I have a PN532 NFC reader, hooked it up to a RPi, and added a Pulse W7002 antenna. It works great. However, I now need an antenna that fits in a 105 x 130 mm space. I have no idea where to begin.

The Pulse antenna uses three loops of 26ga enameled wire, so I picked up a spool. To confirm that it's the right wire, I made a duplicate of the Pulse, and it works just fine. From my (poor) understanding, doubling the number of windings should improve the antenna's performance. So, I made an antenna the same size as the Pulse but with six loops. Its performance is far worse - the card has to be almost touching the coils to be detected while three loops works from a distance of several centimeters.

All that to say, I've experimented and realized that I'm clearly out of my comfort zone, here. How many windings of 26ga wire do I need in order to make a 105 x 130 mm rectangular 13.56 MHz NFC/RFID antenna?

As for my experience, back in college, I switched majors to computer science in the middle of taking Field Theory but after taking Electronics I and II. Basically, I have a decent understanding of electronics but little to none regarding antenna design.

 

I went the "guess-and-check" route - somewhere between 2-2.75 turns seems about right, so I settled on 2.5. I suppose if anyone else stumbles upon this:

1. Get yourself a piece of wood and some wire brads.
2. Pound in the brads in the shape of the antenna you need.
3. Start with 5 turns.
4. Get a ruler, see how far above the antenna you can hold the tag and still read it, and record the distance.
5. Unwind one turn and measure again. Continue until you get the maximum distance.

Not exactly scientific, but it works.

 

Never did it by ticking a finger method, but China now sells the multitude of cheap network analysers be very helpful for read the exact resonances and antenna impedance. One of such is S1201 VNA (about 130 $ for 137-2700 MHz) but even better is two-port measurer NanoVNA (about 50$ for 50 kHz-900 MHz). They together had solved 99% of my practical needs.