A transmitter normally uses an absolute power level of 100 mW. Through the course of needed changes, its power level is reduced to 40 mW. What is the power-level change in dB?

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A transmitter normally uses an absolute power level of 100 mW. Through the course of needed changes, its power level is reduced to 40 mW. What is the power-level change in dB?

2.5 dB
4 dB
−4 dB
−40 dB
None of these answers are correct; where is that calculator?

Explanation: This question involves a reduction in the power level, so the dB value must be negative. Try to find a simple way to start with 100 and get to 40 by multiplying or dividing by 2 or 10. In this case, 100 / 10 = 10; 10 × 2 = 20; 20 × 2 = 40. Dividing by 10 reduced the dB value by 10 dB; then multiplying by 2 increased the total by +3dB; multiplying again by 2 increased the total by +3 more dB. In other words, dB = −10 + 3 + 3 = −4 dB.

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