dBi represents the gain of an antenna as compared to an isotropic radiator, which transmits RF signals in all directions equally. The gain of an antenna represents how well it increases effective signal power in a particular direction, with dBi (decibels relative to an isotropic radiator) as the unit of measure. You must take all of these into account when calculating the EIRP for a specific radio. EIRP represents the total effective transmit power of the radio, including gains that the antenna provides and losses from the antenna cable. Part 15.247 provides details on limitations of EIRP (equivalent isotropically radiated power). Regulatory bodies in other countries have similar rules. Manufacturers of 802.11 products must comply with Part 15 to qualify for selling their products within the U.S. In the U.S., the FCC (Federal Communications Commission) defines power limitations for wireless LANs in FCC Part 15.247. The RF spectrum is limited, so we must control the amount of power must we use. The problem, however, is that RF interference with other nearby equipment would occur more often. In many cases, installers would prefer to use comparatively high transmit power to increase the range of access points. In order to qualify for license free operation, however, the radio devices must limit power levels to relatively low values. Most countries deem 802.11 wireless LANs as license free. The spectrum regulatory body of each country restricts signal power levels of various frequencies to accommodate needs of users and avoid RF interference. Understand the limitations of transmit power and antenna gain limitations so that you don’t mistakenly violate FCC regulations.
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