Low-power communications transmitters can be operated in the United States without radio licenses, provided that their power is low enough that they will not cause interference. Garage door openers, wireless LANs, wireless headphones and motion sensors are all examples of unlicensed low-power transmitters. Part 15 of the FCC Rules provides the details on allowable power levels, and permissible uses. Part 15 lists a wide range of frequency ranges and power limits.
However, for the most part, the frequencies allocated for TV broadcasting may not be used for unlicensed, low-power transmitters. This is because the FCC decided that widespread deployment of consumer products like garage door openers and wireless headsets would eventually pollute the radio spectrum and cause interference to TV reception. This would be particularly likely at the edge of a TV station's coverage area, where the signals are very weak. There are two major exceptions. Biomedical telemetry devices may operate under Part 15 on TV channel frequencies, and wireless microphones can also operate in these frequencies.Biomedical telemetry
WFAA, which operates on channel 8, has been assigned channel 9 for digital broadcasting. When it began channel 9 transmissions on February 27, biomedical telemetry transmissions in the 186–192 MHz band occupied by channel 9 began to receive interference. The telemetry receivers began receiving signals at much higher power than expected, with an unknown signal format, so they basically stopped working.
These telemetry systems are allowed under FCC rules to operate only indoors, and only in health care facilities such as hospitals. They are used to monitor blood pressure, respiration and other patient data. They allow monitoring while patients walk around. They allow one health care worker to monitor several patients remotely, which decreases health care costs. Using them only indoors should decrease interference. And the transmitters are usually tunable, so that in case of interference, they can be tuned to a different frequency.
At any rate, that was the FCC's logic in allowing these products to use TV frequencies, even though you might be able to conceive of a scenario where interference of this sort could cause loss of life. But the FCC figured that hospitals (and their engineers) would know which frequencies were free of interference. That used to be the case, but with digital TV stations coming on the air over the next few years, that's just too optimistic.
The Dallas TV station shut down its transmissions temporarily, once the hospitals figured out where the interference was coming from and notified the station. The TV station had no obligation to do this. One of the key requirements of FCC Part 15 is that unlicensed transmitters have no interference protection rights. Licensed stations can continue operating, and unlicensed devices must accept whatever interference comes along. Most business users of Part 15 devices, such as wireless LANs, are unaware of this limitation.
At any rate, what finally happened in Dallas was that one hospital retuned its transmitters, and the other hospital decided to buy new equipment and now also operates on different frequencies (in this case, channel 12 at 204–210 MHz). But it took a while, after the displays went dead, to track down the interference.
The problem arose in Dallas because the hospitals didn't know that a TV station would start broadcasting on a channel that had always been unused. And the TV station didn't know that any hospitals were using transmitters on its new frequency. Even if it had known, it had no obligation to protect them. The obligation was, and is, on the Part 15 user. The American Hospital Association is notifying its members of this problem. But will it provide hospitals with a listing of what TV channels the new digital television stations will be using, and when?
Wireless mics also operate on TV broadcast frequencies. These devices aren't supposed to be unlicensed, but they are widely sold and used illegally, without a license. Legal users include broadcasters, cable TV operators, TV program producers and movie producers. Illegal users include most rock concerts and live theater performances. (Some wireless mics are perfectly legal for use by anyone, but not those that operate on TV frequencies.)
Wireless mics operate at higher power levels than the medical telemetry transmitters, so maybe they will be less susceptible to interference. And they are often itinerant. But many are fixed and used night after night at particular theaters. I've got tickets to see "Showboat" in July. I'll let you know if I hear anything.