Sometimes a bad idea just won’t go away. Case in point: the proposal to deploy portable unlicensed devices on locally vacant TV channels. (This is often called the "white space" proposal because a spectrum map typically indicates unused frequencies in white.)

The FCC has already approved, in principle, the use of fixed unlicensed transmitters on vacant TV channels — for example, to deliver broadband Internet to homes and businesses. This is not a bad idea, and in fact may offer an excellent way to bring broadband to areas beyond the reach of DSL and cable. An IEEE standards committee, led by one of the engineers responsible for Wi-Fi, worked out a promising scheme to ensure that fixed operations do not interfere with TV reception. Even the broadcast industry, which guards its spectrum with the greatest zeal, has approved the IEEE plan.

But for one group of companies — Microsoft, Google, HP, Intel, Dell, Earthlink, and Philips — fixed use is not enough. Consumers, they say, should be able to operate portable unlicensed devices on vacant TV channels. This is potentially a much greater threat to TV reception. A consumer using a portable device on channel X, in a city where that channel is vacant, might carry the device to another city where channel X is in use. If the device keeps operating on the same channel, it will interfere with TV reception at the new location. Not a problem, say Microsoft et al. — we’ll just build a unit that listens for a TV signal on channel X , and if it finds one, automatically shifts to a different channel. To prove this is possible, the companies built a prototype and sent it off to the FCC engineers for testing.

One problem: the prototype did not work. It failed to detect on-channel TV signals, and if used in actual practice, would have caused interference to viewers watching that channel. The Microsoft group had a ready explanation: the prototype was broken. The companies came close to accusing the FCC engineers of causing the damage when they took the device out of the box.

But there is another possible explanation for why the device failed. To detect a weak signal requires a physically large antenna and a high-quality receiver. Some TV viewers install outdoor antennas that are several feet in size, and use special boosters to amplify signals from distant stations. A portable unlicensed device, in contrast, has a small antenna, a few inches long at most, and inexpensive receiver circuitry. It is no surprise that the unlicensed device could miss a TV signal that might be viewable with the right antenna and receiver. And having missed the signal, the unlicensed device will drown it out.

Late last week, the FCC chairman invited the companies to submit another prototype for testing. Whether such a device can work as needed is a straightforward engineering question, one that can be answered with considerable certainty, one way or the other, through technical analysis and experiment. But just in case the answer comes out wrong, both sides in the controversy are stepping up their public relations and lobbying campaigns. This is Washington, after all, where politics trumps science nearly every time.