Interceptors already do your idea, but radars are a whole different thing.
Perhaps you could make it so that unless it’s over a unit they cannot see any planes, but if you build a radar (make it a cheap and quick type of building that only takes an hour to make) then you can see any plane over that province.
A better idea that would be less broken thought it to make it more expensive and cost more, but allow you to do more damage against planes over that province maybe?
