Looking for a short range detection and identification
Why such a huge range?

When I built my smart cat door 40 years ago I used RF. I used a 10nF polyester capacitor, and wound a few turns of wire around it, and potted it in resin, including a small hook to attach to the cat collar to make a dongle for the cat. Then built an oscillator at its resonant frequency. I built a flat search 'coil' using veroboard (for stability) with copper wires soldered between the strips, total size probably 25x50 mm. With the cat's head in contact with the flap, the dongle was within 100mm of the bottom of the cat flap, where the search coil was. That's enough coupling to increase the loss of the coil, especially since the orientation of the cat approaching the flap ensured the collar, and hence the dongle, was coaxial with the search coil. Rectify the voltage across the coil, trigger on a dip. The point about using a resonant dongle was it could be relatively small and light.