I took a magnetron as inspiration. The cavity resonators kick the RF into subspace radio frequencies channeled through the coil by waveguide and energizing the coil to create the subspace field. The grill glow comes from a waveguide horn coupling to the opposite coil in the other nacelle, an idea I'm borrowing from Probert's claim of how Roddenberry explained the process for TMP (-a Trekyards episode.) One coil would have negative charge, the other a positive, and maybe cycling charges helps control the propulsive effect of the fields. I'm a Gene's Rules kind of guy even if much of Star Trek isn't. This is one possible method out of many.