Last Updated on April 17, 2026 by John Berry
I don’t believe that any discussion about radiocommunications would be complete without a note about the main unit used – the decibel, abbreviated dB.
I regard Recommendation ITU-R V.574-5 (08/2015), Use of the decibel and the neper in telecommunications as definitive. For simplicity I’ve copied here the key applications as they affect my work on other pages on this site.
You should use the symbol dB to indicate a ratio of two powers, two power densities, two other quantities clearly connected with power, or the difference between two power levels.
You should follow the symbol dB by additional information to express an absolute level of power, power flux-density or any other quantity clearly connected with power. In relation to a reference value, for example dBW, this would define a power level with reference to 1 Watt.
The decibel in gains and Losses
Transmission loss (L) is the ratio of the transmitted power (Pt ) to the received power (Pr ):
L = 10 log10 (Pt / Pr ) dB
Where your transmitter and receiver use isotropic antennas as reference antennas, you would use the symbol dBi.
You express antenna gain as the ratio of the power required at the input of a loss free reference antenna (P0) to the power supplied to the input of the given antenna (Pa ) to produce the same field strength in a given direction:
G = 10 log10 (P0 / Pa) dB
The reference antenna on this website is an isotropic antenna. Gain with reference to the isotropic antenna uses the symbol dBi.
Absolute decibel levels
The power level is the ratio, generally expressed in decibels, between the power of a signal at a point in a transmission channel and a specified reference power.
To use a reference power of one watt, you would say that the power level is in “decibels relative to one watt” and you’d use the symbol dBW.
Signal-to-noise ratio is the ratio of the signal power (Ps ) to the noise power (Pn) measured at a given point with specified conditions. You would express S/N in decibels as:
R = 10 log10 (Ps / Pn ) dB
For full definitions and more, see Rec. ITU-R V.574-5 (08/2015).
Practical applications of the decibel
There are several important points I need to make about using a logarithm like the dB in practical work.
Gains and losses add and subtract. That’s the beauty of the dB. Once captured as a dB, there’s seldom any need to revert to the number from which it came.
When talking about both ratios and numbers, it’s general to round up or down to the nearest dB. It makes no sense to talk about, for example, 14.65dB. The difference between 15dB and 14.65dB is small.
If you double the power, add 3dB. In the same sense, if you combine four antennas, phased in a stack, the gain is that of a single antenna plus 3dB, and then it’s plus 3dB again.
Then, if power rises by a factor of 10, add 10dB. In the same sense, if by 100, add 20dB, if by 1000, add 30dB and so on. The same applies in the other direction – for 1/10 of the power, add -10dB, if 1/100, add -20dB and so on.
Typical values
Antenna gains typically lie in the 0 to 40dBi range. A long Yagi might have a gain of about 15dBi. A big dish might have a gain of 32dBi.
Sensitivities of receivers are around -150dBW. It’s negative because the signal needed is very small and well below 1Watt – about 150dB below. A negative number signifies a level below the reference of 1Watt or 0dBW.
Receiver noise figures are in the region 0.1dB to 4dB. A low noise amplifier might have a noise figure of 0.5dB, whereas a typical receiver may have a poor noise figure at 3dB.
Transmitter powers are typically in the region 0dBW to 30dBW: 1W to 1,000W (or 1kW) in power. One tenth of a watt would be -10dBW.
Signal to noise ratios are typically around 10dB for speech. But data modes are typically referenced to a 2.5kHz bandwidth and hence the signal to noise ratio needed is well below 0dB. A transmission system like Q65 can function at -28dB below.
We talk of effective radiated power (ERP) when we add the antenna system gain (or loss) to the transmitter power output. It’s ‘effective’ – we can’t measure it. And when referenced to an isotropic antenna, we give it the symbol dBW EIRP.
Path losses are typically huge numbers – like 190dBi for meteor scatter and 250dBi for EME/moonbounce. Note, they’re positive values of loss and typically subtract in calculations.
Signal strength meters indicate about 6dB per S-point on a scale 1-9. Above S9, S-meters add power in dB, so for example, S9+10dB. S9 is a receiver input power of about -103 dBW at HF and about -123 dBW at VHF and above.
Summary
The decibel is key to any understanding of signals in amateur radio. Many people have a poor understanding of the dB. And yet, with a little work, amateurs can learn to use it to good benefit.
It helps to understand the origins of the unit. The Bell is named after Alexander Graham Bell. It was first used at the Bell Telephone Laboratories in telephony and cable systems in the 1920s. In line with all units named after people, a capital letter should be used. Subsequently the unit was abbreviated to bel. The bel was found to be slightly inconvenient, and one tenth of a bel – the decibel – was standardised.
As an abbreviation, use dB as the norm, but when you write it out, use decibel.