I'm looking for the cheapest way to detect persons wearing a tag while they enter/leave a room. I think I can do this with an antenna inside door mounting (or on the top of the door) and HF passive tag (maybe badges), but I need a reading range of 80cm/1 meter.  I would like to understand what are the appropriate components (antenna, amplifier, receiver) I need to get the code tags found, then pass it to a micro (thought to Arduino) for storing and communicating with the server via ethernet or with a concentrator via rs485. I found several commercial products but they are too expensive for the reality in which they are to be used.

Any suggestions would be welcome. Thanks in advance

Andrea