A Journey Through NFC Antenna Tuning Without a VNA

Fred wanted a better way than an old-fashioned key for his family to unlock the front door, and so he decided to build a keypad entry system. That’s ideal for the rest of the family, but Fred himself has an NFC tag embedded in his hand and wanted to take advantage of that to unlock the door. In order for it to work well, he had to tune the NFC reader’s antenna, and he was able to accomplish that without using a VNA (Vector Network Analyzer).

The problem with a standard NFC reader antenna is that they’re optimized for NFC cards — not tags residing inside a hand. A regular reader still works with Fred’s implant, but the range and sensitivity are low. He wanted to be able to simply wave his hand near the keypad, and have the door unlock. That meant his PCB trace antenna had to be tuned for his implant. Normally, that would require a VNA to tune the inductance, capacitance, and resistance of the antenna circuit, but those are expensive.

The solution Fred came up with was to use a far more affordable LCR meter to measure the results from the different components he selected. Through trial and error, he was able to narrow down the value of the resistor needed to end up with the theoretically-optimal antenna inductance for his situation. He wasn’t able to match it exactly, but Fred reports that the finished reader still works very well with his NFC implant and is able to unlock his door easily.