Measurement of complex Imedance a UHF RFID antenna

[mimorfid]

Hi,
I want to know how to measure the input impedance of the RFID Tag UHF (860-960 MHz) antenna.
is that it always takes lengths (n * lambda / 4)? 84mm 100mm ...
it just is what connects the masses of semi-rigid cables?
how to calibrate the VNA with semi-rigid cable?

 

[mimorfid]

is there another solution for the input impedance?

 

[WimRFP]

It is good the measure the impedance in balanced setup, but that can be done with a single port and a balun also (so you can do a single port measurement).

As you are interested in a narrow frequency range, balun functions can be constructed in various ways.
Some examples you can find here: https://www.edaboard.com/threads/248079/ and in many other places.

Calibration with balun is the best way, but the quality of the calibration depends on how good you can make the open, short en load. If you use very thin cable, all parasitic effects are less. If you have a very good balun (and you know that for sure), you may leave it out of the calibration, and do manual correction afterwards (electrical length, phase, amplitude). Even when calibrating with balun, you may need some manual corrections (because of bad short open and load), depending on what impedance you are measuring.

When measuring the antenna's impedance, the balun should be in the H-plane of the antenna, otherwise still common mode current is induced and these can render your results useless.

 

[mimorfid]

regarding the methodology used in the article (with fixture) .. how can I calibrated my VNA?

 

[WimRFP]

To be honest, I don't know in detail for your instrument. I would recommend you to read the documentation of your VNA. The paper only discusses calibration on page 130 under B and C, but without details.

 

[mimorfid]

I've been measuring my antenna RFID impedance at UHF band recently,i used the
fixture 100mm and calculated the final differential impedance by some formula directly from S11/S22/S12/S21.
BUT, the final calculated impedance is 21+j45 Ohm. The Real part is very similar to my simulation value,the Imag part is smaller to my simulation value which is about 120j .Imag part simulink( +165j) and the input impedance simulink(26+165j).
I can not find where does the error!!??