Does the minimum of the S11 curve at the resonance frequency depend on the resistance of the antenna

[IsuruWickramasinghe]

I was trying to fabricate a flexible NFC antenna for m24lr chip. Initially, I fabricated my design on a copper PCB. It worked perfectly fine with my NFC chip and facilitated NFC communication. I observed its S11 curve using a NanoVNA with a loop probe, and it showed the minimum of the curve within the expected frequency range (around 13.6 MHz).

When I fabricated it on a flexible substrate (PET,) using silver nanoparticle ink, I found that the minimum of the S11 curve was lost. The antenna still worked fine with the NFC communication, but I got a flat curve everywhere, from 5 MHz to 50 MHz (I did not observe beyond that range.)

I measured the resistance of the flexible antenna and found that it has nearly 600 ohms of internal resistance, whereas my PCB antenna had about 2 ohms of resistance. Then, I connected my PCB antenna in series with a variable resistor and observed the S11 curve while varying the resistance of the variable resistor. I noticed that as the resistance increased, the minimum of the S11 curve gradually disappeared. How could this happen, and what could be the reason behind this?

• Could the sensitivity of the NanoVNA be a reason for this?
• Or could the energy loss of resistance be a reason for this?
• Can you explain how a vector network analyzer (VNA) works with loop probes when measuring the S11 curve of an NFC antenna?
• What is the working principle?
  

  Since the antenna is not connected directly to the VNA, I am a bit curious to know about it. I used the instructions in the AN2866 application note as my guide.

[JL. Lebon · Accepted solution]

Hello, 

Normally, the resistance of the antenna doesn't influence the resonant frequency. The effect will be that the Q factor will be flattened, but resonant point should be the same.

What is strange in your case is that despite a very flat S11 curve for your flex antenna, you say that NFC comm is still working, which is very surprising. Are you sure that the S11 measurement is correct? (this can explain why it is working when there is no resonant point around 13.56MHz visible on the curve).

Another possibility may be that the antenna doesn't have the expected inductance. I don't know how you print the antenna, but do you have a fine control of the traces width and spacing? If width and spacing are not exact, inductance value will not match the calculations.

Also, what sometimes happen with printed antenna, is that there is short between traces. This is sometimes difficult to see with bare eyes, but edges of printed ink are usually not very straight and very fine short can occur. This is something you can check with a magnifier.

Best regards.