QRO RF “Safe Distance” Guide for Common HF Antennas (500 W & 1.5 kW)
When you run QRO on HF, the “safe distance” people usually mean is the minimum separation distance needed so that human RF exposure stays below the Maximum Permissible Exposure (MPE) limits.
These distances are for RF exposure compliance, not interference (RFI/TVI).
Controlled vs uncontrolled exposure
In FCC terminology (often used as a reference even outside the US):
- Uncontrolled (general public) ... people who may not be aware of exposure or can’t control it (neighbors, public areas, visitors).
- Controlled (occupational) ... people who are aware and can control/avoid exposure (only where you truly control access and provide awareness).
The FCC MPE limits are time-averaged:
- 30 minutes for uncontrolled exposure
- 6 minutes for controlled exposure
Where these distances come from (and what they assume)
The numbers below are taken from FCC OET Bulletin 65 Supplement B, Table 4a (MF/HF Bands).
Table 4a is a conservative “quick evaluation” method and assumes:
- 100% duty cycle (continuous transmission at the stated power)
- Maximum surface reflection (worst-case ground/roof reflection)
- Distances are the shortest line-of-sight distance from any part of the antenna to the point where MPE is predicted to occur
Antenna gain assumption used here
To keep this guide conservative and simple, the table values below use the 3 dBi row, which is a safe “round up” for low-gain HF antennas such as inverted-L and quarter-wave verticals.
QRO separation distances (conservative, worst-case duty)
Use the Uncontrolled column for neighbors/public areas.
Use the Controlled column only where you truly control access.
| Band / antenna | Freq used (Table 4a) | Uncontrolled @ 500 W | Controlled @ 500 W | Uncontrolled @ 1.5 kW | Controlled @ 1.5 kW |
|---|---|---|---|---|---|
| 160 m (Inverted-L) | 2.0 MHz | 0.7 m (2.3 ft) | 0.5 m (1.6 ft) | 1.2 m (3.9 ft) | 0.8 m (2.6 ft) |
| 80 m (Inverted-L) | 4.0 MHz | 1.3 m (4.3 ft) | 0.6 m (2.0 ft) | 2.3 m (7.5 ft) | 1.0 m (3.3 ft) |
| 40 m (Inverted-L) | 7.3 MHz | 2.5 m (8.2 ft) | 1.1 m (3.6 ft) | 4.2 m (13.8 ft) | 1.9 m (6.2 ft) |
| 20 m (¼-wave vertical) | 14.35 MHz | 4.8 m (15.7 ft) | 2.2 m (7.2 ft) | 8.4 m (27.6 ft) | 3.7 m (12.1 ft) |
| 10 m (¼-wave vertical) | 29.7 MHz | 10.0 m (32.8 ft) | 4.5 m (14.8 ft) | 17.3 m (56.8 ft) | 7.7 m (25.3 ft) |
Note: Table 4a frequency points are near the top of each band, which is a conservative choice.
Quick takeaways (uncontrolled/public distances)
- 160 m: ~0.7 m at 500 W; ~1.2 m at 1.5 kW
- 80 m: ~1.3 m at 500 W; ~2.3 m at 1.5 kW
- 40 m: ~2.5 m at 500 W; ~4.2 m at 1.5 kW
- 20 m: ~4.8 m at 500 W; ~8.4 m at 1.5 kW
- 10 m: ~10.0 m at 500 W; ~17.3 m at 1.5 kW
Adjusting for real operating modes and duty cycle
The table above is worst case. In real operation, your average power over the averaging time is usually lower than your transmitter’s PEP.
ARRL’s RF exposure guidance uses a practical approach: start with PEP at the antenna, then apply a mode duty factor and the fraction of time you actually transmit during the averaging window (ARRL duty factor guidance).
Because these estimates scale approximately with inverse-square behavior, distance scales with the square root of average power.
distance_actual ≈ distance_table × √(average_power / table_power)
If you’re starting from the worst-case Table 4a row for your PEP, a quick approximation is:
distance_actual ≈ distance_table × √(mode_duty_factor × transmit_time_fraction)
Example (practical)
20 m vertical at 1.5 kW PEP, SSB voice, and you transmit about 50% of the time:
- duty factor ≈ 0.2
- time fraction ≈ 0.5
- multiplier = √(0.2 × 0.5) = √0.1 ≈ 0.316
So the uncontrolled distance estimate becomes:
- 8.4 m × 0.316 ≈ 2.7 m (instead of 8.4 m)
This only works if you can reasonably justify your average power and transmit time within the 6-minute/30-minute averaging window.
Practical safety notes for inverted-L and verticals
Even if the MPE distance is small on 160/80 m, don’t treat “small MPE distance” as “safe to touch or stand next to.” Practical hazards around HF antennas include:
- RF burns / contact current if someone touches the antenna or a nearby conductive object
- High voltage near wire ends (often the horizontal/top section of an inverted-L)
- Feedline common-mode radiation creating unexpected “hot spots” closer to the shack
Good practice:
- Keep radiating wires out of reach (height and/or fencing).
- Keep the vertical section of an inverted-L away from patios, paths, and play areas.
- Use an effective current choke and sane bonding/grounding to reduce unwanted RF around the station.
- If you can’t meet an uncontrolled distance on higher bands (often 10/12/15 m), use power limits, time limits, directional choices, or access control for those bands.
Europe note (ICNIRP-based limits)
Many countries base RF exposure rules on ICNIRP. ICNIRP’s 2020 RF guidelines cover 100 kHz to 300 GHz and treat 100 kHz to 30 MHz as a region where both E-field and H-field reference levels can matter for compliance (ICNIRP 2020 guideline document).
That doesn’t make Table 4a “useless” ... it remains a conservative screening tool ... but local compliance may require measurement, modeling, or additional assessment depending on your country and installation.
Mini-FAQ
- Are these “safe to touch” distances? No. These are RF exposure (MPE) separation distances. Touching a radiating conductor can cause RF burns even when you’re outside an MPE distance.
- Which column should I use for neighbors? Use Uncontrolled for any area the public/visitors can access or where you can’t ensure awareness and control.
- Why does 10 m need so much more distance? At higher HF, the far-field behavior and MPE limits drive larger separation distances for the same power and assumed gain.
- Can SSB reduce these distances? Often, yes. If you can justify average power over the averaging window (mode duty factor and transmit-time fraction), distance can drop by √(average_fraction).
- Do chokes change MPE distances? A choke doesn’t “make you compliant” by itself, but it can reduce unintended RF hot spots by keeping common-mode currents off the feedline and out of the shack area.
- Is Table 4a valid outside the US? It’s a conservative engineering screen. Your legal method may be ICNIRP-based and could require additional assessment, especially in near-field scenarios.
Interested in more technical content? Subscribe to our updates for deep-dive RF articles and lab notes.
Questions or experiences to share? Feel free to contact RF.Guru.