FCC Rules Dictate Antenna Use

In the United States, the Federal Communications Commission (FCC) regulates the use of antennas through FCC Part 15.247, which defines power limitations for wireless LANs. The key to applying these rules is to understand EIRP (equivalent isotropically radiated power), which represents the total effective transmit power of the radio, including gains that the antenna provides (and losses from the antenna cable). When using omni-directional antennas (less than 6 dB), the FCC rules require EIRP to be 1 watt or less.

With most radio NICs and access points, you'll be well under the FCC limitations using the transmit power options available to the end user. For example, a radio set at 100 milliwatts transmit power with a 3 dB antenna will only have 200 milliwatts (0.2 watts) EIRP. A 6 dB antenna doubles the EIRP to 400 milliwatts, a 9 dB antenna doubles it again to 800 milliwatts, and so on.

With higher gain directive antennas, the FCC relaxes EIRP limitations. When using antennas having a gain of at least 6 dBi, the FCC allows operation up to 4 watts EIRP, which is 1 watt plus 6 dB of gain. The reason higher EIRPs are acceptable is that the higher gain antennas are more directive, which reduces the possibility of RF interference with other systems.

For antennas with gain greater than 6 dBi, the FCC requires you to reduce the transmitter output power if the transmitter is already at the maximum of 1 watt. The reduction, however, is only 1 dB for every 3 dB of additional antenna gain beyond the 6 dBi mentioned above. This means that as antenna gain goes up, you decrease the transmitter power by less. Thus, the FCC allows EIRP greater than 4 watts for antennas having gains higher than 6 dBi. Of course these higher gain antennas would mostly apply to point-to-point solutions having longer-range requirements, which is not common for most indoor applications.

In summary, be sure to consider antenna selection as part of the wireless LAN deployment. Provide flexibility by purchasing access points accepting interchangeable antennas. This will enable the use of higher gain omnis and perhaps directional antennas to enhance the range and minimize the costs of the wireless LAN.