Decibels
Decibels describe ratios on a logarithmic scale, which keeps gains and losses easy to add up.
Power change in decibels
W
W
Gain of 10 dB
Decibels to power ratio
dB
1.995 times the power
Watts and dBm
W
dBm
100 W = 50 dBm
Worth memorizing
| 1 dB | 1.26 times the power |
|---|---|
| 3 dB | About double the power |
| 6 dB | About 4 times the power, or double the voltage |
| 10 dB | 10 times the power |
| 20 dB | 100 times the power |
| -3 dB | About half the power |
How it works
- Power ratio in dB = 10 log10(P2 / P1). Voltage ratio in dB = 20 log10(V2 / V1), assuming the same impedance.
- dBm is power relative to one milliwatt: 0 dBm = 1 mW, 30 dBm = 1 W, 50 dBm = 100 W.
Learn more: Electronics basics
Sources: Radio Mathematics (ARRL Handbook supplement) (ARRL)
Practice the exam questions: T5 Electrical Principles (Technician), G5 Electrical Principles (General)