Measure the full-strength signal with an appropriate meter; to measure radio signals, for example, a radio signal power meter indicates the strength of radio waves at a particular location in units of milliwatts, microwatts or similar units. Write down the results, labeling them “full strength.”

Measure the attenuated signal with the same meter; this is the signal for which you expect a reduction in power. For example, an antenna picks up a radio signal; right at the antenna, the meter measures 20 miliwatts, but the long cable connected to the cable reduces the power to 5 miliwatts. In this instance, you measure the attenuated signal at the output end of the long cable. Write down the results, labeling them “attenuated.”

Divide the first signal's power by the second signal's power to find the ratio of the two signals. For instance, if signal A has a power of 20 mW and signal B has a power of 5 mW: 20/5 = 4.

Take the log of the the ratio of the signals by pressing the log button on the scientific calculator. For instance: log 4 = 0.602.

Multiply this answer by 10 to find the decibels. For the example: 0.602 x 10 = 6 decibels (dB).

Determine if the decibel reading reflects a loss or gain of power by looking at signal A and signal B. Record a loss if signal A had a greater value than signal B, and a gain if signal B had the greater number. For the example, since the first signal (signal A) measured more than signal B, the result indicated a loss of 6 decibels (dB).