So I have a PAY implant based on a conversion of the PureWrist GO. I have an easy time with all the readers except one that is very ubiquitous in my area: the Verifone MX 915.
I found this Hackaday article for a similar unit, but they stopped at the main board and said nothing about the NFC. I decided to buy a used unit and tear it down to take a look at the NFC antenna. Here is a link to a Google Drive folder with pictures of all the internals. In this thread I’ll just share some of the pictures and what I’ve learned.
Before I took anything apart I decided to investigate the properties of the NFC antenna with a VNA:
I then took the whole unit apart. Here is a process shot about halfway through when I revealed the rear side of the main board:
Then I took the final step and got to the NFC antenna. I was pretty disappointed by what I found:
The actual antenna is just two turns of what looks like 22AWG wire. There is about 8mm of plastic between the windings and the surface of the unit, and it is a bit close to the LCD in spots. The windings are connected to a tuning board with a pressure fit connector, which then travels from the tuning board to the main board via a ribbon cable. They seem to have accounted for many of these issues in the design of the device to get it to squeak past its EMV certification, but it’s far from an ideal setup.
As for positioning, here is a diagram of the antenna location as it would be visible from the outside, along with an example of ideal positions for flex and glass implants:
The top of the unit can present a problem because there is often a metal clamp there. The bottom of the antenna can present a problem because there is often a rubber privacy guard. That leaves the left and right sides (depending on which hand you use). The antenna windings are situated about a centimeter from the edge of the LCD display.
Your mileage may vary, but I find it easier to position the flex antennas in line with the antenna windings for best results. I usually start from the LCD side and slowly work my way outwards. Once you hear a beep and the screen begins to register the presence of the card, stop immediately and leave your hand still in that spot for 3 seconds until the reader says “Enter PIN or Press Enter” or something similar.