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Published: 19 January 2015

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Ferrite rod core inductance.
Calculation of the ferrite rod core coil is enough difficult. The inductance depends on the parameters of the rod, coil sizes, and their mutual size ratio, the position of the coil relative to the center rod.
Inductance of the coil on a ferrite rod is calculating by the follow equation:
where:
 A_{L}  ferrite rod parameter characterizing its ability to increase the inductance of the coil. Usually measured in nH per turn squared
 N  number of turns of the coil
The formula shows that to find inductance of the ferrite rod core coil, we need to determine A_{L}. In the case of pot or a ring ferrite magnetic core, this parameter is usually fixed for a specific core, and its value can often be found in the producer's datasheets. In the case of the ferrite core, this parameter is not fixed and depends on the size of the coil and its relative location on the rod.
Coil placed at the center of the rod and having a length equal to the length of the rod about the parameter A_{L} can be calculated from the following formula:
Where:
 π pi
 μ relative permeability of the rod
 d,l diameter and length of the rod (cm)
Sometimes, this parameter can be found in the reference data producers, such Amidon inc publishes such data for their products. Often, however, we only know the relative permeability of the rod and we have to use for the calculation of the approximate formulas.
In case the coil dimensions smaller than the dimensions of the rod, the result is corrected by the correction coefficient depending on the relative sizes of the coil and the rod, and in this case the parameter A_{L} calculated by the following formula:
where:
By moving the coil relative to the rod coil inductance decreases, this circumstance is taken into account by the correction factor K:
where:
 s  relative offset = offset x divided by half the length of the rod "s = x/(l/2)".
This formula was obtained by regression analysis and is valid for s = 0,05  0,75.
As a result, the inductance of the ferrite rod core coil is determined by the following expression:
The length of the coil winding can be determined by the following formula:
where:
 N  number of turns of the coil
 d_{w}  diameter of the wire
 T = p/d_{w}  ratio of the winding pitch to the wire diameter or "packing factor" is generally in the range from 1 to 2.
In the plugin ferrite.coi for Coil32 is used the above method. The calculation follows the formula [2], [3], [4] and [5] by iteration. Also, one can use online calculator of the ferrite rod cored inductor.
References:
 http://makearadio.com/tech/files/Ferrite_Rod_Inductance.pdf
Special thanks to Andrey V. Cainov for constructive assistance and coauthorship in the development of methods of the calculation.