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[SOLVED] small dia air core coil measure

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kripacharya

kripacharya

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I have some legacy circuits with air core inductors (copper) w/ 19 turns, SWG = 24, dia = 3.20 mm, Length = 12 mm. The measured inductance on a good instrument was 80nH/13.7 milliohms & 93nH/13.37 milliohms.

However, calculations show that it *should* be 272nH, using outer dia, and ~185nH using mean dia.

I need to replicate this board. But what value of L (also Q and C) to choose ?? This is a very simple board for an antenna amplifier for MW/SW/FM bands which works very well - gives > +20dB all the way from 500Khz upto 125MHz FLAT (based on simulations), and I don't want to mess it up.

What is the community opinion/ advice ?
 

272 nH is the result with a commonly used "single layer coil" formula, with 3.2mm mean diameter. I get a similar inductance value (about 250 NH) with a trustworthy field simulator, thus I presume the formula is correct.

https://www.qsl.net/in3otd/indcalc.html
 

Thanks FvM, I knew you would be one of the very few to answer.

The 3.2mm is outer dia, but no matter. The range of inductance if enough. None of these coils is being used in a 'mission critical' way, so the re-affirmation is adequate.

Now I need to convert this to a PCB etched version. Can you point me to some place where I can get design help for this ?

cheers!
 

FvM said:
There are empirical formulas for PCB coils, e.g. https://coil32.net/pcb-coil.html
Click to expand...

I tried this already, and got a decent set of parameters. Then I went further and also tried this.... website.

BIG mistake, since the 2 calculators give significantly different results !!

Dimensions entered on both are (square topology):
Outer dia 'D' = 9.9mm
Inner 'd' = 6.3mm
Pitch 's' = 0.516mm or Track width=track spacing= 0.258mm, depending on the site input requirements.

With the identical dimensions, the coil32.net site says it will be 365nH, while the circuits.dk site says 235-250nH !!

I would have thought these calculators were perfected by now, so where is the mistake I'm making ?
 
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No idea who is right. I would simulate the chosen geometry with FastHenry to get the inductance exactly.
 

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