In the film First Contact, when in the past, the Borg from the future try to contact the Borg in the past's time period by building a transmitter out of the particle emitter on the main deflector dish. The particle emitter is detached from the deflector dish by Worf, Picard and Lieutenant Sean Hawk, and then blown up, to stop the Borg transmitting their message.
Now, fast forward to the end of the film - the crew of the Enterprise have to get back to the future, so Picard instructs whoever it is on the bridge to emit a reproduction of the inverse tachion pulse that opened the temporal vortex that the Borg sphere & the Enterprise went through to get back to the past at the near-beginning of the film.
My question is: With the particle emitter on the main deflector dish destroyed, how exactly did they emit this pulse? Do they carry spare particle emitters on board (doubtful - it's a pretty large piece of equipment to carry spares for), or did they construct a new one somehow? Or do they have a secondary deflector dish powerful enough to do the job? Or is it just a gaping plot hole?...