**S**ometimes you may be unable to find a particular inductor the market. This is actually a problem faced by most of the electronic hobbyists and the problem becomes more serious if your project is RF related. The inductors required for RF circuits (antenna, tuner, amplifier etc) are almost impossible to find in the market and the only solution is nothing other than home-brewing them.

With a little practice and patience you can construct almost all air cored inductors at home. The inductance of an air cored inductor can be represented using the simplified formula shown below and to calculate the inductance of an air-core inductor, the same equation may be used.

**L = [d ^{2 }n^{2}] / [18d + 40l]**

- Where’ L ‘ is the inductance in Micro Henries [µH]
- ‘d’ is the diameter of the coil from one wire centre to another wire centre. It should be specifies in inches.
- ‘l’ is the length of the coil specified in inches.
- ‘n’ is the number of turns.

**Notes :**

- The length of the coil used in the inductor should be equal to or 0.4 times the diameter of the coil.
- As shown in the equation, inductance of the air-core inductor varies as the square of the number of turns. Thus the value ‘l’ is multiplied four times if the value of ‘n’ is doubled. The value of ‘l’ is multiplied by two if the value of ‘n’ is increased up to 40%.

### Winding the coil.

- The coil must be first wounded on a plastic former of the adequate diameter (equal to the required core diameter).
- The winding must be tight and adjacent turns must be as close as possible.
- After the winding is complete, slowly withdraw the core without disturbing the coil.
- Now apply a thin layer of epoxy over the coil surface for mechanical support.
- Remove the insulation from the coil ends.

### Example

Suppose you want to make an inductor which produces an inductance of 10 μH. The diameter of the coil is 1 inch and the coil length is given by 1.25 inches. You will have to find the number of turns of the coil.

Thus substituting the values in the above equation

L = 10 inches

d = 1inch

l = 1.25 inches

n = √{L [18d * 40l]} / d = 26

Thus, the number of turns of the coil will be 26.

Number of turns/inch = 20.8

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