how to design diffrential probe for impedance measurement of RFID tag antenna

I have designed and simulated a meandered dipole antenna for UHF band RFID tag antenna. for the impedance measurement of the same I required the differential probe. so how the differential probe is fabricated and what procedure should be taken for input impedance measurement by using differential probe and VNA.

Many previous threads about the topic, e.g.:
https://www.edaboard.com/thread247900.html
https://www.edaboard.com/thread248079.html
https://www.edaboard.com/thread262572.html
https://www.edaboard.com/thread273612.html

there are differential op amps that actually work pretty high in frequency, like 2.3 GHz for the ADA4927-1 from analog devices. Maybe use those. You could convert to single ended signal after the input op amp.

The easier method is to use a standard single ended VNA and treat each of the differential lines as a separate single ended port. Measure the two port network parameters, then do some math to derive the differential/common mode impedances.

Or do a measurement through a balun, but you will have to disembed the balun from the measurement results.

Thanks for suggestions, but you just told the method of measuring impedance instead I want to know that how to fabricate a differential probe for impedance matching of RFID tag antenna at 915 MHz ......means which type of coaxial probe should be used and what should be the length of cable and from where should i buy the coaxial cable.......if you have idea the please explain it in detailed manner I am new in this field

Look at the equivalent circuit of a differential load with common and differential mode impedance and see that the method can't determine the differential impedance without knowing the common mode impedance. In other words you need a full 2-port measurement to determine both. In case of a symmetrical load, you get S11=S22 and S21=S12, but both values matter for Zdm.

A balun in contrast allows to measure the differential while ignoring common mode impedance.



You find detailed answers in the previous threads linked in post #2.

I see two solutions, the choice depends on the expected impedance range and your calibration options
- using a 50:200 ohm λ/2 coaxial balun as suggested in the threads
- using a 50:50 ohm λ/4 balun, particularly addressed in the second thread

instead of a tuned 50:50 ohms balun, an effective common mode filter might be used

Right, which is why I suggested measuring the two port parameters (not just the one port parameters of each port).

For the coaxial balun method, I wonder how accurate it would be in the case that it's not perfectly constructed. I suppose if it's well characterized, it should be possible but I doubt it's something a standard VNA cal procedure could do.

I somehow overlooked "two-port parameters".

E Kafeman reported some results for antenna measurements with balun which look promising. Calibration with a small chip resistor should be sufficient for antenna measurement accuracy level. In case of the 50:200 ohms balun, we can use a 200 ohm load reference and treat it as 50 ohm load on the VNA side. Finally scale results with factor 4 respectively 1/4.