What metric is used to express a relative increase or decrease in signal strength?

The decibel (dB) is a logarithmic unit used to express the ratio between two values of a physical quantity, most often power. It is inherently a relative measurement. For instance, a gain of +3 dB indicates that the signal power has doubled, while a loss of -10 dB means the power has been reduced to one-tenth of its original level. This makes dB the standard metric for quantifying relative changes in signal strength, such as antenna gain or signal attenuation through a cable, without needing to state absolute power values.

A. W (Watts): Watts are a unit of absolute power. They measure the actual rate of energy transfer, not a relative comparison between two power levels.

B. dBm: dBm (decibels relative to one milliwatt) is a unit of absolute power level. It compares the power to a fixed reference of 1 mW, not to another arbitrary signal level.

D. mW (milliwatts): Milliwatts are a unit of absolute power, equal to one-thousandth of a watt. Like watts, they represent a specific power level, not a relative change.

1. University Courseware:

Massey, J. L. (2007). Lecture Notes for 6.450 Principles of Digital Communications I. MIT OpenCourseWare. In Section 1.5, "Decibels," it is stated: "The decibel (dB) is a dimensionless unit for expressing the ratio of two power levels... Note that the decibel is a measure of a power ratio, not of power itself." (p. 1-11). This distinguishes the relative nature of dB from absolute power.

2. Official Vendor Documentation:

Keysight Technologies. (2017). Application Note: Power Measurement Basics. Publication Number 5992-2595EN. On page 4, under "Decibels (dB)," the document explains: "The decibel is a logarithmic unit that is used to describe a ratio... dBm is used to express an absolute power level referenced to 1 mW." This clearly separates the relative use of dB from the absolute measurement of dBm.

3. Academic Publication:

Rappaport, T. S. (2002). Wireless Communications: Principles and Practice (2nd ed.). Prentice Hall. In Chapter 2, Section 2.7, "The Decibel," it is defined: "The decibel (dB) is a relative measurement that is used to compare the strengths of two signals... A 3 dB gain means the signal has been doubled... The dBm is an absolute measurement of power." (pp. 45-46). This text is standard in university-level wireless engineering courses.