Understanding RF Power ValuesRF signals are subject to various losses and gains as they pass from a transmitter through the cable to its antenna, then through the air (and other obstructions such as walls and doors), to the receiving antenna, through that cable, and finally to the receiving radio. With the exception of the walls and other obstructions, most of these signal-loss factors are known and can be used in the design process to determine whether an RF system such as a WLAN will work. To understand how to evaluate systems, a good understanding of how RF parameters are measured is important. The following sections discuss measurement values such as decibels and RF power, as well as antennas, cables, and RF propagation in a WLAN environment. Decibelsdecibel (dB) scale is a logarithmic scale used to denote the ratio of one power value to another:  An increase of 3 dB indicates a doubling (2x) of power. An increase of 6 dB indicates a quadrupling (4x) of power. Conversely, a decrease of 3 dB is a halving (1/2) of power, and a decrease of 6 dB is a quarter (1/4) the power. Table 2-3 shows some examples.
Power RatingsThe transmitter power rating of most WLAN equipment is usually specified in decibels compared to known values such as mW or watts.Transmit power and receive sensitivity are specified in dBm, where m means 1 milliWatt (mW). A value of 0 dBm is equal to 1 mW. From there you can use the previously mentioned 3-dB rule and calculate that 3 dBm is equal to 2 mW, 6 dBm is equal to 4 mW, and so on. For example, a radio with a rating of 100-mW transmit power is equal to a radio specified at 20-dBm transmit power.Common mW values to dBm values are shown in Table 2-4.
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