The Ofcom EMF licence condition
Since 2021, UK amateur licences have required you to make sure members of the public can't be exposed to electromagnetic fields above the ICNIRP general public limits. You don't need to do anything more if your station never radiates more than 10 W EIRP (6.1 W ERP) averaged over six minutes, and never more than 100 W EIRP at peak. Above that, you need to check the distance at which your signal falls below the limits, make sure the public can't get closer, and keep a record of how you checked. The condition protects the public, not you or your household. Ofcom's calculator doesn't cover frequencies below 10 MHz, which need a different approach because of the near field.
How the calculator works
Average EIRP = peak EIRP × mode factor × fraction of time transmitting
Safe distance = √(average EIRP × reflection factor ÷ (4π × S))
S is the ICNIRP general public power density limit at your frequency. Under the 1998 guidelines it's 2 W/m² from 10 to 400 MHz, rising with frequency to 10 W/m² above 2 GHz, and higher below 10 MHz. The 2020 guidelines relax the limits on HF considerably. The reflection factor of 2.56 allows for a signal reflected off the ground adding to the direct signal, which is the usual conservative assumption.
The mode factor is the ratio of average to peak power. FM and data modes such as FT8 transmit at full power continuously, so they're 100%. Normal SSB speech averages around 20% of PEP, more with heavy speech processing, and CW about 40%.
Limits of this estimate
The formula is a far-field estimate. Close to an antenna, especially on HF, the field is more complicated and the real distance can be shorter or longer. It also assumes the worst case, that the person is in the main beam of the antenna. For a beam antenna, people behind or below it are exposed to much less. If your safe distance is close to the space the public can reach, use a more detailed method, such as the RSGB's calculator or a field strength measurement.
Keeping people at a safe distance
The simplest measures are raising the antenna, moving it away from paths and boundaries, reducing power, or using less gain towards places people go. A vertical on the ground in a small garden at 100 W FM on 2m is the classic case that needs care. A dipole at 10 m on HF is usually fine.
Questions
Do I need an EMF assessment for my station?
Only if your station can radiate more than 10 W EIRP averaged over six minutes, or more than 100 W EIRP at peak. Most stations running 100 W into an antenna with any gain are over that.
What is the safe distance for 100 W on 2m?
With a 5 dBi antenna, 1 dB of feeder loss and FM transmitting half the time, about 3.6 m allowing for ground reflection, under ICNIRP 1998.
Does the EMF rule protect me and my family?
Ofcom's condition is about members of the general public, and it doesn't require you to protect yourself. It's still sensible to keep your family outside the same safe distance.
Which ICNIRP guidelines should I use?
The 1998 guidelines are stricter on HF and are the safer choice. The 2020 guidelines are the latest and allow higher exposure on HF. The difference is small on VHF and UHF.