For the LF-RFID experts: 125kHz antenna design and calculations

Hi,

I’m trying to design an antenna for a 125kHz RFID system, however I’m having a bit of trouble to get a solid understanding of all fators that influence a “smart” choice for a RFID antenna.

The ultimate goal is to maximise read range for 8mm glass tags.

So far I have tried different inductance values for my antenna, 0.750mH to 10mH and different Q-values from 15 to 30. Since the LF-RFID tags are inductively coupled, I have also calculated magnetic field strength in the hopes of giving me a clue why different antennas perform differently. Ideally I’d like to find a formula to estimate the best parameters for my antenna.

Tested antennas:

Antenna diameter: 43mm

  • 0.750mH - Q15 - 1.85mT - 80 turns - 19mm read range
  • 2.600mH - Q15 - 1.23mT - 176 turns - 41mm read range
  • 9.900mH - Q30 - 1.13mT - 288 turns - 40mm read range

First thing that confuses me, is why the first antenna performs worse than the second even though it has a higher magnetic field strength.

Each of your antennas need to be resonate at 125 kHz. What did you do the make them resonant?

Paul

Normally you tune the antenna to one side or other of the resonant frequency of the tag by adding capacitors and measuring the resonant frequency. Note that these capacitors must be of a high voltage type as you can easily get in excess of 100V in a resonant circuit even if you are only driving it with 5V.

The trouble with the glass tags is that they have a very small antenna and you can loose the signal altogether if the angle between tag and coil is anything but perfect.

Paul_KD7HB:
Each of your antennas need to be resonate at 125 kHz. What did you do the make them resonant?

Paul

Correct resonance frequency is adjusted through appropriate resonance capacitor, frequency is verified by scope.

Grumpy_Mike:
Normally you tune the antenna to one side or other of the resonant frequency of the tag by adding capacitors and measuring the resonant frequency. Note that these capacitors must be of a high voltage type as you can easily get in excess of 100V in a resonant circuit even if you are only driving it with 5V.

The trouble with the glass tags is that they have a very small antenna and you can loose the signal altogether if the angle between tag and coil is anything but perfect.

My test setup includes a small rod(not metal) to which the glass tag is affixed, a small tube assures that the orientation is very close to optimal. Since the rod is supported by the tube, I can move the tag closer by a few milimeters at a time and see how often per second the tag is read, evaluating read performance.
Resonant frequency is verified by scope for each new antenna and fine-tuned to within 1kHz of target frequency.