Impedance match an NFC antenna to 50 Ω

Question

I have a hard time understanding how I can impedance match a homemade NFC antenna to 50 Ω.

My antenna is designed by calculating the inductance based on the dimensions, windings, and so on. R_ant is calculated the same with C_ant, so it resonates at 13.56 MHz.

I would now like to impedance match the antenna to a 50 Ω spectrum analyser, but the calculations I have done are far off and definitely wrong. Can some one provide a good guide on this topic? I have included a schematic on the measurements I've done to verify the resonance at 13.56 MHz.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

I could he wrong, but I think that attempting to accurately impedance match a near field antenna to a transmission line or device might be somewhat of a wild goose-chase. One may need to just accept that there will be somewhat of an impedance mismatch, and make the best of it.

With a far-field antenna, the antenna impedance depends upon the frequency of the signal, the dimensions and geometry of the antenna, and the natural impedance of space. When calculating or measuring the impedance of a far-field antenna, an assumption is made that conductive or magnetic objects are not moving into or out of or within the near-field.

The impedance of a near field antenna, when in actual use, will definitely depend upon what is nearby, especially, but not limited to, the companion transmitter or receiver. The impedance will change depending upon how far away such objects are, and their electrical characteristics. Move the receiving antenna a bit, and the impedance seen at the feed-point of the transmitting antenna will change.

That may be one of the reasons why you are having difficulty reconciling your calculated impedance with what seems to be the reality. It may not simply be a mistake in calculation, but a problem taking into consideration all of the variables that lie within the near field.

A useful question to ask is how critical is it really, in your case, for the antenna and transmission line impedances to match, and to what degree? What, in your case, are the consequences of a mismatch?

I am definitely not an expert in near field antennas. I only offer my thoughts on the basis of my general understanding of electromagnetics. So, someone with more specialized knowledge may be able to correct me if I am wrong.

Answer 2

There are lots of application notes about NFC antenna tuning. But most of them are NOT for tuning an antenna connected by a 50-ohm coax.

However, I just saw this one yesterday... the first one to talk about 50 ohm matching.

Search ti.com for application note SLOA241C.

If the link works... https://www.ti.com/lit/an/sloa241c/sloa241c.pdf

Answer 3

Use a NanoVNA or some other VNA to do you measurements.

Search for a YouTube video from W0QE titled "Three Methods to Measure Impedance with the NanoVNA". Pick the method based on the impedance you see... low, near 50ohms, high.

Also, as mentioned in the previous post the impedance you measure is affected by the environment of use. So there are a couple of things to consider here. The NFC tag that you want to read is in range when you do your measurements. You want optimal performance when the antennas are coupled.

Remember that your measured impedance is at a specific frequency. Transform it to the impedance at 13.56 MHz to do your impedance matching to 50 ohms. Try SimSmith (SimNEC) if you haven't already.