GM all,
To build a stable high Q inductor with a predicable value using high
permeability manganese-zinc ferrites, like Fair Rite type 77 for example,
you will need to use an air gap. Even a small air gap drastically reduces
the effective permeability but in exchange you get value stability. Gaps of
a few mils are typical. The design process is a trade-off between gap size
and copper loss, i.e. more gap more predictable the value but less effective
permeability means more turns for a given inductance.
GL and 73, Rudy N6LF
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