By Athena Hessong
Updated Mar 24, 2022
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Decibels (dB) quantify the ratio of signal strengths between two points. When one signal is weaker than another, the difference is expressed as a loss. This loss can be intentional—such as using carpets to dampen noise in a library—or accidental, like signal degradation over a long cable.
Use a calibrated meter to record the power of the unattenuated source. For radio frequencies, a radio‑signal power meter will display the strength in milliwatts (mW), microwatts (µW), or similar units. Label this measurement “Full Strength.”
Repeat the measurement at the point where the signal is expected to have weakened. For example, an antenna may register 20 mW at its input, but a long cable reduces the power to 5 mW at the output. Record this as “Attenuated.”
Divide the full‑strength power by the attenuated power. In the example above: 20 mW ÷ 5 mW = 4.
Using a scientific calculator, press the log button to find the base‑10 logarithm of the ratio. log 4 ≈ 0.602.
Multiply the logarithm by 10 to obtain the decibel value: 0.602 × 10 = 6 dB.
If the full‑strength signal exceeds the attenuated signal, the dB figure represents a loss; otherwise it indicates a gain. In the example, the 6 dB is a loss.
