I am prototyping an RFID system that can detect a specific object about 50cm away. I've tinkered with the idea of proximity sensors but the issue there is the inability to strictly trigger when a specific object is detected. I started reading about RFID tags and figured that could be my solution. I've seen DIYs of tags in animal collars so that pet doors are opened when the collar is detected, and my system is something similar. The main difference would be the range. The object would essentially have the tag. I need my reader to be about 50cm away, and technically the signal can't be linear since the object can be in different locations within that 50cm radius. Also, the tag should be thin, not bulky so I would prefer avoiding an active tag unless I don't have a choice.
As much as possible, I'm trying to keep the reader compact, preferably can fit in a 2x4 inch enclosure that is also only about an inch thick.