The dB measures the power of a signal as a function of its ratio to another standardized value. The abbreviation dB is often combined with other abbreviations in order to represent the values that are compared. These are are two examples:
dBm : The dB value is compared to 1 mW. Therefore, if someone set the CCK Transmitter Power of their access point to 50 mW, it equals to 17 dBm and covers a range of approximately 150 feet. If the Transmitter Power is set to its maximum value, then the Cisco Access Point (AP) covers approximately 300 feet.
dBw : The dB value is compared to 1 W.
The dB abbreviation can be used in order to describe the power level rating of antennas:
dBi : For use with isotropic antennas.
Note: Isotropic antennas are theoretical antennas that transmit equal power density in all directions. Isotropic antennas are used only as theoretical (mathematical) references, and do not exist in the real world.
dBd : Refers to dipole antennas. Isotropic antenna power is the ideal measurement to which antennas are compared. All FCC calculations use this measurement (dBi). Dipole antennas are more real-world antennas. While some antennas are rated in dBd, the majority use dBi.
The power rating difference between dBd and dBi is approximately 2.2, that is, 0 dBd = 2.2 dBi. Therefore, an antenna that is rated at 3 dBd is rated by the FCC (and Cisco) as 5.2 dBi.
Antenna gain ratings are also in decibels which is a ratio between two values. An antenna rating is typically to the gain of an isotropic or dipole antenna. An isotropic antenna is a theoretical antenna with a uniform three-dimensional radiation pattern (similar to a light bulb with no reflector). dBi is used to compare the power level of a given antenna to the theoretical isotropic antenna. The U.S. FCC uses dBi in its calculations. An isotropic antenna is said to have a power rating of 0 dB, for example, zero gain or loss when compared to itself.
For more related information, refer to these documents:
Refer to RF power values for more information about the Radio Frequency (RF) values.
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