In rural areas, as little as 36MHz of the nearly 300MHz allocated for broadcast TV is used, Genachowski said. Even in large cities, half of the broadcast TV spectrum is typically unused, he said Wednesday.

Genachowski's announcement received a cool reception from the National Association of Broadcasters (NAB), with the trade group noting that TV stations turned over a quarter of their spectrum in the digital TV transition. The FCC sold that spectrum, in the 700MHz band, in an auction that ended in March 2008.

"As a one-to-many transmission medium, broadcasters are ready to make the case that we are far and away the most efficient users of spectrum in today's communications marketplace," Dennis Wharton, NAB's executive vice president of media relations, said in a statement. "We look forward to working with policymakers to help expand the roll-out of broadband without threatening the future of free and local television, mindful of the fact that local TV stations just returned more than a quarter of our spectrum following our transition to digital."

Genachowski, in Wednesday's speech, didn't focus on how to reuse government-held spectrum. White said he understands why Genachowski avoided the issue. Getting government agencies to give up spectrum will be difficult, he said.

While TV broadcasters hold less than 300MHz of spectrum, U.S. government agencies have exclusive rights to more than 620MHz of spectrum, and the government shares another 970MHz with commercial users, the said.