Geometry factors for some common Fair-rite binocular ferrite cores

Designing with some common Fair-rite binocular ferrite cores can be frustrating because different parameters are published for different material types, and some are controlled for different parameters.

An approach is to derive the key geometry parameter from the published impedance curves and published material complex permeability curves.

For example, the above curves for a 2843002402 (also common known as a BN43-2402) were digitised and iteratively Calculate ferrite cored inductor (from Al) used for find the value of Al that gives the observed value for Z at 10MHz on the chart above.

The calculated geometry statistic ΣA/l captures the permeability independent geometry of the core and can be used with different core materials using Ferrite permeability interpolations to find the complex permeability at the frequency of interest and Calculate ferrite cored inductor – ΣA/l or Σl/A to find Z (or Y).

Core BN designation ΣA/l (m)
2843000202 BNxx-202 0.00288
2843000302 BNxx-302 0.00211
2843001502 BNxx-1502 0.00139
2843002402 BNxx-2402 0.00154
2843010402 0.00308

Above is a table of ΣA/l (m) which can be used in Calculate ferrite cored inductor – ΣA/l or Σl/A for three common small binocular core sizes.

A side note: Fair-rite publishes Σl/A parameter for some of its cores, most toroids for instance, though in non SI metric units. Σl/A is the inverse of ΣA/l, but can be used in the calculator when adjusted to metre dimension and the appropriate drop down selected.


I wish to design a small common mode choke to make antenna analyser measurements on 1.8MHz.


An BN73-2402 is a candidate.

Above is an estimate of complex permeability.

By trial and error, it is found that 7 turns gives sufficient Zcm for the application. So a BN73-2402 wound with 7t of small gauge twisted enamelled wire and then SOL calibrated provides a facility for measurement of the antenna isolated from the test instrument (an analyser with USB attached computer).


An BN73-202 is another candidate.

Above, 5t gives sufficient Zcm.

Above, 5 circles of 1.4mm will fit within a 3.8mm circle, so 5 twisted pairs of 0.5mm ECW will comfortably with within the core aperture. A larger transformer, a little longer wire path, thicker wire accommodated.