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Calculating inductance of PCB antenna used with an st.com RF chip?

Flyback

Well-Known Member
Thread starter #1
We are going to use the M24LR64E-R RF comms memery IC. ..(you can load its memory by pointing an RF transmission at it).

Considering the antenna that we have to put in with it, page 13 of AN2972 provides an equation whereby the inductance of a “square spiral” antenna can be calculated…however, the equation on pg 13 doesn’t include many of the necessary parameters, eg, the coil trace thickness, the spacing between traces, the permeability of FR4, etc etc. Therefore, the “eDesignSuite antenna design tool” from st.com has to be used….

..However, when I use this tool, I put in 10 turns, and it draws only 5 turns. Why is this?
ok sorry, ive tried it again, and its now ok...
In fact, when I put in 10 turns without having filled in any other parameters, it does then actually draw 10 turns, but its when i add the length and width parameters etc that it then halves the number of turns that I put in.
ok , ive tried it again, and its now ok...


..So my question is, ..is this tool reliable?…and how can I calculate the antenna inductance if this tool is not reliable?

M24LR64E-R datasheet:
https://www.st.com/resource/en/datasheet/m24lr64e-r.pdf

AN2972: App Note on making antenna for RF chips like M24LR64E-R
https://www.st.com/content/ccc/reso...df/jcr:content/translations/en.CD00232630.pdf

eDesignSuite antenna design tool:
https://my.st.com/analogsimulator/html/#/home
 
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BobW

Active Member
#3
I tried to run the design tool, but it wouldn't let use it until I'd created an account with ST and given them my entire life story (No thank you). So, I have no idea how accurate it is.

If you wish, I could run your parameters on some inductance calculation software that I have, and you could compare the results. In any event these tools will only give you an approximate value (some tools are more accurate than others, of course), and you'll still have to prototype the result to get the true characteristics.
 

Flyback

Well-Known Member
Thread starter #5
Thanks.
To summarise, we are trying to use the ST25DV04K-IER6S3 (13MHz Near field Radio receiver chip) to receive data to our PCB….this data is then retrieved by the microcontroller on this PCB.

The st.com software app which calculates the antenna for this chip, has made the antenna 2cm by 2cm , and we don’t have room for that on any of our PCBs. Therefore, we have decided to use a 1cm by 1cm antenna (as shown) and we are trying to compensate for the small size by having the antenna spiral down through the 4 layers of the PCB …thus giving us more turns, but without having the wide diameter.

Also, we have made make/break links between the coils of each PCB layer, so that we can experiment and see which number of turns receives the near field radio wave the best.

Do you think that our ploy is bogus?

ST25DV04K datasheet
https://www.st.com/resource/en/datasheet/st25dv04k.pdf
 

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BobW

Active Member
#7
Based on the PCB diagram, and the 1 cm outer dimension that you provided, I measured an inner dimension of 7 mm.
Turns per layer: 6
No. of layers: 4
Average space between layers: .5 mm (estimated based on typical 1.5 mm thick 4 layer board)
Trace width: .33 mm (estimated from diagram)

From the above parameters, my software gives an inductance value of 6.64µH.

I should also point out that the sensitivity of an antenna is proportional to its effective aperture, which is proportional to its physical size. Since you've gone down from 2x2 cm to 1x1 cm, you have 1/4 of the aperture of the recommended antenna, and therefore about 1/4 of the sensitivity.
 
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#8
Actually you could have 5 layers of coil. Stamp antenna but really just the old fashioned loop antenna. The feed throughs will kill it, use thru hole if needed. Commercial outfit used that technique for ferrite coils. Make it oval or rectangular to get size up. Trace size and spacing will determine Q.
 

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