FCC rule allows end-users to change out antennas on their Wi-Fi and other gear legally if the manufacturer has performed the right tests and the antennas conform to certain guidelines: Jim Thompson alerted me to FCC rule 04-165 issued July 12, 2004, which has some substantial changes for devices that use unlicensed spectrum, most significantly Wi-Fi. The rules affect devices that operate under Part 2 and Part 15 rules, and we're most concerned with Part 15, which governs 900 MHz, 2.4 GHz, and several bands in 5 GHz.
The most notable general applicable of this rule change is in section 2, which starts with point 18: "Replacement Antennas for Unlicensed Devices." Until now, the FCC has required that any antenna to be used with a device operating under Part 15 rules had to be tested and certified as part of a system. There was no mix and match proviso. Further, the FCC required unique connectors for each manufacturer, and required new connectors to be designed as the existing ones became commonplace.
"Wait," you may ask--"I can go to HyperLink Technologies or other companies and buy antennas with the right connectors and attach them to my Wi-Fi gateway. If it's illegal, how can I buy this gear?" Simple. It's legal to sell antennas; it's illegal to use them. It's the same logic that guides the sale of bongs and switchblade kits. It's opposite to the logic that underlies the Digital Millennium Copyright Act. The antenna/switchblade law essentially says that the seller isn't responsible for all the uses to which a purchaser may put a product. The purchaser is obliged to know local and federal rules and conform to them.
Thus, attach the antenna, and you're a pirate. Assemble the switchblade, and you're a criminal. Forget that there have been approximately zero prosecutions for the use of these antennas on home or business systems. But no one wants to be in de facto violation of a law, especially businesses that may considering building out Wi-Fi as part of their operations. The lawyers might look askance, and the companies might have to pay enormously higher fees to purchase legal antennas--if they're available. Those fees help cover the companies' cost in certifying the antennas as part of a system, but also represent their lock-in market for legal use.
The FCC rule doesn't suddenly make all antennas legal for all systems. Instead, they have chosen a clever middle ground. For new devices--or, presumably for recertification of old devices--manufacturers will be allowed to test the system with high-gain antennas of each major type, like omni, patch, yagi, and so forth. Once the device is certified, the manufacturer can release the characteristics of the antennas they tested for both their in-band and out-of-band signal patterns and strengths. (Out-of-band transmissions are the inevitable but not intentional frequencies that are broadcast on at typically very low levels due to harmonics and other technical radio issues.)
Thus, if Linksys certifies its WRT54G with a very high-gain yagi antenna within the Part 15 rules, then a user can add a lower-gain yagi that has all its parameters within those levels and be perfectly legal.
Jim Thompson provided a longer, detailed explanation via email:
"Let's say you have a (warning, plug alert!) HS3000 that has been tested with a 9dBi omni. Were you to find an 8dBi omni from a different manufacturer, with similar out of band gain (i.e. it doesn't generate more gain in the restricted bands, thereby causing a system that would otherwise comply with the restricted band limits to 'go illegal'), you could use it.
"You could also attach a 2.2dBi omni, as long as it didn't have more gain out of band than the antennas with which the device is certified. You can repeat the above paragraph substituting 'yagi' or 'patch' or 'grid' antenna everywhere 'omni' occurs.
"What you can't do is certify (let's say) with a single 2.2dBi omni, and then have your customer attach a 13dBi yagi, (without recertification), nor could you say, certify with a 13dBi yagi and have your customer attach a 13dBi patch (or omni)."
I have some suspicion that the recent array of Linksys add-on antennas were certified under this new rule and delayed for release until such point that the rule could go into effect. This rule would dramatically reduce the cost of re-certifying gear for more antennas, and it makes it possible for Linksys to sell a huge matrix of their own antennas at no additional testing cost beyond the initial certification. On the other hand, it also makes it easier for third parties to sell antennas legally for Linksys's devices, but sales of legal antennas for illegal uses has seemed to curtail sales before this.
Remember that until a device is retested under these rules--and who knows if manufacturers will pay to retest the current generation of equipment--you're still technically violating the law by mixing and matching antennas. Watch for more news on this front, as devices are certified until these new rules.
The FCC decided to leave the connector rules intact, even though manufacturers argued that it's so easy to get their "proprietary" connectors from third parties, that the current rules just added cost and complexity. The FCC demurred, noting that it wanted to make it just hard enough to make adding an antenna an intentional act. It gets lost in the furor over unscientific concerns about the risk of Wi-Fi and 2.4 GHz electromagnetic radiation that microwaves can injure humans at sufficiently high gain--far, far higher than the Part 15 rules allow.
The FCC said in regards to the connector issue, "...our concern that removing this requirement might have the unintended consequence of allowing uninformed consumers to inadvertently attach an antenna which causes the device to emit at levels in excess of the limits for human exposure to radio emissions." Good enough. It preserves the market for pigtails, that's for sure.
This section also revises the rules about integral antennas for 5 GHz (802.11a and other uses) devices. The FCC will now allow externally detachable antennas for these devices, which adds flexibility.
The FCC tweaked a number of other rules, including one that appears to reflect a change in thinking from its one-off approval of Vivato's beam-focused Wi-Fi--I'm trying to better understand that section. In another part, they tweak measurement standards. Part 5 covers changing rules for frequency hopping to make it possible for future Bluetooth flavors to work legally in the U.S.
Interestingly, spectrum etiquette in unlicensed bands was discussed in part 6, but the FCC declined to take any action. They gave props to Microsoft for a proposal they might implement in the future that would reduce the noise of devices that aren't actively transmitting. Some of these principles are already embodied in 802.11h, which was a required extension for 802.11a to operate in the 5 GHz band in Europe.