Anti-Air unit don't have a range at all.
However once any air power attacked your army and that army that you have has an anti-air it will give you a huge advantage and the air power of your enemy will be likely destroyed
And there is something else i noticed about anti-air that is if your enemy is patrolling over your land and that land has an anti-air the anti air will attack this aircraft of your enemy.
I hope that helps you, and sorry for my bad English .
