Back when I was in college, I built a sound generator that turned on when my carport came on at night. Joys of being an electrical engineering student
I spoke to an instructor at UC Davis vet department about what frequencies that cats find annoying which are outside the human range. With that knowledge in hand, I set to work on a building the generator.
I installed it over my car (a 1990 Chevy Lumina Z34.........what a POS that was!) and pointed the speaker straight down from the roof of the carport.
This did the trick! I could watch the neighborhood cats cross the street when they came to my driveway rather than go near that carport. This looks to be something similar that is available commercially.
http://www.contech-inc.com/products/catstop/
Hydramada
SPF1867