The Plumber's Choice 4 in. Anti-Siphon Sillcock Frost Free Outdoor Faucet with 1/2 in. MIP/Sweat Connection and 3/4 in. Hose Bib SILLVAC4 - The Home Depot
Anti-Siphon Valve Basics | BoatUS
plumbing - Leaky anti-siphon valve on exterior faucets - Home Improvement Stack Exchange