If you have a 125KHz RF reader that allows an external antenna, you can build it to fit your needs. In my case the activation tag on the cat's collar is not always hanging nicely down, but sometimes hangs on the side of his head. If your reader has a small range it will not always detect the cat even when he is in the exact same place every time. I chose to make an antenna large enough that the cat can stick his head through, which activates the reader almost 100% of the time.
Guides for the construction of antennas are available for download on the web (e.g. http://ww1.microchip.com/downloads/en/devicedoc/51115f.pdf
). They help quite a bit, but I still ended up just trying a bunch of things to see what worked best.
An antenna coil is as simple as this: http://img101.imageshack.us/i/coil2sm.jpg/
, just a wire wound up a bunch of turns with the two ends connected to the RF reader.
I used 24 Ga magnet wire and a 1.25" diameter tag to test the antenna.
The following worked well (range is in distance from the plane of the coil):
1.6" diameter, 90-110 turns, no range measurement
3.1" diameter, 75 turns, no range measurement
4.1" diameter, 48 turns, no range measurement
6" diameter, 43 turns, range 3-3.5"
9" diameter, 35 turns, range 4"
10" square shape, 31 turns, range 1"
So the largest range was with a 9" diameter coil. Larger than that, the range first went down to zero and then disappeared from the center. If you draw power from the Arduino, use the 9v adapter, not USB only, because the latter provides less power. Also important is when the coil is complete, the wire should be guided away from the coil.