How to Design an Air Core Inductor

An air core coil inductor is made by winding several turns of enameled wire around a none ferromagnetic material core (like plastic, ceramic, or other non-magnetic forms, as well as only with air inside the windings).

These coils have lower inductance compared to the ferromagnetic core coils. Despite of this characteristic, air core coil inductors are often used at high frequencies because they are free from energy losses called core losses that occur in ferromagnetic cores (which increase with frequency).

In air core coils with winding not rigidly supported on a form (core), a side effect called 'microphony' may occur. 'Microphony' refers to the mechanical vibration of the windings which in-turn causes variation in the coil inductance.

The major advantages of air core coils are:

1. The inductance is independent of the electrical current through the coil since there is no ferromagnetic core to get saturated as the current increases.

2. There is no iron loss as in case of ferromagnetic cores, thus giving a better Q Factor and low distortion with increased frequency.

3. They can be operated at high frequencies (upto 1 Ghz).

The inductance of an air core coil inductor can be calculated using the equation shown in the above figure.

here,

    L = Inductance (in uH)
    a = Diameter of the coil (in Inches)
    b = Length of the coil (in Inches)
    c = Diameter of the winding wire (in Inches)
    n = Total turns of wire

Note: The formula is valid only if the value of b is greater than 0.4a.