Good info Tom, I'd like you to try -61 material, but I don't have anything
but T50 in that ferrite.

W4ZCB

I have an interest in making RF filters with low distortion (for
example, as preselection filters for receivers). I've found that very
often I need to use air-core coils to get the low distortion I want,
but it would be convenient if I could use powdered iron or ferrite
core toroids to get higher Q at low frequencies without having to make
the coils physically big.