 The supplier states AL = 95 nH for the T106 toroids. From the geometry (Do=27, Di=14, h=11.5 mm), we can calculate magnetic circumference lt = pi*(27+14)/2 mm = 62.4 mm and cross section at = 11.5*14 mm^2 = 161 mm^2. Using AL = µr * µ0 * at / lt we get µr = 95nH / 3.14nH = 30. This low value indicates that the cores indeed consist of iron powder rather then ferrite material.    My original idea was to assemble a thin "broomstick" coil around a lr = 1 m long stack of ~ 90 cores, surrounded by a single-layer solonoid of n = 5000 turns (0.2 mm wire) and 3 cm diameter. The core cross section of the rod is then  ar = pi/4 * (Do-Di)^2 = 4 cm^2, which at 8.27 kHz would allow 4.1 V per turn at 0.2 Tesla flux.   We have to consider the effect of the air gaps due to the enameled cores. Assuming a gap 2 * 0.05 mm every 11.5 mm we get µgap = 11.5/0.1 = 115. Due to the rod's large length-to-diameter ratio, the influence of the demagnetisation factor µd ~ lr^2 / ar = 5000 is negligible, and we get µreff = 1/(1/µr + 1/µgap + 1/µd) = 1 / (1/30 + 1/115 + 1/5000) = 24.and a total inductance  L = µreff * µ0 * ar / lr * n^2 = 12 nH * n^2 = 0.3 Henry.   Unfortunately this is much less than the 1.4 Henry required to resonate my small 270 pF antenna at 8.27 kHz, so the coil would need to be either thicker or much longer.   Anyway I have ordered some cores to make measurements, and also some of the red double-height ferrite (?) sleeves from the same supplier.   All the best,Markus (DF6NM) -----Ursprüngliche Mitteilung----- Von: DK7FC <[email protected]>An: rsgb_lf_group <[email protected]>Verschickt: So, 17 Apr 2016 12:31 amBetreff: Re: LF: TXing on VLF again, Iron powder cores... `Hi Clemens, Am 16.04.2016 21:42, schrieb Clemens Paul: > There's a typo: 25 should read 52. Yes, sorry, a typo. It can also be read on the cores in my images ;-) > Really cheap cores. > Yes indeed! I called them and asked if they have 500 more of them available. They (she) said, they have MUCH more then 500 of them available :-) > The question is: > Are they iron powder or ferrite like the seller says? > The seller is wrong here. T106 describes just the geometry of the core. Material 52 can be 'read' from the colour combination. > According to the spec'd AL value they should be iron powder. > Did you *measure* AL? > No i didn't. I just observed how much the resonance frequency of my large coil and that small coil drops when i put them 'into the field'. This is what counts. Markus did some calculations, maybe he will report you here. 73, Stefan > 73 > Clemens > DL4RAJ > > > `