Ferrite K mix

Among forum experts, there are ready recommendations for the ideal ferrite material (or mix) for a balun, often without knowing any detail of the application.

The ‘magic’ mixes include K. Perhaps they are devotees of Sevick.

Over some years I have searched for manufacturer’s data on K mix, and found only two references:

  • Amidon who give a very brief table summarising characteristics, inadequate for RF inductor design; and
  • Ferronics who give characteristic curves, albeit in less common format.

Problem is that Ferronics µi is 125 against Amidon’s 290… so their K materials are different.

One has hoped that an interested competent person might have made measurements of some samples from Amidon to give full characteristic curves, it isn’t that hard.


Ferronics describe their K mix as a cobalt-nickel ferrite.

Screenshot - 21_02_2015 , 16_43_01

Above are Ferronics curves, and you might at first think that since they have characterised their K material to this level of detail that they have good knowledge of product.


Clip 110

Above is the Ferronics-K data converted to complex permeability, a more common format.

Clip 111

The above curves for Fairrite-61 (nickel-zinc ferrite), fairly similar to the Ferronics-K curves, so one might conclude that although they are not identical, they are fairly similar in behaviour.


As mentioned, Amidon does not show characteristic curves for their mix K products on their website. Though I say “their product”, it is probably manufactured by another party, unknown.

The volume resistivity given is typical of nickel-zinc ferrite, as is µi in range.

The short form characteristics are somewhat similar to TDK K5 and later SY20 materials.

It has been suggested that Amidon-K is very similar to Fairrite-52 (nickel-zinc with µi=250) but without seeing credible Amidon-K detailed curves, I cannot comment.

I would be willing to crunch the numbers on a measurement taken of an Amidon-K product. I must be sure it is recent Amidon product, and the measurements must be from a reliable, quality one or two port VNA or equivalent antenna analyser. I do not regard any AIM products currently meets the requirement.


  • It seems there are at least two ferrite products in the market labelled as K mix with significantly different characteristics.
  • When buying baluns advertised as using the magic K mix, which of these two characteristics is the magic one, and which are you buying.