The ranger emergency frequency is a low-powered radio broadcast used by Reilly's Rangers (specifically Butcher) to call for help from their position atop the Statesman Hotel. This is a critical portion of the Reilly's Rangers quest, and is one way to begin it, the other being to talk to Reilly herself in Underworld's The Chop Shop.
The range of this station is extremely limited to a small radius around the Our Lady of Hope Hospital and Statesman Hotel where the Rangers are trapped. It can also be received right outside The National Archives. stand close to the door of The National Archives to get the signal. One can also get the signal near the Metro Central exit on Pennsylvania Avenue.
The station only broadcasts the distress message one time, before cutting to static. It will always begin the message as you activate the station and before having VERY long pauses of static (presumably because the Rangers are stuck fighting). The message will eventually restart, but this can take a very long while, so pay attention when it first plays.