Hacksaws are what you build if you don't want Ravens to kill a building, by the way.
You'll probably need quite a few of them, placed so they have time to hit the ravens as they come in. They do enough burst damage to kill one raven each, so build appropriately. Obviously this isn't a sustainable long term defense, and there's ways around it (perhaps even literally -around- it, they are very short range) but they're also significantly cheaper than a raven and do a pretty good job on anything else that comes close enough too.
A little later on, you can build a few chainsaws and they tend to ensure that any planes that get close enough at the very least will take losses - but planes are supposed to be able to go into your base and kill things, that's their intended role.
Your air defenses influence a) how many planes it will take, and b) how many planes will come back.
Razors are lovely, but I view them mostly as tough, light, anti-gunship AA, since they just don't do damage fast enough against planes. Threshers are a great multi-purpose AA, which you have to pay appropriately for - they're the Stinger of AA turrets.
Faraday are also AA. Howso? Planes stop when they get stunned. -Everything- can hit a stopped plane, including a stardust.
