Decibel (dB)
The decibel is the unit expressing the ratio of two power values on a logarithmic scale: dB = 10·log₁₀(P2/P1). 3 dB means twice, 10 dB ten times and 20 dB a hundred times the power. Since gains and losses in RF are written in dB, addition is used instead of multiplication; that is how a link budget calculation fits on a single line.
The decibel is not an absolute value, it is a ratio. An antenna's "3 dBi gain" or a cable's "2 dB loss" only makes sense when it is stated what it is relative to.
- Equivalents to keep in mind: 3 dB ≈ 2 times, 6 dB ≈ 4 times, 10 dB = 10 times, 20 dB = 100 times the power.
- Negative dB is loss: −3 dB is the power dropping to half.
- Additivity: 2 dB of cable loss, 65 dB of path loss and 3 dBi of antenna gain are added and subtracted directly.
In terms of range the key conversion is 6 dB: in free space, adding 6 dB to the link budget doubles the theoretical range, losing 6 dB halves it. That is why losses of 2–3 dB may look insignificant in the field, yet when they accumulate they noticeably shorten the real distance. A long antenna cable, a badly positioned antenna and a polarization mismatch accumulate in exactly this way.
Context in which this term is used: Wireless remote control installation: pairing, relay, safety