The Civil Alert System Broadcast is being broadcasted from a ham radio inside the military bunker at the base of the satellite dish. You can turn off the ham radio inside to disable the signal. Inside the bunker you’ll find locked ammo box and crate.
Way more fun to explore and a whole lot bigger in the Automatron DLC...depending on your level(60 or better) there is a X-01 full suit of armor behind a locked cage or at least a T-60 suit.
Found the site early on(level 30 or so) and got just as far as the bunker main room. Wasn't until I was into the Automatron DLC that it opened up a whole new underground world full of raiders, robots, etc. Lots of loot to scavenge after clearing it.