C.2 dBm AND dbW

The decibel can also be used to express a specific power level, provided we have a reference or zero-dB level. In other words, a power (P₁) is expressed as a ratio with a particular reference level of power (P_R) such that:

d B = 10 \log_{10} (\frac{P_{2}}{P_{R}})

(C.4)

There are two main references: dBW and dBm.

dBW

A power level expressed in dBW uses 1 W as the reference:

10 \log_{10} (\frac{1 W}{1 W}) = 0 d B W

(C.5)

The capital ‘W’ in dBW means that the power is in reference to 1 W. For example, to convert 20 W to a dBW figure:

P_{d B W} = 10 \log_{10} (\frac{20}{1}) = 13.01 d B W

(C.6)

This convention is widely used in satellite link budgets, radar systems, and high-power RF applications.

dBm

For lower power levels, the more practical reference is 1 mW:

10 \log_{10} (\frac{1 m W}{1 m W}) = 0 d B m

(C.7)

For example, 10 W corresponds to:

P_{d B m} = 10 \log_{10} (\frac{10}{0.001}) = 40 d B m

(C.8)