Matching the differential impedance

[MikeObs]

Hello,

I have created a board similar to the NFC03A1, and 6 different exchangeable antennas to be able to use the desired one. I matched all of the antennas to approximately impedance of 45-50 Ohms. Showcasing one here:

 

Now I received a question on why didn't I match them to the impedance of 27 Ohms, which is stated in a datasheet to be the differential impedance. I found in the other post here, where you guys state that: The target impedance can be between 27 +0j and 50 + 0j.

In that case I would be in the limits and should be no problem, my question is though...

Why there could be this variety of impedances ranging from 27 to 50 ohms and what does it affect when I used higher impedance that the differential one from datasheet?

Thank you so much for your time, I am so sorry but I am new to the antenna designing and just trying to understand it as much as possible.

Mike

[Henry Crane · Accepted solution]

Hello Mike,

based on theory, the maximum power transfer between the reader and the antenna circuit occurs when antenna circuit input impedance is tuned to 27 Ohm.

Let's use a simple model:

• CR95HF:a 3V open circuit voltage generator in series with 27 Ohm output resistance
• Antenna circuit: a variable resistor Zin

the picture here attached show the variation of the power transferred to the antenna (Pout in blue), the power dissipated in CR95HF output resistance (Pcr95HF in red) and the total power in green.

As shown on the graph

• for Zin = 27 - deltaZin (Zin <27 Ohm) => strong decrease of transferred power, strong increase of CR95HF internal dissipated power a,d strong increase of total consumed power
• for Zin= 27 + deltaZin (Zin > 27 Ohm) => slight decrease of transferred power, strong decrease of internal dissipated power and slight increase of total consumed power.

Tuning the reader antenna in free space corresponds to maximize the generated magnetic field strength for tags at long distance.

During communication with tags, the reader antenna input impedance varies in presence of a tag. Worst case is tag put on reader antenna. Maintaining a high power transferred to the tag (for instance in case of application using the energy harvesting feature) at 0mm as well as a long read range lead to increase the antenna circuit input impedance in free space.

For all these reasons as well as experimental results, we recommend to tune the antenna between 27 and 50 ohm.

I hope this helped, else feel free to tell us.

best regards,

Henry Crane RFID/NFC support team.