I made a simple repeater by repurposing the antenna of a comemrcial NFC NDEF keyfob. Cut away the MOB chip and replaced it with a 22pF capacitor. I measured the resonant frequency using a NanoVNA, and I got close enough (peak at 13.9 MHz). Taped it to the back of my phone (the case hides it), and it improves the read performance of my Flex quite a bit.
