Not Quite Normal

CATaLOG | Adding an external antenna

Originally the RFID reader was mounted on the bottom of the cat flap on the inside of the door.  This proved problematic for a number of reasons.

  • The reader didn't always read the tag
  • In windy and rainy conditions the reader could get wet

I decided to disable the onboard antenna and attach an external one to try to increase the range - as well as protect the reader from the elements (and cats), using this howto as a rough guide.

I already had an external antenna from the dodgy RFID reader I tried to use before I bought the Parallax one, so I cut the traces and soldered the external one in place. Unable to find my uber fine soldering iron bit, this proved quite a challenge. You need to be get enough heat on the trace to melt the solder well, but not melt / overheat any of the tiny SMT components. Once done, I ended up with the setup below - the PCB antenna section above the wires can be cut off to reduce the size of the reader if need be.

external antenna

Happily the reader still worked and the system was more reliable but still not good enough.  Sometimes the cats run in so quick, the reader just isn't powerful enough to read the fast moving tag.  To try to solve this, I made a new - slightly smaller coil with more turns, to hopefully make the signal more concentrated in the area where the tag will be.

I used 34 SWG copper wire to make the coil, and a toilet roll as a former to wrap the wire around.  The inductance of the coil is important - even at 125khz.  The first coil I made didn't work at all, so I used a coil inductance calculator to give me a rough idea of the number turns I should be doing. The target inductance from the howto above and some PDF specifications for other 125khz antennas suggested it should be between 500 and 1000 uH. Entering the standard UK bogroll diameter as 45mm, and the length as 5mm (as it's just a ring of wire effectively), told me 93 turns should give me around 700 uH - a good middle figure to try.

The new coil is the one on the right - the old coil is the one on the left and is slightly mangled as it was held together in a square with hairspray and I had to bend it to fit it on the catflap.

new antenna

The new coil works better than the old one - I've also mounted it on a marmite lid to make it closer to the tag as the cat passes through, but I think it's still missing the cats 5-10% of the time.  The next thing to try will be more turns to try to make the signal stronger, but still get the inductance within the 500 - 1000uH range.

← Return to CATaLOG project page