Is that really the same? Can you fire off a gun in your backyard at the sky? Isn't that illegal as the bullet comes back down to earth it can kill.

Meanwhile ultrasound is just lot's of sound waves and doesn't go far, and can't kill anything. I can fire ultrasound up into the air without a license or it being illegal.

But doing it intentionally, whether as a triggered booby trap or an aimed shot, and taking down a drone would likely still be illegal if shooting a drone down with a gun is illegal.

I can’t remember for sure whether it is illegal to do so over your property, but I think it is.

Are you allowed a high fence (assuming neighbours don't object)? What if it's virtually invisible, like nylon fishing line? What if the fence is incorporeal, like an ultrasound "barrier".

Fence height restrictions are quite common. Not sure about whether they are limited to "visible" fences. I doubt it.

Use a defensive drone that fires vertically upward into the airspace over your property?

> can't kill anything

Unlike a large drone fully powered up hitting an unsuspecting person.

Or you can use a stick

https://www.youtube.com/watch?v=uhbuEUoxntQ