The location of the antimatter generator in the secondary hull of the Enterprise-B
The antimatter generator was a type of power generator used on the Excelsior-class starships in the late 23rd century. It was located on the lowest decks of the secondary hull, in the vicinity of the antimatter fill port and antimatter storage.
In 2293, the location of this generator was labeled in the USS Enterprise-B master systems display on the ship's bridge. (Star Trek Generations)