I am trying to design a custom antenna for an RFID circuit I am building.

I want to make a ~20 cm circular antenna to read a tag that passes through. I still can't have success.

Reading this http://ww1.microchip.com/downloads/en/AppNotes/00710c.pdf and other sources I keep reading about the "resonant capacitor" that is required to use for the antenna to work.

I am almost sure this is what is missing in my case.

I wonder, how does this resonance capacitor work? what does it do?

Please consider I am still fairly new to electronics in case this is a very basic question.


1 Answer 1


The capacitor combined with the coil (inductor) forms a LC circuit or resonator. The inductor in this case is your antenna. The frequency at which the circuit has the optimal (maximal) response, its resonance frequency, is determined by the value of the capacitor and inductor (see the resonance frequency in the linked Wikipedia article). Thus, if you know at what frequency you want your circuit to work and you know the inductance of your antenna then you can calculate what capacitor you need.

  • \$\begingroup\$ Thanks! Now I get it. Does this mean I dont need to change the capacitance if I change the size of the antenna? Since I'm basically building an antenna with the same impendance but in a bigger factor \$\endgroup\$
    – academo
    Mar 13, 2022 at 7:15
  • \$\begingroup\$ Possibly, you will have to add a capacitor though, else you just have an inductor and not a resonator. I suggest you measure (or calculate) the inductance of your coil and see what capacitance you need to obtain the desired resonance frequency. \$\endgroup\$
    – Julian
    Mar 13, 2022 at 10:50

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