The origin of the broadcast is a ham radio in the basement of a small trailer northwest of the relay tower. Go toward the broken section of plane, and it is just over the next ridge.
The trailer is guarded by a single raider. There is a tiny bunker accessible from a hatch on the south west side of the shack. Inside is the ham radio broadcasting the signal, a single dose of Psychotats, a cooler with random contents, a single 10mm round, and a baseball bat.
Raider radio signal appears only in Fallout 4.