NFC Antenna Design/Matching without VNA

Question

I would like to design a circuit around the W7002 antenna (Dataheet) and the NXP PN532 NFC Reader Chip. Since I never worked with antenna design before I do not have a VNA and I also don't have experience in antenna matching. The goal is to get an antenna which is able to read desfire tags through a plastic cover at around 7cm distance. Even a shorter distance like 4cm would be fine, but generally a longer distance is better. Fortunately for the antenna I want to use there are many values already provided in the datasheet so it shouldn't be that difficult to calculate the necessary values. I tried to use Adafruit's circuit as a "template" and I understand that I need to adjust C1, C2, C7 and C8. The damping resistors R3 and R4 might need to be changed aswell and lowered a little (to 1 Ohm each). What would be a good starting point to get the correct values for those Capacitors? I assume once I have a general idea I can try increasing/decrasing the values on a prototype and see if that increases or decreases the range? Also I am not sure what the purpose of C5, C6, C9 and C10 is, those are labeled "NC" so are these non-existent in this case?

Would I contact the antenna manufacturer or NXP as the chip manufacturer in this case for further assistance?

Answer 1

If you copy the adafruit design, you probably only have to change/tune the parallel resonance caps C7 & C8.

All the other components are just there for impedance transformation and you do not need to change that.

But C7 & C8 define the resonance frequency together with your (custom?) antenna inductance (L). This is the sweet spot.

Even if you had a VNA, you would not necessarily be able to get better results than the try and error tuning approach. This is because the RFID card heavily detunes the resonant circuit when approaching.

You will notice that a little variation of the parallel caps do not influence the reading distance that much, once you found a good reading distance. The size and windings of your coil have a much bigger impact.

Answer 2

If you have an oscilloscope and function generator with a similar frequency range you can manually sweep the frequency until you find the peak and trim from there

However if you are not exactly concerned about the best possible range and just want something that works. You can clone an existing demo board's antenna design.

One of my earlier projects did just that. Measured out the dimensions, trace thickness and length. And played with the pcb files until I matched it. Measured the existing capacitors to get the same values. And worked as well as the demo board. Based on later measurements with the right tools it was a little off the mark. But it functioned for my purposes.