AN2866 App Note Square Spiral Antenna inductance not in agreement with online eDesignSuite calculation

[Ted Jackson]

I tried to compare a modeled square spiral antenna from the AN2866 app note (Sec 4.3 page 11):

https://www.st.com/resource/en/application_note/cd00221490-how-to-design-a-1356-mhz-customized-antenna-for-st25-nfc--rfid-tags-stmicroelectronics.pdf

with the result of ST's edesign calculator:

https://my.st.com/content/my_st_com/en/products/nfc/st25-nfc-rfid-tags-readers/st25-nfc-rfid-readers/st25-nfc-readers.html#edesign-suite

But the results I got are about three orders of magnitude different. Note that the antenna size is a bit large at 10cm/10cm. Am I missing something?

[Henry Crane · Accepted solution]

Hi Ted,

the simplified formulas documented in Application note can be found in literature and give an approximation of the self resonant frequency of the coil. Edesign suite antenna module calculates the imaginary part of self impedance to take into account self inductance in combination with stray capacitance. antenna equivalent inductance is then given using formula L=X/(2.pi.f) with f=13.56MHz.

For information, tolerance of antenna module on reactance calculation is 5%, validated by comparison with Electromagnetic simulations done on chosen antennas. When antenna topology can lead to extra error due to algorithm limitation, a warning message is displayed.

to summarize, application note formulas are fast and easy to use but only give an evaluation of antenna self inductance (inductive part of antenna impedance) and Edesign suite antenna module is "limited" to rectangular coils but allows to get a more precise value of antenna reactance by taking into account inductive and capacitive part of antenna impedance.

best regards,

Henry Crane NFC/RFID product support team