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When previously using iron-powder toroids I've always had close correlation between measured inductance values and those obtained from the published tables and on-line calculators. So when doing some development of a [40m/30m] LPF for my QMX I was surprised to measure inductance differences between 10% and 20%.? For example the calculator at toroids.com shows 440nH for an 11-turn coil on a T30-6 core with 25mm pig-tails. But with the T30-6 supplied with my kit I'm measuring 529nH with pig-tails of just 10mm. It's the biggest discrepancy I have ever seen. It's not a problem for a one-off design checked on the bench. But if this is typical then there may be consequences for production repeatability of LPFs, especially any that use a "notch" to pull down the response at second harmonics. Has anyone else measured similar discrepancies?

(I'm a 30+ year professional user of VNAs and am freshly calibrated at the measurement reference plane with SMA short, open & load. And I am counting the turns by the number of passes throught the toroid centre.)

20% is a typical tolerance and if you have an inductance meter it’s better to go back to the theory calculated inductance values and check them as you wind?

(That said it’s common to see ‘off values’ result ?in practice for output (LPF) filters as they are tweaked to provide matching as well as filtering )

The statement below about a 20 Percent tol on IP toroids is WRONG.? There is no such generic number.?

For type -6 material which is the most common material for HF use look at -

Note this is for a specific mafr but it appears generic toroids also "yellow - 6"? are made of Ni Cabonyl to the? MM specification.
The tolerance given is Plus minus 5 percent on AL.? that's for type -6 material for Type 18 my hardcopy manual says the tolerance is plus minus 10%

MM base their AL on a fully wound? toroid. and then back calc from there (measured uH at freq) to get the AL
If? you don't fully wind but also don't evenly space the windings around the core the effective AL will be less than tabulated.?