Strange results when Measuring NFC antenna parameters for matching

[KRauccio]

Hi,

I am trying to measure the parameters of an external differential antenna I designed. I have been using the application note AN4974 in order to figure out how I can measure these parameters and I have some confusion on the setup and the results I am receiving so far.

I've looked at other posts and the way I understand it is that I should connect my VNA in single-ended mode (ant1 connected to signal and ant2 connected to the shielding/body of my SMA connector). Is this correct?

I have taken a measurement of a flexi antenna I would like tuned and I also tried to measure the antenna of the st25r3911b-disco board to verify if I measured the flexi correctly. My issue is that the measurements both look completely different from what I see on the application note. Using other posts as a reference, my antenna looks somewhat similar. although, my values are strange compared to others and the app. note.

The first picture is my flexi antenna and the second is the antenna of the st25r3911b-disco board.

First of all, the low frequencies are very jagged and I'm not sure why. My only guess is maybe there's something wrong with my calibration process. I'm also guessing this may be why I'm measuring negative ohms... The antenna inductance looks okay for both cases. I expected my antenna's inductance to be high. My other confusion is that the inductance at the resonant frequency is very high compared to the app. note and other references.

Am I understanding the procedure for measuring correctly? Is it just the calibration of my VNA?

Any help or pointers would be appreciated.

Thank you,

Kaitlyn

[Travis Palmer]

Hello Kaitlyn,

From the pictures you are showing, i would also assume that there is something wrong with the calibration or scaling of the VNA. The S11 curve is normalized to 50Ohm and should not leave the smith chart.

The smith chart of the ST25R3911B-DISCO can be found in AN4914. It should be an inductive matching, which means that the resonance circle is in the upper half of the smith chart.

You can also check the calibration by measuring a 50Ohm resistor (better would be a 50Ohm calibration kit).

It is also possible to take a capacitor (e.g. 390pF) and see how the curve looks like.

When re-calibrating, i would propose two different settings:

1.) Antenna measurement: 1MHz to 200MHz (dependent on the self resonance of the antenna)

2.) Matching measurement: 5MHz  to 25MHz 

BR Travis

[KRauccio]

Hi Travis,

I have been in talks with my analyzer's manufacturer after finding out that it's not calibration or anything I can affect. The manufacturer thinks it is damaged and it will be sent out for a repair.

On another note, I found out that my analyzer has a minimum frequency of 10MHz rather than 1MHz. With this is mind, how might this affect tuning? I was planning on using the antenna matching tool provided, but I wonder if a DC resistance at 10MHz is suitable. Is this okay or is there maybe a method of estimating?

Any advice would be appreciated.

Thank you,

Kaitlyn

[Travis Palmer]

Hello Kaitlyn,

Sorry to hear about the issue with your VNA.

The calculation of the matching impedance is described in AN4974 chapter 4.2 and AN5592 chapter 3.1

You would need to adapt the formulas to use 10MHz instead of 1MHz. 

I would expect an minor error on the Q-factor / damping calculation (due to the different DC resistance) based on the self resonance frequency of the antenna. Maybe neglect able since fine-tuning is anyhow required afterwards.

As example on the ST25R3911B-DISCO antenna the "DC" resistance at 1MHz is 0.4Ohm and at 10MHz 0.6Ohm.

If we just use the AMT and calculate the default ST25R3911B-DISCO antenna using 400mOhm:

and 600mOhm:

BR Travis