There are two often used ways of avoiding RADAR (RAdio Detection And Ranging): Stealth and Jamming.
My previous blog post covered stealth. This one will cover jamming.
World War Two era planes weren't equipped with stealth technology to avoid radar, because it didn't exist yet. The air forces of the world had to figure out ways to avoid radar, and thus they figured out how to jam radar.
World War Two era bombers were easily picked up by radar, so to confuse the towers, the planes released aluminum chaff. From the tower's point of view, all the signals from the chaff looked the same as the signals looked from a squadron of bombers. by jamming the tower with false signals, the airplane escaped without being tracked by the tower.
Modern jammers work differently, but have the same purpose. modern radar jammers spam the radar source with false signals on the same frequency as the airplanes flying in the range of the radar. The towers still pick up the planes, but they can't distinguish the fake and real signals.