessarily behave like a simple plane wave. The relationship between E-field, H-field, power density, antenna current, height, ground, and human position can be messy.

Simple free-space equations assume a clean far-field environment. Real amateur stations are often the opposite: low wires, end-fed antennas, loops around houses, inverted-L antennas, ground-mounted verticals, balconies, metal gutters, roofs, common-mode feedline current, and people standing inside the near field.

NEC-based investigations into amateur EMF compliance show why this matters. The adjusted free-space method often used for simple screening can be conservative above 10 MHz for the dipole cases studied, but below 10 MHz it can underestimate fields for low HF antennas. The same work also notes that shortened or loaded antennas can produce stronger near fields than full-size antennas and are not reliably represented by the simple adjusted free-space method.

Where the screening distances come from

The main table below follows the FCC OET Bulletin 65 Supplement B style of quick evaluation for amateur stations. The simplified adjusted free-space method uses this form:

S = 2.56 × P × G / (4πd²)

where S is the plane-wave-equivalent power density, P is power delivered to the antenna, G is antenna gain as a linear ratio, and d is separation distance. The factor 2.56 accounts for a worst-case direct-plus-reflected field assumption.

The table assumes:

• FCC general-public exposure limits
• 3 dBi antenna gain
• 100% duty cycle
• Maximum surface reflection using the 2.56 factor
• Power delivered to the antenna
• Frequencies near the upper edge of each HF band
• No reduction for SSB, CW, receive time, or lower average power

These are deliberately conservative screening numbers for many ordinary stations, especially above 10 MHz. They are not loop-specific, vertical-specific, end-fed-specific, or house-installation-specific near-field calculations.

Conservative FCC general-public screening distances

The 2.5 kW and 5 kW columns are engineering scaling examples. Operation must always remain within the power authorized by your license and jurisdiction.

How to read the table

The values are continuous-carrier, worst-case screening distances. If the table says 4.46 m for a 100 W station on 10 m, that does not automatically mean your 100 W SSB station is unsafe at 4 m. It means that, under the deliberately severe assumptions used in the table, the general-public screening distance is 4.46 m.

If your actual average power is lower because of operating mode and transmit-time fraction, the distance may be reduced where the applicable rules allow time averaging. If your antenna is low, loaded, indoors, near a house, or below 10 MHz, the table may also be too simple and a better assessment method may be needed.

QRP stations: low exposure, not zero exposure

At 5 W or 10 W, whole-body exposure distances are usually modest. That is why QRP portable work is generally easy to manage from an RF-exposure point of view. But “QRP” does not mean “no hazard.” It means lower power.

Be careful with:

• portable whips close to the body
• magnetic loops beside the operator
• indoor wires passing near desks, beds, or occupied rooms
• end-fed antennas with accessible counterpoise or feedline current
• loading coils and loop capacitors within reach
• long continuous-duty digital transmissions

When a calculated value is only a few centimeters, do not treat it as a precision “safe distance.” The practical interpretation is simpler: keep energized conductors, coils, capacitors, counterpoise wires, and feedline hot spots out of reach.

Typical 100 W stations

A 100 W transceiver is not automatically a serious exposure problem, but it is also not automatically exempt from thought. Under the conservative continuous-duty assumptions in the table, a 100 W station gives about 0.30 m on 160 m, 0.60 m on 80 m, 1.10 m on 40 m, 2.16 m on 20 m, and 4.46 m on 10 m.

The upper-HF distances are larger because the FCC general-public exposure limits become more restrictive as frequency increases through HF. This is why a station that looks harmless on 80 m can require more separation on 10 m, even at the same power.

Example: 100 W SSB

Assume a 100 W PEP station, SSB voice, and transmitting about half the time during the averaging period. If we use an illustrative SSB mode factor of 0.2, the average-power fraction is:

0.2 × 0.5 = 0.1

The distance multiplier becomes:

√0.1 = 0.316

So the 100 W continuous-duty distance becomes roughly the same as the 10 W continuous-duty distance. On 20 m, for example:

2.16 m × 0.316 ≈ 0.68 m

This is an example, not a universal rule. Speech processing, compression, ALC behavior, contest operating, digital modes, AM, FM, RTTY, FT8, and tuning carriers can all change average power.

Distance scaling with power

Exposure distance scales approximately with the square root of power:

d₂ = d₁ × √(P₂ / P₁)

This is why a station going from 100 W to 1 kW does not need ten times the distance. It needs roughly √10, or about 3.16 times the distance, under the same assumptions.

The same rule can be used for average power when time averaging is allowed:

d_actual ≈ d_table × √(average power / table power)

Or, starting from a PEP-based table:

d_actual ≈ d_table × √(mode duty factor × transmit-time fraction)

How antenna gain changes the table

The table uses a 3 dBi gain assumption. For other gain values, distance scales with the square root of gain. A practical multiplier from the 3 dBi table is:

A higher-gain antenna increases the distance in the direction of maximum radiation. For rotating beams or directional arrays, consider the complete swept exclusion zone, not only the direction the antenna happens to point during one QSO.

Do not simply label a 6 dBi case as “full-wave loop.” A loop’s near field depends on shape, height, orientation, current distribution, feedpoint, nearby structures, and where people can stand. Far-field gain alone does not define the near-field exposure situation.

Different antenna types need different caution

Verticals and inverted-L antennas

Verticals and inverted-L antennas can place high RF current close to the ground and high RF voltage near wire ends, top-loading sections, or loading components. Keep the vertical section away from patios, paths, gardens, play areas, and places where people can touch the radiator or coupled metalwork.

Dipoles and inverted-V antennas

Dipoles are often straightforward above 10 MHz when mounted with reasonable height and access control. On 160, 80, 60, and 40 meters, low dipoles and inverted-V antennas deserve more care, because simple adjusted free-space screening can become unreliable.

Full-wave loops around houses

A full-wave loop routed around a house can place conductors near upstairs rooms, gutters, walls, windows, balconies, and roof access points. The closest exposure point may be beside a vertical or sloping section, not directly under the lowest wire. A low loop at QRO should not be approved or rejected from a simple gain table alone. Use proper modeling, a recognized calculator, lower power, or access control.

Loaded and shortened antennas

Shortened antennas, loading coils, compact loops, and high-Q matching networks can create strong local fields. They may require more careful assessment than a full-size antenna at the same power. Keep coils, capacitors, and high-voltage wire ends physically inaccessible.

End-fed and off-center-fed antennas

End-fed and off-center-fed antennas often depend on the feedline, counterpoise, or station environment as part of the RF return system. If common-mode current flows on the outside of the coax, the feedline itself becomes part of the radiating structure and must be considered in the exposure assessment.

Converting distance into antenna height

For a straight horizontal wire over level ground, with people able to walk underneath, a simple geometric screen is:

minimum conductor height ≈ 1.8 m + required separation distance

For example, if the required separation distance is 3 m, the wire should be roughly 4.8 m above the standing surface if people can walk directly below it.

This shortcut does not work well for sloping wires, vertical loop sections, balconies, accessible roofs, ladders, maintenance work, nearby metalwork, or feedlines carrying common-mode current. In those cases, map the accessible human space in three dimensions.

What failure of the table really means

Failing this table does not automatically mean “someone will be injured.” It means the installation does not pass this simplified screening method under the stated assumptions.

The correct conclusion is:

The installation needs a more accurate evaluation, lower average power, more distance, better access control, or a different antenna arrangement.

The table can overestimate exposure in some cases, especially above 10 MHz. It can also underestimate exposure in some low-HF near-field situations. That is exactly why careful wording matters.

When a more detailed evaluation is needed

Use a recognized calculator, pre-assessed configuration, NEC modeling, competent measurement, lower power, or access control when:

• the antenna is low and operates below 10 MHz
• the antenna is indoors, on a balcony, or close to a house
• the antenna is a compact loop, loaded antenna, or shortened radiator
• people can approach wire ends, coils, loop capacitors, or vertical conductors
• a roof, gutter, fence, mast, tower, or other conductor is nearby
• common-mode current on the feedline is suspected
• several transmitters or antennas may operate at the same time
• the simple screen fails but you still want to operate at the proposed power
• local rules are ICNIRP- or Ofcom-based rather than FCC-based

Grounding, bonding, and chokes

Grounding does not make the intended antenna field disappear. A ground rod is not an RF-exposure shield, and bonding does not cancel the near field around the radiator.

A good common-mode choke can still help. It can reduce unintended feedline radiation, remove RF hot spots near the shack, and stop the coax from becoming part of the antenna system. But a choke does not magically make a high-power antenna compliant. It only helps ensure that the antenna system radiates where you expect it to radiate.

Lightning protection, bonding, mains safety, and RF exposure are related station-safety topics, but they are not the same calculation.

Practical assessment procedure

• Identify which exposure rules apply in your country.
• Determine whether each accessible area is general-public or controlled.
• Calculate the maximum power delivered to the antenna on each band.
• Include mode duty factor and transmit-time fraction only where allowed and justified.
• Map where people can realistically stand, sit, climb, or work.
• Apply the screening table using realistic antenna gain.
• Check whether feedlines, counterpoises, loading coils, or metalwork are part of the RF field.
• Use recognized tools, modeling, measurement, lower power, or access control when the screen is not enough.
• Record the assumptions and power limits used for your station.

Practical takeaway

At 5 to 10 W QRP, whole-body exposure distances are usually small, but close-body antennas, indoor wires, compact loops, loading components, and feedline current still require common sense.

At 100 W, conventional outdoor HF stations are usually manageable, but upper HF, continuous-duty modes, indoor antennas, and antennas close to people can require real separation.

At 500 W to 1.5 kW, RF exposure becomes a normal part of station engineering. Low antennas, house loops, loaded antennas, and accessible radiators need careful review.

At 2.5 kW to 5 kW, generic screening tables are only a starting point. You should expect to need height, distance, access control, reduced average power, or installation-specific evaluation.

If any energized radiator, loop, coil, capacitor, counterpoise, or feedline hot spot is within one or two meters of people during transmission, do not assume it is fine just because the station is “only HF.” Prove it, move it, reduce power, or block access.

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