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Measuring Line Loss in Decibels

The concept of decibel measurement, or dB, is covered more in Chapter 2. But for now, it’s easy to think of it as the higher the number, the stronger the signal. Remember that negative numbers descend as they get higher. Transmission line loss is represented as negative dB. Wi-Fi radio transceiver effectiveness is described as a measurement of power output and receive sensitivity. Generally, these two measurements are expressed as power in milliwatts (expressedas mW, meaning 1/1000 of a watt) or as “dBm” (decibels related to 1 mW). Decibel measurement can be confusing. But there are two key concepts to make this easy to understand:
_ Decibels are relationship-oriented
_ Decibels double by threes
Relationship-oriented means that there is no set value for a dB. The trailing letter in a dB measurement defines the relationship. For example, dBm means decibels related to 1 mW of power. 1 dBm equals 1 mW.When you know the value of the relationship, decibels are easy to calculate. Doubling by threes is due to the logarithmic nature of RF energy. When comparing a signal of 1 dBm (1 mW) to a signal of 3 dBm (2 mW) you see that it’s double the power. This doubling nature of power measurement or line loss makes it easy to see how a cable can quickly reduce the RF signal to almost nothing.