Help? Making rfid antennas, math is not my forte. Resonance and capacitance

[Satur9]

Hey @erischilde

First off check out this thread for some pointers:

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. [PXL_20210701_210325620~2] Once you have an idea of what you …

It sounds like you already have a handle on tank circuits. You shouldn’t need a capacitor in this case, because the LED has around 20pF of capacitance in the junction for red and green, and about 45 for white and blue. Check out this thread for an inductance calculator:

Coil Inductance Calculator

I’ve been designing inductor coils to serve as NFC antennas a bunch lately. I tried some online calculators and they were hit-or-miss. I combed through some Application Notes published by the popular NFC chip manufacturer NXP and discovered some reliable information on designing PCB trace inductors for NFC applications. I used this information to create a small java applet that runs in the command line. Here is everything you need to run it zipped up: Inductance_Calculator.zip (106.1 KB) In th…

Here’s a good LC tank calculator

If you have a 20pF LED you can use the inductance calculator to create an inductor that fits your maximum dimensions and is about 6.89uH. A 45pF LED would need an inductor of about 3.06uH. You can indeed put a capacitor in parallel to the LED to allow for smaller inductors, but you would need ones in the tens of pico Farad range not the hundreds.

If you want any help learning how to make flex PCB antennas feel free to hit me up. They’re expensive up front (~$150 for MOQ 300) but they’re much more reliable. We have an unofficial dangerous things Discord which has robust voice chat and screen sharing capabilities that I usually use to get people started. If I can motivate myself soon I’ll be releasing a long YouTube video explaining the process.