Basic NFC Loop Design on PCB

Question

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 50Ω.
(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?

5. 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!

Answer 1

You don't need to match the output from the coil to 50 ohms if you are using 13.56MHz BUT, you do need to have a load on the receive power coil that is representative of the power you might be wanting to extract when powering a passive NFC device.

At 13.56 MHz, the dominant field is magnetic i.e. it is an alternating magnetic field that transfers all the power from coil A to coil B and as such you need to make life easier by resonating the coils as tuned circuits. This is done by series or parallel capacitors added to each coil.

However, the basic idea for transferring power is embodied in the following: -

From this you should be able to realize that the two coils can be identical or different size (identical is probably the easiest to get power through however). Because either coil is working as a magnetic loop antenna, either termination can be your 0V reference. Earth (ground) as a concept is unimportant for this type of antenna.

Answer 2

There are plenty of resources out there, and all in easy reach of a simple google search.

If you are using an existing chip to handle the NFC protocol, then you can be certain that the will offer a few documents describing how to design and tune the loop. Some of the ones that I've found useful in the past are the following:

NXP:

• Antenna Design and Matching Guide - AN11755
• Hardware Design Guide - AN11756

Nordic Semiconductors

• nRF52832 NFC Antenna Tuning - nWP-026

Panasonic

• NFC Design Navigator https://b2bsol.panasonic.biz/semi-spt/apl/en/tool/nfcdesignnavigator/

  This is a wizard that will help you design the Loop. It works well, but at extreme dimensions the inductance and tuning capacitor values are way off. I expect it to be correct when you enter more normal values.

It is really hard to mess up an NFC loop to the point that it doesn't work, but if you are confused by all of the documents linked here, just buy a spare mobile phone NFC antenna, from eBay or a mobile phone repair shop; they are cheap, and give you a good hint on where to go, additionally, will also help you to tune the loops that you have designed.