As one of many many side projects I'm working on, I've realized adding an NFC sticker and reader to our products would make lives a lot easier. Essentially the goal is that the user can attach an accessory to the main unit, and automatically transfer the data like model, calibration date, and calibration data (the accessories are completely passive). Less room for user entry error, fewer steps for the user, and I can greatly simplify the UI on the main unit.
But I feel like I'm missing something (or several things) from how to design and tune NFC antennas.
Things I know from various application notes and youtube videos:
1) I want my antenna to be between 0.5uH and 4uH, and reasonably close to 1uH
2) I need an NanoVNC/LiteVNC to make measurements
3) I need a matching circuit and EMC filter
4) I will probably be using an NXP reader chip, likely MFRC631?
5) The complications below will make any estimation of the inductance of my coil way off, but I have to start somewhere
Things I'm confused about:
1) Some application notes say I want a symmetrical Smith Chart, while others seem to be okay with sweep being mostly capacitive, but pokes into inductive at the target impedance.
2) Do I want the target impedance to be on the resistive line, or in the inductive side (some ANs have it on the line, others have it inductive)
3) What is my target impedance? I'm seeing 17, 20, 25, and 50, so I'm not sure what my actual goal is.
4) If I want to add a 50R 20cm sma/u.FL extension to the antenna to save space, where is best to put it? And do I need to match 50R on both sides of the extension?
Some added complications:
1) The antenna needs to fit inside a donut shaped area, the PCB has a max diameter of 50mm, and there's a 25mm diameter ground plane with non-nfc related components in the middle.
2) In operation, the NFC antenna will be inserted about 2cm inside the end of a 54mm diameter aluminum cylinder
3) The NFC sticker will be ~5mm from the coil, and not in the middle (since it can't block the components in the middle). The sticker will be oriented correctly with the coil at least.
The complications may make the whole thing impossible, but I at least want to try it out.
I've attached my first poor attempt at matching a coil I made in kicad with this to figure out number of coils, diameter, width, etc:
https://www.translatorscafe.com/unit-converter/en-US/calculator/planar-coil-inductance/I understand it could be 100% wrong, so feel free to make fun of it. And is there's a best location to add the extension with as few components on the PCB with the antenna? Do I need multiple matching circuits to do that? Space is very tight already in the ground plane area