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I'm designing a small board that integrate a PN512 IC along with other components (MCU, etc). I need to build a proof-of-concept device for a presentation with the suit people and everything is ready but the PN512 antenna/matching circuit design. My guidelines for a PCB antenna are:

  1. It must be single layer. (I've seen single layer designs with a 0 ohm resistor as a jumper over the traces)
  2. It can't be more than 10x10cm or 4x4in.
  3. I need it to successfully transmit data between the PN512 and an external NFC device, 5cm (2 in) or less range will be more than fine.
  4. I own a 200MHz scope to play with (Sadly, no network analyzer).
  5. My knowledge of the "black magic" behind this is very limited, but I'm learning as i go.

So my question is:

It is possible to build an antenna with the above requirements? Do you know of any (open) design available to get as starting point?

I've found many design guides for 2.4GHz antennas, but nothing helpful for the 13.56MHz range. Any help will be highly appreciated!

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Datasheet for the suit people would be amusing to see :-) –  Anindo Ghosh Dec 8 '12 at 21:40
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2 Answers 2

up vote 5 down vote accepted

How about this document about antenna design for the PN512? The rectangular antenna on page 47 looks like it will meet your needs.

Texas instruments also makes some fairly decent HF antenna design guides.

I've fabricated some of these rectangular antennas for 13.56MHz NFC before (the ones specified by TI), they're fairly simple and work well as can be expected.

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+1 for app note. Cool! Nice find. –  DrFriedParts Dec 9 '12 at 23:56
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Context

I could give you all the math and physics, but I think you just want the "do this" answer, so here goes:

The NFC part of your design problem relaxes a number of the usual antenna design requirements since it is "near-field" (the NF in NFC). From an end-user performance metric (can it read/write the tag), it isn't super sensitive to the impedances and the antenna impedances don't change a lot given the small-scale and close proximity. Basically, it's all bad, so a little more or less bad doesn't matter a lot. NFC was designed to discourage long-range reading (snooping).

Solution

Follow the Philips (NXP) reference design for the front-end and then draft an antenna something like:

  • 2" x 2" loop
  • with a few turns (say 4)
  • with as filamentary a conductor as possible (your PCB fabricator's minimum track width)
  • with minimum gap between turns (your PCB fabricator's minimum gap width)

Implementation

  • To minimize cross-overs, spiral inwards.
  • Obviously, ensure that there is no metal near the loop and specifically inside of it.
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