Once you know the complete combination of transmitter power level, the length of cable, and the antenna gain, you can figure out the actual power level that will beradiated from the antenna. This is known as the effective isotropic radiated power (EIRP), measured in dBm. EIRP is a very important parameter because it is regulated by governmental agencies in most countries. In those cases, a system cannot radiate signals higher than a maximum allowable EIRP. To find the EIRP of a system, simply add the transmitter power level to the antenna gain and subtract the cable loss.
Suppose a transmitter is configured for a power level of 10 dBm (10 mW). A cable with 5-dB loss connects the transmitter to an antenna with an 8-dBi gain. The resultingEIRP of the system is 10 dBm - 5 dB + 8 dBi, or 13 dBm.
You might notice that the EIRP is made up of decibel-milliwatt (dBm), dB relative to an isotropic antenna (dBi), and decibel (dB) values. Even though the units appear tobe different, you can safely combine them because they are all in the dB "domain".