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.)
