----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
>
> OK..
>
> Let's say we want L from around 0.1 to 20-30 uH in a 1,2,4 kind of
> sequence
> (so we'd like to choose cores with a permeability such that we can get an
> exponential sequence with integer numbers of turns) For instance, the LDG
> AT 11 uses T106-2 cores and the following scheme
> Turns uH
> 1 .11
> 2 .22
> 3 .39
> 4 .59
> 7 1.25
> 11 2.5
> 17 5.0
> 25 10
>
> Note well that the uH doesn't track the number of turns squared... (which
> it should...).
You're not counting turns correctly, Jim. If you add 1 to your # turns
above,
then the inductance numbers work out according to N^2. I made this same
mistake when I was measuring toroids recently. You need to count the
number of times the wire passes through the center of the toroid. Thus 1
turn looks more like a 1/2 turn, but it is really a full turn. Likewise 2
turns
looks more like 1 turn, etc.
I would start by looking at Fair-Rite products type 61 material (ui = 125)
for transmitting applications in the HF bands. It has fairly low loss up
to about 20 MHz. Above 20 MHz it starts to get lossier, but you might
be able to get away with it. T106-2 sounds like an iron powder core.
I haven't worked with these much. Permeability is low compared to ferrite
cores, but might be high enough to get you into the 20uH range with a
reasonable number of turns (pretty sure this is what Alpha uses for the
low-frequency portion of their HF amplifier tank inductors).
Mike, W4EF..............................................
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