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How to Measure Antenna Parameters with a Mini-VNA

How to Measure Antenna Parameters with a Mini-VNA

Mar 11, 2025

Intro

The goal of this page is to show how a mini VNA-like tool, can be used to measure the typical NFC Loop Antenna. Those parameters together with the provided QucsStudio model, refer to the screenshot here below, will provide a easy way to match the NFC output stage:

QucsStudio model PTX105R_Eval_1v0_SP_0306.sch
Qucs Studio QucsStudio Homepage (qucsstudio.de)

RS_ANT (Antenna Serial Resistance) and XS_ANT (Antenna Serial Inductance)

  1. Setting Up the VNA:

    • Set the VNA format to Smith mode
    • Set the start and stop frequency to 10-16MHz
    • Perform a single port calibration
    • Include cable and whatever is necessary to connect the antenna in the calibration 
    • Set a marker at 13.56MHz to display the R+jX values (RS_ANT and XS_ANT respectively)
    • Connect the antenna (with NO matching component)
    • Read out the antenna parameters 

  2. Measuring ANT_SR (Antenna Self Resonance):

    • Set the stop frequency to 200MHz
    • Perform the calibration again and connect the antenna
    • Identify the parallel resonance as the first horizontal axis crossing on the right side
    • Use the marker to read out the frequency. If the VNA yields multiple loops, lower the stop frequency and repeat the process, re-calibration is required each time
    • Read out the frequency by moving the marker in the crossing position
  3. Plug the measured parameters in the QucsStudio model
    2. Press the simulation button 
    3. Read the resonance frequency from the Smith Chart
    4. The goal is to find out the capacitors value in order to have exactly 13.56MHz as resonance frequency, as you see here below:

Note


  • While the measurement remove the antenna matching components: remove matching components to avoid measuring the antenna plus components.


Additional Notes:

  • In the QucsStudio antenna model, CP_ANT is connected in parallel and will be indirectly measured by ANT_SR. Refer to the PTX100R/105R antenna matching guidelines for IoT applications for more details.
  • The "Rp_se" are the parallel resistors in the QucsStudio model, part of the Antenna Matching block. They are there for tuning the Q-factor and do not require measurement



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