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