Dog Door

Hi all, I have made a dog door opener using sensors to detect the dog inside or outside. I want to have the dog with a collar that a antenna can read instead of the sensors. The dog is large so he can carry a couple of batteries if needed. Does anyone know what i should buy and where? I need some range between the collar and antenna for it to work properly.

Thanks

Sounds kinda like RFID with the antenna being a loop around the door itself.