On Monday, something began to happen that is either startling or irrelevant, depending on your point of view about what was once the dominant form of communication in America: over-the-air television.

On that day, the broadcast signal for WMFP, an independent television station in Lawrence, Mass., that can be seen in southern New Hampshire, will disappear from channel 18, and the channel will merge with WWDP in Norwell, Mass. No big deal, except that this shift will represent the start of a national wave of channel changing.

By 2020, almost 1,000 stations, at least 25 of which can be picked up by antennas in parts of New Hampshire, will have either moved to new channels, including such Boston stalwarts as the CBS affiliate WBZ and public television giant WGBH, or have disappeared entirely, as did WBIN in Concord last year.

This will be unprecedented in the history of American broadcasting – but in the era of cable television and Internet video streaming, does it really matter? Who watches TV over a home antenna anymore?

Plenty of people, says Gary Underwood, owner of New England Antenna in Epsom, and for one good reason: Over-the-air television is free.

Underwood has been selling and installing antennas of all types since 1990 in a business that his father, Ralph, started in 1949. He says interest in TV antennas perked up after the national switch to digital broadcasting in 2009, and has only grown. These days, he estimates that 90 percent of his antenna customers are buying one for the first time, rather than upgrading.

The National Association of Broadcasters says that viewership of broadcast TV has been growing slightly recently after years of decline. And while such industry organizations aren’t exactly objective when examining their own business, he said data from the Nielsen TV ratings company says about 77 million people still watch TV through antennas.

“It has grown over the last three or four years, and streaming services are a lot of what has spurred this. They’re paying monthly subscriptions for Hulu or Netflix, then looking around … and realize they can watch the local stations over the air,” said Steve Gardner, spokesman for the National Association of Broadcasters, an industry group. “Many of us switched away from it at some point for cable, but are coming back. It’s different for millennials; for them this is kind of new and exciting to discover: You can watch television over the air for free!”

So if viewing is starting to rebound, why are some channels making it harder by moving around?

Blame digital broadcasting, which shrank the size of over-the-air signals when broadcasters had to switch from analog technology in 2009. This change freed up much of the broadcast spectrum, the set of frequencies that were assigned to different broadcasters back when the television system was created.

The federal government has since auctioned off most of that freed-up spectrum for roughly $20 billion. Most of the different frequencies will be used by companies to provide wireless Internet and cell phone service. The broadcast channels who use the auctioned portions of the spectrum will have to shift position out of it.

“This is not determined by how big or how small your station is, just your place in the frequency spectrum. It affects network stations and independents,” Gardner said.

The actual effect on viewers of channel switching is less than it would have been a decade ago before “smart” TVs came on the market. If a channel disappears, then running a rescan with your TV or converter box will find it, assuming that the new frequency is accessible where you live.

That last proviso is one of the weaknesses of broadcast television, especially the further north you go in New Hampshire. Although antennas have improved, they can only pick up signals that reach the area.

“Around Concord isn’t the best for reception, unless you’re in a high area,” Underwood said. “Around Concord you sometimes can’t get the Boston networks, and all their subchannels. If you live south of Concord – Manchester, southern New Hampshire, northern Massachusetts – you can get about 35 to 40 channels.”

The channel switch will take place in phases between now and 2020, because it can be a pretty complicated process for the stations themselves. Some moves can only take place after an earlier move has happened in order to free up the spectrum space, and there are mechanical bottlenecks.

“Some stations have to physically alter the tower. There are only about 12 crews in the entire country that know how to do this, who are trained and equipped to make these changes. Part of the reason for sequencing is the availability of tower crews to make these changes,” Gardner said.

Depending on which channels you watch and when they’re scheduled to change, Gardner said, “You may have to rescan your television as many as 13 times to get all the channels.”

But according to Underwood, you almost certainly won’t have to replace your over-the-air antenna after a channel switch.

“Antennas that are 35 years old will work, as long as it’s a high-gain antenna. You can use the same antenna you used for analog,” he said.

The one exception involves WGBH, the PBS station in Boston that many still think of being Channel 2 even though it’s been on Channel 19 for a while. It will be moving so far down the spectrum that some recent antennas, known as VHF high-band antennas, may not be able to access the signal. In that case, buying a VHF low-band antenna may be necessary.

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