I'm no expert on 13MHz-carrier data-transmission but i do come from a background of transmitting power magnetically to rotating circuit boards and receiving modulated data back at 80MHz so if I've misunderstood anything please let me know.
•Could it be acceptable to put the components (NFC controller, LED > driver) on the same PCB as the NFC antenna?
Yes this is acceptable but keep circuits as far distant as possible so that the receive side stands the best chance of working effectively.
•What would be the "best" position relatively to the antenna in this
case? (i.e. one that would limit eddy currents and inductance
increase)
According to the very long-winded document you linked, the antenna Q needs to be a bit less than 30 and this is easily checked by using standard oscilloscopes and signal generators. Q is centre frequency (i.e. 13.56MHz) divided by 3dB bandwidth and for a Q of 30 bandwidth will be about 450kHz.
So you need to make sure your centre frequency is spot on and that you have chosen Rq values to limit the Q sufficiently or you'll be possibly breaking some regulations about emissions levels.
I'd position the antenna and compensate for eddy current effects (frequency and Q changes) with a slight retune then add or remove a little bit of Rq to give a Q of 25 to 30.
•How to efficiently shield the main controller board? Will a sheet of
copper do the job(instead of ferrite)?
You may not need to do much (if anything) here but I'd recommend using the proper ferrite sheet (Ferroxcube used to sell it because I've used it). Using copper sheet may detune and slightly desensitize the antenna depending on how close to the antenna it is.
•Is there an advantage to have different length and width for the
antenna?
Yes, the squarer the better. Bearing in mind this is magnetic coupled data and you should be looking for a loop with as much inductance as possible but not so much that it's self resonant frequency is lower than (say) 15MHz.
As a simple idea about inductance - inductance is defined as total flux produced per amp. The bigger the cross sectional area, the more flux is produced per amp. A long thin loop however has less inductance than a square loop of the same area.
In summary,
- When using a NFC standard loop coil antenna, sized about 4x4 cm, the theoretical maximum working distance is 20 cm. In practice, the range for reliable communication is much smaller, usually about 5 cm (4 times smaller).
How could you improve the range without tweaking the power level and sensitivity of the NFC devices (master/transponder and slave/tag),
Answering to your specific concern,
- To achieve a practical 60 cm range, you may try to create a near field with a theoretical range several times bigger, for instance, 180-240 cm, for which you need a circular coil of about 2.5-3.4 meters in diameter.
- You can also try a square shaped coil of equivalent area to the circular coil.
If you finally try, please, update or answer your own post and let us know about your findings.
Best Answer
If you copy the adafruit design, you probably only have to change/tune the parallel resonance caps C7 & C8.
All the other components are just there for impedance transformation and you do not need to change that.
But C7 & C8 define the resonance frequency together with your (custom?) antenna inductance (L). This is the sweet spot.
Even if you had a VNA, you would not necessarily be able to get better results than the try and error tuning approach. This is because the RFID card heavily detunes the resonant circuit when approaching.
You will notice that a little variation of the parallel caps do not influence the reading distance that much, once you found a good reading distance. The size and windings of your coil have a much bigger impact.