The USS Enterprise-E's particle emitter, located in the center of the navigational deflector
A particle emitter was a component of Starfleet vessels, located in the middle of the navigational deflector. It was used notably on Sovereign-class starships.
The Borg building an interplexing beacon on the Enterprise's particle emitter
In April 2063, the Borg began to construct an interplexing beacon on the USS Enterprise-E's deflector to contact reinforcements from the Delta Quadrant. While assessing the Borg's actions in environmental suits, Captain Jean-Luc Picard told Worf and Hawk that it looked like the Borg were placing the beacon on top of the Enterprise's particle emitter. (Star Trek: First Contact)
In the Second Year of Hell, Kathryn Janeway stabilized the particle emitters of the USS Voyager from deflector control, allowing Tuvok to generate a deflector field that protected the ship from a micrometeoroid storm. (VOY: "Year of Hell, Part II")