Loop in ground (LiG) – #5 – small broadband RF transformer – discussion of ferrite material choice

At Feasibility study – loop in ground for rx only on low HF – small broadband RF transformer using medium µ ferrite core for receiving use – 50:200Ω I laid out a design using Fair-rite a #43 ferrite smallish binocular core. #43 is a medium permeability NiZn ferrite.

I have been asked by several correspondents why I used #43 when the consensus of online experts is that #75 is a clearly better choice for the application.

Let me say that almost all such articles and posts:

  • are absent any quantitative measurement of their proposed design;
  • they tend to use medium to large toroids; and
  • the few that expose their design calcs treat permeability as a real number that is independent of frequency.

#75 mix is a high permeability MnZn ferrite and subject to dimensional resonance in the frequency range of interest for this application, a problem exacerbated by using larger cores.

Permeability is a complex quantity that is frequency dependent and any analysis that pretends otherwise is not soundly based.

Above is a plot of  Fair-rite’s published permeability complex data for #75. Note the rapid descent of the real component µ’ to zero, a strong sign of dimensional resonance in the measured core. Dimensional resonance is a function of cross section dimensions and larger cores may have dimensional resonance evident well below 1MHz.

The chosen small binocular #43 cores do not exhibit dimensional resonance in the frequency range of interest, the windings are a practical number of turns, the performance is predicted by a simple model, and the measured performance documented in the article reconciles well with the predictive model.

References / links

  • Snelling, E C. Soft ferrites properties and applications. Iliffe books 1969.