Decibels and power
A decibel is a ratio. Every 3 dB doubles or halves the power, and every 10 dB multiplies or divides it by ten. That's why radio amateurs use decibels: you can add up gains and subtract losses along the whole path instead of multiplying fractions.
dBm and dBW are decibels with a fixed reference. 0 dBm is one milliwatt and 0 dBW is one watt, so dBW is always 30 less than dBm.
dBW = 10 × log₁₀(power in W) = dBm − 30
Watts = 10^(dBW ÷ 10)
Volts RMS across 50 Ω = √(watts × 50)
ERP and EIRP
Your transmitter's output is not what the antenna radiates. You lose some power in the coax and connectors, then the antenna concentrates what's left in some directions at the expense of others. Effective radiated power tells you how strong your signal is in the antenna's best direction, compared with a reference antenna.
- EIRP compares with an isotropic antenna, an imaginary point that radiates equally in every direction. Use gain in dBi.
- ERP compares with a half-wave dipole, which has 2.15 dB more gain than an isotropic antenna. Use gain in dBd, or subtract 2.15 from dBi.
ERP = EIRP ÷ 1.64
For example, 100 W into a dipole through 1.8 dB of total loss gives an EIRP of about 108 W and an ERP of about 66 W.
Ofcom's EMF threshold
Since 2021, Ofcom has required radio licence holders who transmit above 10 W EIRP (6.1 W ERP) to make sure people aren't exposed to radio frequency fields above international guideline levels, and to keep a record of how they checked. Ofcom says you don't need an assessment if your average power is at or below 10 W EIRP and your peak power isn't above 100 W EIRP. You still need to keep a note of your average power.
The average depends heavily on mode. A steady carrier such as FM or FT8 is on full power the whole time you transmit, while normal SSB speech averages only about a fifth of the peak. The calculator uses typical figures for each mode. It's a starting point for your own records, not an EMF assessment. Ofcom's website and the RSGB's EMF guidance explain how to do a full assessment.
Questions
How many watts is 30 dBm?
1 watt. 40 dBm is 10 W, 50 dBm is 100 W, and 60 dBm is 1 kW. Each 10 dB step is ten times the power.
What's the difference between ERP and EIRP?
They measure the same thing against different reference antennas. EIRP compares with an isotropic radiator, ERP with a half-wave dipole. EIRP is always 2.15 dB, or 1.64 times, higher than ERP for the same signal.
Do I need an EMF assessment for my station?
In the UK, you need one if your average power is above 10 W EIRP, or your peak power is above 100 W EIRP. Most stations running 100 W into an antenna with any gain are over that, so most amateurs need to keep an EMF record.
What power can a UK Foundation licence use?
Since February 2024, Foundation licensees can use up to 25 W on most bands, Intermediate up to 100 W, and Full licensees up to 1 kW on the primary bands. Some bands have lower limits, so check your licence schedule.