I have always thought that using ferrite toroid cores for radio projects at HFs and above was not necessary, because all of the RF circuit parts lists call for low permeability powdered iron cores. I’ve never given it much thought; about the only thing that crossed my mind was that the powdered iron cores may have less temperature coefficient than ferrite because they have much less influence on the magnetic field (a -0 core is non-magnetic and is the same as an air core, so it has almost no thermal coefficient).
In this blog post, at the end, below the cat pics, the author compares the characteristics of two types of cores. He doesn’t say anything about thermal coefficient. But the -43 ferrite medium mu core compares favorably with the other core at HFs and above. This is the first time I’ve seen any comparison of a ferrite core, even though it is not complete.
I just bought some powdered iron cores, just to see how they work in RF tuned circuits. Now that I’ve read this article I may also try some of the ferrite cores that I’ve used for Joule Thiefs.
