MIKE DUFFY wanted to be ready to switch from over-the-air analog TV broadcasts to an all-digital signal that will occur across the country next February. His Chattanooga, Tenn., hilltop home was nearly in the line of sight to a nearby TV station antenna and he had never had a problem getting analog TV signals.

He assumed he would have no difficulty with digital signals. So he hooked up a digital converter box to his old TV and attached the rabbit-ears antenna he had always used. His new digital picture was sharp and detailed  for as long as he remained sitting in his chair.

When he stood up the picture would deteriorate into blocky images or disappear completely. “It did seem to be affected by the position of people in the room.” he said.

Mr. Duffy had discovered that over-the-air digital signals behave differently from analog signals. That won’t matter to most Americans come the switchover. They either own a digital TV or get their signal through cable or satellite. But 17 million households have an analog TV that receives its signals over the air  13 million of them use a rabbit-ear antenna. That means that not only do they have to buy a digital converter box; they may have to buy a new antenna. An additional 18 million homes have TVs that get over-the-air signals, but the residents have other TVs that are connected to cable or satellite services.