Shep asked for it to be done and Zalenka(?) made it happen within seconds, so that suggests easy manipulation of the shield execept it's pre-programmed state.
Also, when the city shield shrunk itself to protect different parts of the city, it provides further proof that shrinking the shield to a certain place is more than possible but has infact happened.
Basically the emmiters were specifically designed for that use. And if you detonated a gate ontop of the cityshield, the shield would only take on about 60-70% of the damage IF that, considering the way the gate explodes.