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Got very different output (nH) from different inductance calculator! :(

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Willen

Well-Known Member
I used three different inductance calculator. Result makes me irritated!
Details was:

diameter= 4mm (2mm radius)
High/length= 7mm
Turns= 8

1st calculator: I used a calculator by sm0vpo (harry lythall's) and got 119.27nH.
2nd calculator: I used a small software program and got 162.8nH.
3rd calculator: I used a calculator based on web page and got 114.53nH.
The 3rd calculator is based on this formula N^2 x r^2 / 9r+10H and I did by hand and got same as 3rd calculator.

But among 3 calculator values are different. 2nd is VERY different! I learnt: 'nH' on RF impedance matching network is VERY critical for desired output. If I used these confused values may be I will be hell :)

Review once and suggest me please for best!
 
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What is the wire size? It has a significant effect on inductance when the number of turns is small, as in your case. It appears that none of the calculators ask for that information, and so they must be making there own assumptions, which are quite likely wrong.

The formula: N^2 x r^2 / 9r+10H, is Wheeler's long coil formula, and is not accurate when the coil length is not much greater than the diameter.
Try this one:
**broken link removed**
It doesn't make any assumptions. and it accounts for wire size and turns spacing.
 
What is the wire size? It has a significant effect on inductance when the number of turns is small, as in your case. It appears that none of the calculators ask for that information, and so they must be making there own assumptions, which are quite likely wrong.

The formula: N^2 x r^2 / 9r 10H, is Wheeler's long coil formula, and is not accurate when the coil length is not much greater than the diameter.
Try this one:
**broken link removed**
It doesn't make any assumptions. and it accounts for wire size and turns spacing.

Wow! It's advanced!

I calculated with 21 SWG, 24 SWG and 30 SWG wire but got same inductance- 115nH. So I think it is not VERY critical for general coil designing.

BUT it might be the best while designing a coil which has large diameter that its lenght. Because you said- Wheeler's formula is not accurate if coil has large diameter than its lenght.
 
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Be careful, that calculator uses either AWG or millimeters for conductor size. If you enter SWG instead of AWG your results may be slightly off (though probably not by much). Difference between AWG and SWG is explained here:
**broken link removed**
 
Be careful, that calculator uses either AWG or millimeters for conductor size. If you enter SWG instead of AWG your results may be slightly off (though probably not by much). Difference between AWG and SWG is explained here:
**broken link removed**

I used a table which has SWG vs. mm. Thus I filled:

21SWG= 0.81mm
24SWG= 0.55mm
30SWG= 0.31mm
 
I calculated with 21 SWG, 24 SWG and 30 SWG wire but got same inductance- 115nH. So I think it is not VERY critical for general coil designing.

I think you are referring to the "Base Inductance" value which does not account for conductor size, and is 115 nH in every case. The actual values, when corrected for conductor size, are:

21SWG= 0.81mm, L=100 nH
24SWG= 0.55mm, L=108 nH
30SWG= 0.31mm, L=119 nH

These numbers are for an operating frequency of 0 Hz (DC). Therefore, there will be a very small change in inductance at higher frequencies.


Edit:
Just had a look at: https://www.daycounter.com/Calculators/Air-Core-Inductor-Calculator.phtml
and I see that it also uses Wheeler's long coil formula, which is fine as long as you're aware of it's limitations (and most people aren't), but when you have the processing power of a modern computer available, why not use a better formula that's accurate for any coil geometry?
 
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Hi,

There's another curve from Bunet that provides the inductance when the diameter is less than three times the length. It's pretty good and includes the build height which means it considers wire diameter too. Not sure if it can be found on the web or not though.

But with all of these formulas you might expect some error anyway. The usual procedure is to create a coil based on a formula that has slightly higher inductance than desired, then fine tune it in the field by separating the turns slightly as you measure some parameter like frequency, peak response, etc. That's the ONLY way to get it right unless you dont need an accurate inductance value.
 
There's another curve from Bunet... Not sure if it can be found on the web or not though...

I'd be interested in seeing that one.

Sadly, virtually everything I've found on the web is Wheeler's long coil formula. The only exceptions are:
1. http://hamwaves.com/antennas/inductance.html which uses Lundin's very accurate empirical formula (error < 4 ppM), and includes corrections for conductor size.
2. **broken link removed** (my own calculator, so I'll admit to being biased), which uses the theoretically exact elliptic integral formula, plus conductor size corrections.
3. **broken link removed** which uses an empirical formula that I developed (error < 20 ppM), but doesn't include corrections for conductor size, so there is some loss of accuracy, but still not too bad.

I confess to being a bit of an inductance evangelist. (We all have our foibles.) Several years ago, I was trying to do some calculations for some coils that didn't fit the usual coil geometry, and the numbers refused to work out. I became obsessed, did some research, and found out that there were far more accurate formulae than those currently available on the web, but it appeared that those people who'd created the various calculators, were far more interested in creating calculators than actually providing something that gave accurate results. So I wrote my own calculator(s). I've actively sought feedback from people who have the facilities to measure actual values accurately, and compare them to the calculated values. So far, the feedback shows that theory=practice, generally to less than 0.2%.
 
I think you are referring to the "Base Inductance" value which does not account for conductor size, and is 155 nH in every case. The actual values, when corrected for conductor size, are:

21SWG= 0.81mm, L=100 nH
24SWG= 0.55mm, L=108 nH
30SWG= 0.31mm, L=119 nH

These numbers are for an operating frequency of 0 Hz (DC). Therefore, there will be a very small change in inductance at higher frequencies.


Edit:
Just had a look at: https://www.daycounter.com/Calculators/Air-Core-Inductor-Calculator.phtml
and I see that it also uses Wheeler's long coil formula, which is fine as long as you're aware of it's limitations (and most people aren't), but when you have the processing power of a modern computer available, why not use a better formula that's accurate for any coil geometry?

Opps sorry! Yes I used 100MHz frequency (because i am going to use these inductor on FM Tx) and 24SWG conductor and got 'Corrected Inductance: 103.3nH' wow! It's another different value and I think it's very accurate than others', isn't it? Thank you!
 
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