Unlike other manufacturers, iPhones use a specific NFC Antenna setup.
Typically there are two antennas in the iPhone. One supporting usual stuffs like NFC Payments and standard NFC communitation. The second one is typically a part of the Qi charging setup which requires NFC check to prevent that Qi destroys an NFC Card.
NFC 1 - A standard NFC Antenna which a part of the Qi coil.
NFC 2 - Typically an NFC Chip antenna (it depends on iPhone version).
It is known that some iPhones (espiecially iPhone 12 and iPhone 13) might have in issue with a basic NFC card detection.
The reason for that is that some types of iPhones uses a full Low Power Card Detection. Which means that the iPhone is just sending "analog" RF pings and it is checking whether there is enviroment change in its proximity.
If so, the iPhone starts to send "digital" RF pings which will activate the NFC card/NFC Tag. On the other hand, This feature saves a power consumption but decreases a detection distance.
Below, you can see an example of the standard "digital" RF ping. This RF ping is part of the NFC Discovery loop and contains polling for all technologies as 14443-Type A, Type B, FeliCa ans ISO 15693. The lenght of digital RF ping is typically around 80 ms.
While the "analog" RF ping is just very short RF pulse without any technology "inside" which takes approx. 0.5 ms.
The standard "Digital " polling sequence looks like below:
The NFC Reader/Phone sends only the digital polls which is reliable but also very power consuming.
The oposite if a pure "Analog" polling known as LPCD, which looks like below:
The NFC Reader/Phone sends short analog pulses and "scan" his surroundings. Once there is some change, e.g., NFC Card placed. This detunes the NFC Antenna, and reader sends the Digital ping. This method is very power saving, but not so reliable. Also the detection range is reduced.
A very good trade-off between both methods is a hybrid mode. In hybrid mode, e.g,, every 4 th ping is the digital one. Wwhich means, that the NFC card can be easily detected also if the load change/detuning is not strong enough.
