Safe Distances for QRO HF Antennas
When you run high power on HF, RF-exposure safety becomes a real engineering constraint. This article explains what happens around high-power HF antennas, why low antennas can easily exceed public RF-exposure limits, and provides practical tables for safe separation distances for loops, verticals and dipoles at 1 kW, 2.5 kW and 5 kW.
(Indicative values only — always perform your own exposure evaluation for your installation.)
Understanding RF-exposure on HF
HF antennas generate strong electric and magnetic fields in the near field — and the near field on HF is large. On 80 m, the reactive near field extends out to ~13 m. On 40 m, it’s still ~6–7 m. Inside this region, fields can be significantly higher than simple far-field equations predict.
Regulators use two categories:
- Controlled / occupational exposure — you and informed operators
- General-public exposure — neighbors, visitors, people near your property
Most ham installations must meet the general-public limits wherever people can reasonably be.
Why full-wave loops can be problematic at low height
A loop placed 3–6 m above ground — especially one routed around a house — places large parts of the conductor within 1–2 m of human-accessible space. At 1–5 kW, this is well inside the mandatory safety distance on most HF bands.
On 40 m in particular, a 1 kW loop often requires ~5 m separation from people. At 2.5 kW or 5 kW, required distances rise to ~8–11 m. A 3–6 m-high house loop cannot physically meet those limits.
Being above the limit doesn’t automatically mean injury — but it does mean the installation is considered unsafe and non-compliant.
Approximate Safe Distances for QRO HF Antennas
Values assume 100% duty-cycle, general-public exposure, and typical gains: vertical ≈1 dBi, dipole ≈3 dBi, loop conservatively ≈6 dBi.
| Band | Power | Vertical | Dipole | Full-wave loop |
|---|---|---|---|---|
| 160 m | 1 kW | 0.5 m | 1.0 m | 1.5 m |
| 2.5 kW | 1.0 m | 1.5 m | 2.0 m | |
| 5 kW | 1.5 m | 2.0 m | 3.0 m | |
| 80 m | 1 kW | 1.5 m | 2.0 m | 2.5 m |
| 2.5 kW | 2.0 m | 3.0 m | 4.0 m | |
| 5 kW | 3.0 m | 4.0 m | 6.0 m | |
| 40 m | 1 kW | 2.5 m | 3.5 m | 5.0 m |
| 2.5 kW | 4.0 m | 5.5 m | 8.0 m | |
| 5 kW | 5.5 m | 8.0 m | 11.0 m | |
| 20 m | 1 kW | 5.0 m | 7.0 m | 9.5 m |
| 2.5 kW | 7.5 m | 11.0 m | 15.0 m | |
| 5 kW | 11.0 m | 15.5 m | 21.5 m | |
| 10 m | 1 kW | 10.0 m | 14.0 m | 20.0 m |
| 2.5 kW | 16.0 m | 22.5 m | 31.5 m | |
| 5 kW | 22.5 m | 31.5 m | 44.5 m |
Converting distances into minimum antenna height
If a person can walk under the antenna, head height is ~1.8 m. So the required height is:
minimum height = 1.8 m + safe distance
Example for an 80 m loop:
- 1 kW → bottom wire ≈ 4.3 m
- 2.5 kW → ≈ 5.8 m
- 5 kW → ≈ 7.8 m
A loop at 3 m bottom height fails these requirements on all QRO levels.
What improves safety
- Increasing antenna height
- Reducing average power (duty-cycle aware)
- Restricting access under/near radiators
- Using proper RF-exposure calculators (ARRL, RSGB/Ofcom)
Practical takeaway
A full-wave loop around a house at 3–6 m height cannot meet general-public RF-exposure limits above a few hundred watts on most HF bands, and becomes clearly unsafe at 1–5 kW.
If any part of your antenna is within 1–2 m of people during QRO operation, assume it is not compliant until proven otherwise with proper calculations.
Mini-FAQ
- Do loops have higher near-field intensity? Yes — ARRL notes that HF loops can produce stronger near-field fields than dipoles, requiring larger safety distances.
- Do SSB and CW reduce exposure? Yes — average power is lower than 100%-duty modes, but worst-case evaluations must still assume 100% unless controlled-environment rules apply.
- Is a low loop ever safe at 5 kW? Only if people cannot get within several meters of it — which usually means it must be mounted far above roof level or on a tower.
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