EMR Safety — Protecting People from RF
Why RF Safety Matters
At 400W with a directional antenna, you're generating significant RF fields near your antenna. Radio waves at sufficient intensity can cause tissue heating — and you're legally required to ensure your station complies with safety limits.
The Basic Calculation
In the far field (more than a few wavelengths from the antenna), power density follows the inverse square law:
\( S = \frac{P \times G}{4\pi d^2} \quad \text{W/m}^2 \)
Where P = power (W), G = antenna gain (linear, not dB), d = distance (m).
Example: 400W into a Yagi with 10 dBi gain (= 10 linear):
- At 5 metres: S = (400 × 10) / (4π × 25) = 12.7 W/m² — potentially exceeds limits!
- At 10 metres: S = (400 × 10) / (4π × 100) = 3.2 W/m²
- At 20 metres: S = (400 × 10) / (4π × 400) = 0.8 W/m² — safe for most limits
Safe Distance
Rearranging for distance:
\( d = \sqrt{\frac{P \times G}{4\pi \times S_{limit}}} \)
Rule of thumb for 400W: Keep people at least 10-15 metres from a directional HF antenna during transmission. The exact distance depends on antenna gain and the applicable exposure limit for the frequency.
Australian Requirements
You must comply with ARPANSA RPS3 (Radiation Protection Standard). Key points:
- Exposure limits vary with frequency — check the specific limits for your band
- General public limits are stricter than occupational limits
- Duty cycle matters — SSB and CW have lower average power than FM because you're not transmitting continuously
Practical Safety Measures
- Keep antennas high — height is your best friend for reducing exposure at ground level
- Use minimum power — not just good practice, it directly reduces exposure
- Consider duty cycle: SSB/CW conversations have much lower average power than FM or digital modes that transmit continuously
- Restrict access to areas near antennas during transmission
- Never touch antenna elements or feedlines during transmission — RF burns are painful!
Near-field warning: The formula above only works in the far field (more than a few wavelengths away). Close to the antenna, fields are complex and can be much stronger than the formula predicts. On 80m, a few wavelengths is 500+ metres — so for HF antennas, you may always be in the near field at ground level. Use conservative estimates.