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The tube or winding can be coated in glue or varnish to help with this. Cardboard (say, paper towel tube) can be used, but it's not dimensionally stable and your winding will come loose. Easily found at McMaster Carr or the like (.if you don't mind paying for it). As for winding form, they probably used phenolic tube or something like that. The lower Q won't affect the calculation much. (If you're getting warnings and no output, try increasing pitch or decreasing wire size a bit. Also see the self resonant frequency, or the estimate at least. Longer means higher impedance peaks, but less efficient (inductance per wire length, which peaks around 1:1). There's probably k ~ 0.1 between sections, giving a similar (~10%) increase above that figure. The inductance of the chain will be slightly higher than the sum of each section in series, due to mutual inductance. It is more accurate than anything you can physically measure, and includes frequency dependent effects. Estimate the dimensions and enter them here: This is the best calculator I know of anywhere, for single layer solenoids. The spool winding has deeper resonances, and at lower frequencies, making it unsuitable for the inductor facing the EUT but where it is, it doesn't need much impedance (compare to the 5 ohm resistor it's effectively in parallel with), so it's a fine way to save space. I think you'll find it's the other way around, the 250uH are on the spools (more layers = more inductance in a smaller package), while the 50uH is sectioned, in single layers.