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NFC loop using PCB

Anthony Gonzaga
Prodigy 10 points

Hi all!

I'm a college student learning the basics of NFC loop antenna design.

As a starting point, I'm trying to design a PCB-based rectangular NFC loop operating at 13.56 MHz, matched to 50 Ohms. 
(NFC "inductive" loop -->> matching network -->> SMA connector).
I plan to produce two antennas, and after fabrication, I'm planning to test these by making these two "touch" and see its reaction/performance through a Vector Network Analyzer 
I've read quite a number of application notes, and currently have some questions in mind. I hope someone can help me with these:

1) Can I make the the two antennas ("reader" and "tag") identical (same geometry, etc)? Or should I make the tag antenna smaller?

2) Since, I'll be using a VNA to examine its performance, I need to match the loop impedance to 50 Ohms. I'm currently planning to do it using discrete R and C components. Is there a better way to do it? (I mean, discrete and surface mount R and C are available in standard values, I'll be having a problem if my required R and C matching components have values far from standard.) 

3) The loop has two "ends". One terminating inside the inner loop, and the other outside the outer loop. I need to connect both of these to the SMA, right? Does it matter which end connects to ground and which end connects to the signal? 

4) I'm planning to use HFSS to simulate my design. Are there any specific reports/tests I should examine before implementing the design?

4) Can you recommend appnotes and articles that I can further read and study to help me with my goal? (URLs with a good discussion on Basic NFC loop design)


I hope someone could guide me.

Thanks a lot!

--

-Tony tony

  • 0
    TI__Guru* 91206 points

    Tony - 

    Have a look at these: 

    and

    should get you started. 

    and yes, when making an antenna (actually in this case its a magnetic dipole) - for measurement connection and use for 50 Ohm system - it is considered single ended, which means one side will be connected to ground. 

    regarding the question about the tag size vs. antenna size. 

    this is transformer coupling you are asking about, so coupling factor comes into play - typically you want this to be as close to 1 as you can get it with the smaller tags, but generally, the reader antenna is always going to be bigger to get larger field out there. tradeoff is with larger reader antennas and smaller tags, you end up with "read hole" in the middle of the field (think of it like a classic doughnut in that case - and you want to make a filled one - hope that analogy makes sense to you)

    if you have access to HFSS and have the time to learn it, great! share what you learn - most of us older guys who never had the simulators just do it the old fashioned way - but that tool generates some cool graphics for your school reports - so have fun with it!