I try to understand inductors and inductance.

I create a multi-layer circular coil with 35 turns of enamelled copper wire ~21AWG (0.75mm precisely) with a diameter of 8.5cm.

    Maths results:

    Inductance          : 0.16mH/160.45μH
    Number of Turns     : 35
    Coil Inner Diameter : 8.5cm
    Wire Length         : 934.62cm/9.35m
    Turns per Layer     : 20
    Number of Layers    : 1.75
    Coil Length         : 1.5cm
    Thickness           : 0.375cm
    Coil Outer Diameter : 9.25cm

Then I realize an inductance meter from this tutorial using my arduino and the LM239N comparator instead of the LM339 and a 100nF capacitor instead of 2μf. This counter inductance returns me 171.23μH. Here, I am very close to 160.45μH.

enter image description here

But ,I create a multi-layer rectangular antenna of 105 turns of enamelled copper wire ~21AWG (0.75mm precisely).

Maths results:

Inductance          : 0.923mH/923.272μH
Number of Turns     : 105
Width               : 4.589cm
Length              : 5cm
Wire Length         : 2013.69cm/20.14m
Turns per Layer     : 16.67 *
Number of Layers    : 6.3   *
Thickness           : 1.4cm
Coil Length         : 1.25cm

  |          ↕b        |⋱↘h
  |  +--------------+  | '
  |  |\_____________|  | |
  |  ||             X  | |
  |  ||             |  | |
  |  +-------Y------+  | |
  |                    | |
  .--------------------. |

   X: 4.589cm
   Y: 5cm
   b: 1.4cm
   h: 1.25cm 

 * I use 10 turns per layers.

The inductance meter returns me also 171.23μH for this coil (yes exactly the same value of first circular coil). I don't understand why my second coil is so far of ~1mH (my second coil maths calculation) and the same as my first circular coil result.

The LM239N have the same LM339's response time used in the inductance meter tutorial.

The range of inductance meters is given for : 80μH to 30mH.

How to correctly mesure inductance please. Thank you for your help.


1 Answer 1


The whole tutorial doesn't account for parasitic effects as e.g. parasitic capacitance which is relative big in multi-layer rectangular inductor.

How to correctly mesure inductance please.

  • Do research what kind of parasitic elements a multi-layer rectangular inductor has, make a model of it and estimate the range of the value of each element
  • Connect the multi-layer rectangular inductor to a spectrum analyser
  • Using the estimated values of the parasitic elements and the results of the spectrum analyser, you can determine which element(s) is/are dominating and estimate/calculate the value
  • Simulate your model by using the values found in previous step and verify if it approaches reality (the result of the analyser) and iterate by tweaking the values in order to have a better fit for the real case
  • \$\begingroup\$ Thank you for your reply. But I do not understand why the values are the same (171.23μH) in my case for my circular and rectangular coil, I do not think that it is due to parasites. I only have an analog oscilloscope from 1970 (I think) and I can not clearly see the resonant part precisely and I need to learn about how to use this oscilloscope surely. \$\endgroup\$
    – Ephemeral
    Commented Aug 22, 2019 at 20:41

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