Why decibels
Decibels express ratios on a logarithmic scale, so a system's gains and losses simply add up. For power, dB = 10 × log₁₀(P₂ ÷ P₁). For voltage or current, dB = 20 × log₁₀(V₂ ÷ V₁), because power goes with the square of the voltage. 3 dB is double the power, 6 dB is double the voltage, and 10 dB is ten times the power.
Handy figures
- +3 dB = ×2 power, +6 dB = ×4 power or ×2 voltage, +10 dB = ×10 power, +20 dB = ×100 power.
- Going from 5 W to 100 W is +13 dB, a bit over two S-units.
- Losing 3 dB in coax wastes half your power.
Adding signals
When you add gains and losses along a chain, simply add the dB values. But two separate signals or noise sources that aren't in step add by power, not by dB: two equal noise sources together are 3 dB higher, not double the dB. The power sum does that. For absolute levels in dBm, use the dBm and watts calculator, and for S-meter readings, the S-meter converter.
Questions
How many dB is double the power?
3 dB, or more precisely 3.01 dB.
How many dB is double the voltage?
6 dB, because power goes up four times.
What is 10 dB as a ratio?
Ten times the power, or about 3.16 times the voltage.
How do I add two signals in dB?
Convert to power, add, then convert back. Two equal signals add up to 3 dB more than one.