In 2004, the FCC relaxed their rulings on the certification of antennas used on a Wi-Fi access point.  Previously, antennas were certified by the AP manufacturer and could not be changed or amended or the end-user would be in violation of the FCC ruling.  With the 2004 update, the FCC relaxed their rulings allowing for antennas of the same style/pattern (i.e. omnidirectional, patch, etc.) and the same or lower gain to be considered certified as long as the gain did not exceed the certified gain of that AP.

Each AP manufacturer has their product tested at max power with external antennas and should publish the maximum allowable gain for each antenna type for use with their products.

See the below video training and excerpts from the FCC ruling and a Cisco White Paper regarding the FCC changes.

FCC Regulations Update

In 1994 the FCC added regulations to the 2.4 and 5 GHz WLAN bands, requiring unique connectors or permanently attached antennas. A unique antenna connector was defined as a nonstandard RF connector, not readily available to the general public. At the same time, only antennas certified with a specific WLAN were permitted. The only exceptions to these rules were for transmitters that were certified for installation by a professional installer.

While the requirement for unique connectors remains, the regulations for certification of antennas have changed with the rules introduced in October 2004. These regulations permit any user to install any antenna that is of the same family or style, and equal or lower gain, than any certified antenna. For example: if a 10-dBi patch antenna is certified for use with a specific WLAN transmitter, any patch antenna with a gain of 10 dBi or less may also be used, regardless of its manufacturer. Or if a Yagi directional antenna with a gain of 13.5 dBi is certified with a transmitter, any Yagi antenna with 13.5 or less gain may be used with that transmitter.

View the entire Cisco White Paper here

FFC Regulations, Title 47, Part 15, Subpart C, §15.04

(c) An intentional radiator may be operated only with the antenna with which it is authorized. If an antenna is marketed with the intentional radiator, it shall be of a type which is authorized with the intentional radiator. An intentional radiator may be authorized with multiple antenna types. Exceptions to the following provisions, if any, are noted in the rule section under which the transmitter operates, e.g., §15.255(b)(1)(ii) of this part.

(1) The antenna type, as used in this paragraph, refers to antennas that have similar in-band and out-of-band radiation patterns.

(2) Compliance testing shall be performed using the highest gain antenna for each type of antenna to be certified with the intentional radiator. During this testing, the intentional radiator shall be operated at its maximum available output power level.

(3) Manufacturers shall supply a list of acceptable antenna types with the application for equipment authorization of the intentional radiator.

(4) Any antenna that is of the same type and of equal or less directional gain as an antenna that is authorized with the intentional radiator may be marketed with, and used with, that intentional radiator. No retesting of this system configuration is required. The marketing or use of a system configuration that employs an antenna of a different type, or that operates at a higher gain, than the antenna authorized with the intentional radiator is not permitted unless the procedures specified in §2.1043 of this chapter are followed.

View the Entire FCC Regulation here