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.

Duffy had discovered that over-the-air digital signals behave differently than analog signals. That won’t matter to most Americans come the switch. 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 on top of the TV set. 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.

Estimates from a computer simulation run by Centris, a market research firm in Los Angeles, found that more than 9 million households that now get programming over the air could lose one or more stations they now receive.

Although digital broadcasts will provide a superior picture and more channels than old analog broadcasts, digital reception is more easily blocked by hills, trees and buildings than analog reception. Furthermore, analog degrades gradually, with the picture displaying snow or ghosts (image echoes) as the signal becomes weaker.

But the digital signal stays uniformly crisp until the signal gets weak; then the picture suddenly drops out, a phenomenon that engineers call the “cliff effect.”

An antenna is a simple device, an assembly of metal rods that turns electromagnetic waves into electrical currents. They come in various sizes and styles because the length of the rods are designed to catch particular frequencies.

“The bigger the antenna, the more bars, the better at receiving a weaker signal,” said Dave Wilson, director of technology and standards for the Consumer Electronics Association. “The main reason for most of the different shapes is that many times you want the antenna to not just convert the electrical field, but to amplify a weak signal, or ignore an interfering signal.”

For a simple device, the antenna has generated a lot of misinformation. For example, you may hear about digital antennas or HDTV antennas. But you do not need an HDTV antenna to get digital high-definition signals. Digital HD is broadcast on the same UHF and VHF frequencies as analog TV. The antennas need not be different. A special antenna is not required to receive the new extra channels from a station. If your antenna receives the local PBS channel, for example, it will receive all the digital subchannels.

For those locations suffering the cliff effect, a new antenna is needed to pull the signals in. For TV reception on UHF and VHF frequencies, an indoor antenna like rabbit ears may work if the signal is very strong. A weaker signal may require a disc-shaped outdoor antenna that receives signals from all directions. An even weaker signal will require a V-shaped outdoor antenna, perhaps with a motor to point it directly at each broadcast signal tower.

Before buying an antenna, buy the converter box and attach it to your old antenna and to your TV. You may get all of the stations you want. (The boxes are available at electronics stores. You can get a coupon for a $40 discount at dtv2009.gov, but don’t wait too long; there is a limited supply of them.)

As Duffy found, picking the right antenna can require some experimentation. After the rabbit ears, he tried different receivers and antennas in different locations. He graduated from a $25 indoor antenna that didn’t solve the problem to a $30 outdoor antenna with a 40-inch boom mounted on the garage. Reception is now good, he said, but not perfect, which has him thinking about a rotating antenna or a signal amplifier.

The Consumer Electronics Association and the National Association of Broadcasters sponsor a Web site to help people through the process, antennaweb.org. The site uses software that determines which aerial you need based on your location relative to station antennas. It also advises you where to point it.

Also check antennaweb.org to make sure your antenna suits the post-February frequency. Some digital stations that are now available may move from UHF to VHF after February 2009.

At the site, click on the “choose antenna” button, then fill in your street address. The site organises types of antenna by a color code, from yellow for low-strength multidirectional antennas to violet for large directional antennas that may also need an amplifier to pull in weak signals.

It is easier finding the right spot for an antenna. “The best placement is outside on a mast as high up as possible,” said Wilson of the Consumer Electronics Association. Make sure the antenna is properly grounded. (Another suggestion: Make the switch before the February changeover; it is not good to walk on an icy roof.)
Not a do-it-yourselfer? Antennaweb links to techhome.com, which can locate an installer in your area. But buyer beware, though the installers are on the association Web site, they aren’t required to be certified, bonded or experienced to get on the list.