Need help finding right ID tag and reader

I'm helping a friend build an automatic dog door. He would like it to open when his dog stands nearby and he thought RFID might be a good option. Researching, My first idea was to use RFID technology but all that I could find was passive rfid witch had alot of arduino support but only had about an inch of range and active RFID that had way to much range and little arduino support that I could find. I tried looking at a few other tag ID solutions but none of them seemed like they could work.

I understand that I may have to build and tune my own antenna (which I think I can do if necessary). Are there any suggestions for inexpensive RFID modules that I can attach an external antenna to?