I found this data a year ago -
"The easiest way to identify most ferrite materials is to wind about four turns of wire through the core and then measure the lowest frequency at which the value of reactive impedance equal resistive impedance i.e. X=R. There will be some variation between different batches and sizes of materials. But if you can plot the results graphically you can easily identify the 'signature' of each material.
Here are my references for some common ferrite materials
FT240-77 0.74MHz
FT240-31 3.5MHz
FT100-33 7MHz
FT240-43 17MHz
FT240-K 22MHz
FT240-52 31MHz
FT240-61 58MHz
Iron powder has a slightly different 'signature' it usually has a very low resistive component, which peaks to a higher value near self resonance. The more lossy the material the broader and lower value of resistive peak is apparent.
T200-52 40MHz Lime Green (& Blue or Red) colour common in PC switch mode power supplies - moderate loss
T200-26 60MHz Yellow & White colour common in PC switch mode power supplies - high loss
T200-2 60MHz Dark Red colour used for HF tuned circuits (& Ruthoff Ununs) - high Q low loss
T200-1 70MHz Blue colour not common - moderate loss
T200-6 100MHz Yellow colour used for VHF tuned circuits - high Q low loss
To illustrate this point here are some loss measurements made on ring cores recovered from switched mode power supplies.
In each case the windings were 5 turns of 1mm wire bifilar wound as a 1:1 transformer. As you can see there is great deal of variation between the results depending upon the type of core material.
Powdered iron cores are popular for high power baluns, but they don¡¯t offer much inductance per turn of wire, so their effectiveness when used as baluns can be limited. Ferrite materials provide a much higher impedance value per turn of wire and are much more effective over a wider frequency range, but they can be very lossy when connected to a mismatched load, and heat up to a point where irreversible damage occurs."
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At this web address:
Using G8JNJ's data for matching the crossover frequency where X=R for the toroid being tested, I believe I've determined the mix type for a few unknown cores. The two I was most interested in were both roughly 2 inches diameter, but one was thick, heavy and all black and the other is painted gray with yellow around one edge. The X=R points were very close to the same value for each of those two cores, and were the same as Mix 77.
G8NJNJ's page was the only place that I found the crossover X=R frequencies data listed. I feel very grateful to him for publishing the data.
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Doug, K8RFT
