Willis Tower

A file photo of the Willis Tower, taken in January 2012. (Antonio Perez, Chicago Tribune / September 30, 2012)

The installation of a new television antenna atop Willis Tower knocked out some over-the-air TV service Sunday morning.

Stations that broadcast from the tower turned off their signals around 6 a.m. to allow a helicopter to lift parts of a new ABC-7 antenna onto the 110-story building. The maneuver was complete by 9:30 a.m., said ABC-7 engineering director Kal Hassan, and he believed most stations were back on air by 10:20.

The process didn't affect cable viewers, but did pose problems for people who receive their TV channels through an antenna.

Rhonda Sawtelle of Bloomingdale said she woke up Sunday to find many of her stations off the air. By late morning, most of her channels were back.

ABC-7 stayed on the air because it has a backup antenna at the John Hancock Center.

Hassan said the lift was planned in advance and originally scheduled to occur during the Olympics. That date was pushed back, he said, because it would have meant forcing NBC off the air during the London games.

mitsmith@tribune.com

Twitter: @ChicagoBreaking