Right now I have build four 125 kHz RFID reader antennas, so it was time to make a little first comparison, on how they perform. With each antenna I read two RFID tags and measured the read distance of each one of them. The first one is a RFID Card by Sparkfun (8,5 x 5,5 cm) and the second is a smaller RFID tag sticker ( 2,3 cm).

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1. Big Antenna ( 20 cm, 65 turns):

  • RFID card: few lucky readings with 0 cm, with the tag being close to the coil
  • RFID sticker: no reading

2. Medium Antenna A ( 10 cm, 65 turns):

  • RFID card: 7,5 cm
  • RFID sticker: 2 cm

3. Medium Antenna B ( 10 cm, 45 turns):

  • RFID card: 12,5 cm
  • RFID sticker: 9,5 cm

4. Small Antenna ( 5 cm, ~ 90 turns):

  • RFID card: 8 cm
  • RFID sticker: 4,5 cm

The diameter to turns ratio of the big antenna just doesn’t fit to any of the used RFID tags. The medium antenna A showed a decent read range for the RFID card and a rather short distance for the RFID sticker tag. Surprisingly the medium antenna B showed the best results for both tags, with even having less turns than the other 10 cm diameter antenna. The small antenna showed the second best read range results. So what do we learn out of that? Bigger and more turns is NOT better! The antennas depend on many different parameters and it can get really confusing, which one have greater effect on the read range than others. I still have to figure out more about the parameter dependencies, getting more into antenna tuning, especially involving the Q factor more into my precalculations.

Similiar results were found in this article on instructables, which compares different RFID reader antennas.

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