Designing NFC "Antennas"

I was talking over antenna design with another forum member and figured it would be good info to share.

I would start by understanding what the circuit we’re building actually is, an LC Tank Circuit. There’s a resonant frequency where the coil/chip combination will naturally draw the most power and have the highest coupling efficiency. The farther your “resonant peak” is from 13.56MHz, the less power is being delivered to the chip.

Once you have an idea of what you want to build, you can use some calculators to figure out the dimensions of your coil that will create the correct inductance. Here’s an LC Tank calculator.

There are three relevant values for your circuit, and unfortunately you need at least two to figure out the third. To find the target inductance, you need to identify the chip and look up it’s input capacitance on a datasheet (standard values from 35-70pF, but some can be as low as 15pF or as high as 100pF). Then you need something like the NanoVNA pictured adove to measure it’s resonant peak. With those two and the calculator, you can determine the inductance you need to meet. If you can’t find the capacitance anywhere, you can always hook the chip up to a known inductance and measure the peak, then you have the capacitance.

Once you have the target inductance you can use a calculator to estimate the dimensions of the coil. For 3D coils with an air core I’ll use a calculator like this. For ones with a different core material it gets more complicated. I also designed a PCB trace inductance calculator that can be really useful for flat coils. You can expect values around 3uH for most NFC coils.

Once you have the target inductance and necessary dimensions, you can get to winding (or CAD’ing). You can buy some enamel coated magnet wire of different thicknesses, but 28AWG will work for most of your purposes. Make sure you have enough before you start winding, especially for LF coils that could be 30mH! If it’s a 3D coil you can 3D print a coil form to wind around and still treat it as an “air core” inductor. Hook it up and check the resonant peak. It’s always a good idea to wind too much so if the peak is low you can just remove a bit of wire to reduce the inductance and bring the peak up.

That’s pretty much it. Blue LEDs are around 45pF and red ones are around 15pF. Conventional NFC coils are usually palm sized. Because we’re shrinking them but still need the same inductance, we’re increasing the number of turns, which increases the impedance (AC version of resistance).

You can buy a NanoVNA on eBay for $60. It’s not required to get an LED to blink, but if you want to have any idea what’s going on with an RFID chip you’re gonna need it. Here’s a video I made of how to set one up for NFC testing.

If you want to really go into more depth with the NFC theory and calculations here’s a good paper.

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Was talking to someone else about inductance and what affects it’s value so I’ll repost here:

If you’re interested in the literal math behind physics I always recommend you check out hyperphysics first
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/indcur.html

There’s a lot here, but the important thing is the equations at the end, specifically which variables are in the numerator and denominator, which ones are constants, and which ones have exponents (which would make them nonlinear).

So Inductance is:
L = (u * N² * A) / l

u is the magnetic permeability of the core. If it’s air it’s constant. If it’s different grades of iron you can multiply the inductance simply by winding it around different stuff

N² is number of turns. You can see it’s exponential, so double the turns means 4 times the inductance

A is area. A wider coil linearly increases the Inductance

l is length, and it’s in the denominator. A shorter coil linearly increases the Inductance

As a result, a perfectly flat wide disc coil wound around a flat disc of ferrite would have a really high inductance. A straight wire is the exact opposite of that and has a very low Inductance.

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Was helping someone make a DIY field detector coil for HF with just an LED and some wire. Thought I would repost here:

  1. Figure out roughly what dimensions you want the inductor to be (circles are easiest but some similar coil shapes couple better)
  2. grab some enamel coated magnet wire (maybe 0.32mm diameter) and some pico farad capacitors (or LEDs which are about 20pF)
  3. Use an LC tank circuit calculator like this one:
    https://goodcalculators.com/resonant-frequency-calculator/
    plug in 13.56MHz as the frequency and some capacitor values that you have handy (more capacitance means wider frequency range while more inductance means better coupling). Figure out a rough inductance that matches with your available capacitors (ex: 47pF capacitor + 2.93uH inductor)
  4. Once you have a target inductance use an inductor calculator like this one
    Coil Inductance Calculator
    If you want a flat coil change it to Multi-layer, single-row coil and Centimeters. Try out some values until you find a size/turns that matches your target inductance
  5. take your enamel wire and cut more than you would need to wind the coil (you can calculate the linear length easily enough from the circumference). Wind the coil. You can wind it on some tape for stability or add cyanoacrylate glue afterwards to bind the windings together. Don’t use metal tools or you’ll scrape off the enamel which can cause a short
  6. use a soldering iron to burn off the enamel from the wire tips. Tin the iron tip, press it to the wire, add more solder until the bead engulfs the wire, then slowly move it along the wire to clean it.
  7. solder an appropriate capacitor (or LED) to the wire tips

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This is for @Taggart to help explain winding your own LF coils. I just posted here to consolidate information for reference later. Here’s a link their original thread.

First it’s always a good idea to check the datasheet for the chip if you have it for the input capacitance. Luckily DT hosts the ATA5577C Datasheet for just this reason. If you check out the Electrical Characteristics section you’ll see the available options.

I’m under the impression 330pF is the most common, but if you measured 175pF then let’s go with that. That would mean it’s probably a 130pF chip with a bit of extra capacitance from the leads of your meter. Next you could use this calculator to figure out that you would need a 12.47mH inductor to match this chip.

t5577Ant

Even if you suspect the capacitance to be higher, it’s always better to undershoot. Capacitance and Inductance both lower the resonant peak. If it turns out higher than you thought, you can just remove turns from the coil to compensate. If it turns out lower than you thought, you can add a parallel capacitor.

I usually calculate 3D coil dimensions using this calculator from 66pacific, just because I’ve verified it several times and know it works. You can use whatever you want. Calculate a suitable number of turns for the dimensions you have, and start winding. Always good to overwind, because you can easily remove turns. If you’re having trouble reaching such a large 12.47mH inductance in the available space, either use a smaller gauge wire since turns are exponential, or add a strip of thin ferrite around the interior of your winding channel to change the magnetic permeability of the core.

If you really want to investigate the performance more than that, you’re going to need a VNA like pictured above. I’ve posted a video in this thread about how to set one up. It will tell you how far off you are from the target resonant peak. Otherwise, just keep trying until you get the maximum read range.

Here’s an example of an inductance calculation using a 25mm diameter coil with 17 turns that result in it being 4mm long. The resultant inductance is 44.6uH

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A post was merged into an existing topic: Help designing 125khz passive inductance coil for T5577