DIY RF Inductors--The ABCs of inductors （1）

Published:2011/7/24 22:29:00 Author:Li xiao na From:SeekIC

By B. Kainka

Electronics hobbyists often wish to copy a circuit for which suitable coils or fixed inductors are not readily available. However, you can wind just about any type of inductor if you only know how. Or you can take inductors from old equipment and modify or adjust them. All you have to do is determine is how many turns you need.



Low-value inductors are primarily used in RF circuits. A general distinction must be made between inductors with magnetisable cores (made from ferrite or iron) and ’air-core’ inductors, which are wound on insulating forms or entirely without any sort of coil form.

Air-core inductors

Let’s first turn our attention to air-core inductors. Figure 1 shows an example of an inductor for a shortwave resonant circuit, which has 20 turns, a diameter of 16 mm and a length of 35 mm. It has an inductance of around 3 /JH, and with a variable capacitor having a maximum value of 300 pF it has lower frequency limit of approximately 5.3 MHz. How can this be calculated? Read on to learn more... (and by the way, there’s also a simple utility program to make things easier).



For a ’long’ inductor with 2 > D and n turns, a cross-sectional area A in m2 and a length I in m, the following relationship generally holds true:

L = (μ₀ x n² x A) ÷1

where p0 is the magnetic constant or permeability of free space and has a value of

4JT x 10^-7 henry/meter
1.2466 x 10^-6 henry/meter.

Although this formula is strictly true only for infinitely long inductors, it can be used as a satisfactory approximation for inductors with lengths down to 1 = D.

For an inductor with a given number of turns, the magnetic coupling between the individual turns increases as the length of the inductor decreases, which yields a greater inductance. By reverse token, increasing the spacing between the turns of an inductor decreases its inductance, and this is sometimes used to tune inductors.

For inductors having a circular cross section, the above formula can be simplified to the following approximate formula, where the diameter D and length 1 of the coil are given in millimeters:

L = n²xD²÷1 [nH]

This formula includes the approximation JI2 = 10, which introduces a small error (approximately 1.3 %). Extreme accuracy should anyhow not be expected, since the inductance depends in part on the shape of the coil, particularly the ratio of its length and diameter, as well as the thickness of the wire and even its surroundings. Consequently, for many purposes it is adequate to be able to calculate the inductance of an air-core inductor within a 10 percent tolerance margin.

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