Sec. 15.5(b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful interference is caused and that interference must be accepted that may be caused by the operation of an authorized [licensed] radio station, by another intentional or unintentional radiator, by industrial, scientific and medical (ISM) equipment, or by an incidental radiator.All radio equipment (including the supplied antenna in Part 15 systems) must receive FCC certification/authorization for it's intended service, as different specifications apply to each radio service. In almost all cases any modifications to the system (such as using a different external antenna) would void the "implied license for the device to transmit." Or in other words, void the FCC certification thus making it illegal to use any such modified gear. See:
Sec. 15.204 External radio frequency power amplifiers and antenna modifications (b) A transmission system consisting of an intentional radiator, an external radio frequency power amplifier, and an antenna, may be authorized, marketed and used under this part. However, when a transmission system is authorized as a system, it must always be marketed as a complete system and must always be used in the configuration in which it was authorized. An external radio frequency power amplifier shall be marketed only in the system configuration with which the amplifier is authorized and shall not be marketed as a separate product. (c) Only the antenna with which an intentional radiator is authorized may be used with the intentional radiator.Part 15 users must obtain (manufacture specific) Part 15 certified external components (antennas, coaxial cable, amps ect). If you mate an antenna to a radio, and that particular combination has not been certified together, (and it's not for personal use) you're in violation of Part 15. Even when the antenna is functionally identical, or even identical in the case of an OEMed antenna that just had a radio vendor part number applied to it (and a price markup).
However there may be be an exception for personal (non-commercial/non-business) users under section 15.23:
Sec. 15.23 Home-built devices. (a) Equipment authorization is not required for devices that are not marketed, are not constructed from a kit, and are built in quantities of five or less for personal use. (b) It is recognized that the individual builder of home-built equipment may not possess the means to perform the measurements for determining compliance with the regulations. In this case, the builder is expected to employ good engineering practices to meet the specified technical standards to the greatest extent practicable. The provisions of Sec. 15.5 apply to this equipment. It also appears that running a network traffic dump or Wardrving may be illegal per:
Sec 15.9 Prohibition against eavesdropping.
Except for the operations of law enforcement officers conducted under lawful authority, no person shall use, either directly or indirectly, a device operated pursuant to the provisions of this part for the purpose of overhearing or recording the private conversations of others unless such use is authorized by all of the parties engaging in the conversation.
Sec. 15.247 Omni-Directional Antennas (b) The maximum peak output power of the intentional radiator shall not exceed the following: (1) For frequency hopping systems operating in the 2400-2483.5 MHz or 5725-5850 MHz band and for all direct sequence systems: 1 watt. (2) For frequency hopping systems operating in the 902-928 MHz band: 1 watt for systems employing at least 50 hopping channels; and, 0.25 watts for systems employing less than 50 hopping channels, but at least 25 hopping channels, as permitted under paragraph (a)(1)(i) of this section. (3) For systems using digital modulation in the 902-928 MHz, 2400-2483.5 and 5725-5850 MHz bands: 1 Watt. [Note: The maximum gain of omni-directional antennas at this 1 Watt, is inferred to be 6 dBi from the paragraphs below, or 36 dBm maximum. 1 Watt is 30 dBm, plus the 6 dB gain, results in about 4 Watts Effective Isotropic Radiated Power (EIRP)] Directional Antennas [In addition to the above:] (4) Except as shown in paragraphs (b)(3) (i), (ii) and (iii) of this section, if transmitting antennas of directional gain greater than 6 dBi are used the peak output power from the intentional radiator shall be reduced below the stated values in paragraphs (b)(1) or (b)(2) of this section, as appropriate, by the amount in dB that the directional gain of the antenna exceeds 6 dBi. (i) Systems operating in the 2400-2483.5 MHz band that are used exclusively for fixed, point-to-point operations may employ transmitting antennas with directional gain greater than 6 dBi provided the maximum peak output power of the intentional radiator is reduced by 1 dB for every 3 dB that the directional gain of the antenna exceeds 6 dBi. Example: Max Transmitter RF power (dBm) Antenna Gain (dBi) EIRP (W) 30 6 3.98 29 9 6.35 28 12 10.14 27 15 15.81 26 18 25.23 25 21 40.28 24 24 62.79 23 27 100.2 (ii) Systems operating in the 5725-5850 MHz band that are used exclusively for fixed, point-to-point operations may employ transmitting antennas with directional gain greater than 6 dBi without any corresponding reduction in transmitter peak output power. (iii) Fixed, point-to-point operation, as used in paragraphs (b)(3)(i) and (b)(3)(ii) of this section, excludes the use of point-to- multipoint systems, omnidirectional applications, and multiple co- located intentional radiators transmitting the same information. The operator of the spread spectrum intentional radiator or, if the equipment is professionally installed, the installer is responsible for ensuring that the system is used exclusively for fixed, point-to-point operations. The instruction manual furnished with the intentional radiator shall contain language in the installation instructions informing the operator and the installer of this responsibility. Sec. 15.407 Point to multi point (d) Any U-NII device that operates in the 5.15-5.25 GHz band shall use a transmitting antenna that is an integral part of the device. (e) Within the 5.15-5.25 GHz band, U-NII devices will be restricted to indoor operations to reduce any potential for harmfull interference to co-channel MSS operations.
The 5 GHz U-NII frequency bans is segmented into three 100 MHz bands for operation in the US. The lower band ranges from 5.15 -5.25 GHz, the middle band ranges from 5.25-5.35 GHz and the upper band ranges from 5.725-5.825 GHz. The lower and middle band, accommodate 8 channels in a total bandwidth of 200 MHz and the upper band accommodates 4 channels in a 100 MHz bandwidth. The frequency channel center frequencies are spaced 20 MHz apart. The outermost channels of the lower and middle bands are centered 30 MHz from the outer edges. In the upper band the outermost channel centers are 20 MHz from the outer edges.
In addition to the frequency and channel allocations, transmit power is a key parameter regulated in the 5 GHz U-NII band. Three transmit power levels are specified: 40 mW, 200 mW and 800 mW. The upper band defines RF transmit power levels suitable for bridging applications while the lower band specifies a transmit power level suitable for short-range indoor home and small office environments.