Decibels compare power levels

The watt is a standard unit of power. We use decibels (dB) to compare two power levels. For quick estimates, we can use the rule of thumb that 3 dB gain corresponds roughly to twice as much power and -3 dB (a 3 dB loss) corresponds to roughly half the power.

So, for example, a 2 watt transmitter would be 3dB above a 1 watt transmitter and a 10 watt transmitter would be 3 dB above a 5 watt transmitter.

Consider the following example, which compares various power levels with a 1 watt transmitter:

Power (watts) 1 2 4 8 16 32 64 128
Decibel gain relative to 1 watt 0 3 6 9 12 15 18 21

We can also use this approach to estimate decibels. For example, how many decibels above a 1 watt transmitter is a 100 watt transmitter? How many times must we double 1 to arrive at 100? As shown below, if we add three decibels for each doubling of power, we reach 100 watts at between 18 and 21 decibels. (The exact value would be 10*log(100/1) = 20)

Power (watts) 1 2 4 8 16 32 64 100 128
Decibel gain relative to 1 watt 0 3 6 9 12 15 18 20 21

Note that doubling power is actually equivalent to a gain of 3.0103 decibels, not 3 as in these estimates.

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