PN5190 antenna with a coax cable

[Pionner786]

Hello,

I am designing a NFC reader for a commercial product using the PN5190. My application necessitates a customized antenna with a 1.5-meter extension cable.

The current solution I have implemented is as follows: PN5190 - EMC circuit - first matching circuit - BALUN - 1.5m 50-ohm coax cable - second matching circuit - antenna. This setup is functioning well for me. However, in an effort to reduce the antenna size, I aim to eliminate the circuit part (the second matching) on the antenna side. And I found a solution in AN13892 (Section 8.2, which is for PN7160 but I believe the methodology is the same).

According to this document, I can treat the "BALUN-coax cable-antenna" combination as an antenna in the standard design procedure. I followed this approach but encountered a problem: the measured impedance of the "BALUN-coax cable-antenna" turns out to be capacitive. I am unsure of the reason for this and am at a loss as to how to proceed with the circuit design.

Or, are there any other way I can remove second matching on antenna side?

I would greatly appreciate any technical support or insights to help me resolve this issue and continue with my project. Thank you!

[EduardoZamora]

Hello @Pionner786,

Hope you are doing well.

My recommendation will be to keep using your first setup; this is, having one matching circuit right after the EMC filter and another Matching circuit at the other end of the coax cable.

For the second approach you are considering, same steps described in PN5190 antenna design guide could be followed for the tuning components calculation and matching adjustment (PN7160 frequently asked questions, Section 8.2.1).

Regards,
Eduardo.

[Pionner786]

Hi thanks for your reply. Could you please explain the reason why I should keep first setup? That will be great. (as PN7160 frequently asked questions recommand second setup)

And I still want to figure out why my cable+coil measurements are capacitive. 

[EduardoZamora]

Hi,

Antenna circuit without BALUN can be used to reduce the BOM.

You could use the hints from PN7160 documentation for antennas with coax cable. The length of the cable and loading conditions may affect the impedance.

Also, you may find interesting the NFC Antenna Design | NXP Semiconductors online training series.

Regards,
Eduardo.