Broadcasters are questioning the viability of the Defense Department agreeing to turn over a portion of its spectrum to wireless carriers. The plan, which officials estimate would cost $3.5 billion, says the Defense Department would share spectrum with broadcasters, but no specific plan or timetable has been released. "At first blush, the DOD proposal doesn't seem to make much sense," NAB spokesman Dennis Wharton said. "While we are not opposed in principle to sharing where it works, DOD has done none of the legwork necessary to even begin the conversation."