Broadcast tower LP8
There's a radio antenna and a generator enclosed behind a metal fence, the latter's switch can be turned on to emit Signal Echo Foxtrot. The sealed cistern entrance is at the base of the hill on the northeast side (facing the monorail / highway ruins).
Inside the sewer chamber, there is a copy of Big Book of Science.
This is a random encounter location. The exact spot is on the north-northwest corner of the gate that surrounds the broadcast tower, and will be triggered within a radius of approximately 20 - 25 running strides.
Broadcast tower LP8 appears only in Fallout 3.