For 20 years we have always had a family dog, along with cats, so we never had a problem with cats coming onto the property except at night and if we let the dog out then the cat would be gone in a flash. Also we had a couple of neutered male cats that were very good at guarding our property.
But in the last few years, we decided not to have another dog and to just enjoy our cats. This has created a situation where 2 neighborhood cats want to come over and fight with our cats, and our cats have sustained a few injuries.
I have had to become a lot firmer with the trespassing cats to let them know that they are not welcome on our property. This has helped somewhat as the cats are learning that our property is not available for them as part of their territory.
A couple of effective techniques include banging on a tin plate when I see them and spraying water towards them. I think motion sensor water sprinklers or sound alarms are a great idea if you don’t have cats yourself.
It doesn’t take them long to learn how serious you are about the boundaries of your space. My love for animals has made my resolve weak in the past, but as I have witnessed the distress it causes my 4 cats, and the subsequent injuries, my resolve has strengthened.
I noticed a helpful write-up from our city about steps to take if cats are not welcome and they mentioned a few good ideas that I might be able to use. One was using pvc tubing on top of the fence and the other was putting up some brackets with netting that is angled outwards to prevent cats from coming into our yard.
Here is a humorous video about a man who decided to work with the pvc tubing concept to keep his cat inside his yard space.
Purrfect Fence has a fence extension product that could be used to keep cats out of the yard by mounting the brackets for the levers to angle outwards. If they are angled inwards then they will effectively keep your own cats inside the yard and will likely keep most cats out. Also Cat Fence-In has a product that is designed to keep cats in and cats out.
Keep in mind that there are some fence height restrictions. Check your city or rural bylaws, for fence height, type and easement regulations. Most people have enough space to add a portion on the top of their existing fence to make a cat proof fence.