# Inductors, Inductance calculation

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:
MULTILAYER CIRCULAR COIL
-------------------------

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

Maths results:
MULTILAYER RECTANGULAR COIL
---------------------------

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.