Antenna Circuit Design for RFID Applications” is a great work on RFID antenna design by Youbok Lee (Microchip Technology Inc., 2003). It provides all the mathematical equations for calculating different parameters for many different antenna and circuit setup designs. Furthermore it sums some other really helpfull tips:

Two tuning methods:

  • Voltage Measurment Method: Set up a voltage signal source at the resonance frequency. Connect a voltage signal source across the resonant circuit. Connect an Oscilloscope across the resonant circuit. Tune the capacitor or the coil while observing the signal amplitude on the Oscilloscope. Stop the tuning at the maximum voltage.
  • Impedance Measurement Method (S-Paramenter): Set up an S-Parameter Test Set (Network Analyzer) for S11 measurement, and do a calibration. Measure the S11 for the resonant circuit. Reflection impedance or reflection admittance can be measured instead of the S11. Tune the capacitor or the coil until a maximum null (S11) occurs at the resonance frequency, fo. For the impedance measurement, the maximum peak will occur for the parallel resonant circuit, and minimum peak for the series resonant circuit.

Relevant parameters for the read range of RFID devices are:

  1. Operating frequency and performance of antenna coils (determined by antenna & tuning)
  2. Q of antenna and tuning circuit (determined by antenna & tuning)
  3. Antenna orientation (determined by antenna & tuning)
  4. Excitation current (determined by reader module circuit)
  5. Sensitivity of receiver (determined by reader module circuit)
  6. Coding (or modulation) and decoding (or demodulation) algorithm (determined by communication protocol)
  7. Number of data bits and detection (interpretation) algorithm (determined by firmware)
  8. Condition of operating environment (electrical noise), etc.