A subspace emitter coil was a component of Danube-class Starfleet runabouts in the 2370s.
In 2370, in preparation for his and Major Kira Nerys' mission to Cardassia IV to rescue Li Nalas from the Hutet labor camp, Chief Miles O'Brien installed field buffers around the USS Rio Grande's subspace emitter coil to make the runabout look like a Lissepian transport to Cardassian sensors. (DS9: "The Homecoming")