Did you look at the B-H curves for the material you're using? Any magnetic
material will be non-linear to some degree. Normally the lower the peak flux
density you operate the material at the more linear the B-H curve. Generally
ferrite will have lower losses compared to powered iron. However powered
iron has more tolerance to small DC bias in the core without saturating the
material. Which brings up another point you should not operate the core with
a DC current flowing through the winding if you can avoid it. The best you
can do is find the combination that provides the inductance you need while
operating the core material at the lowest flux level you can get away with
using which tends to suggest using a high permeability material using the
least number of turns to get the needed inductance. The flux density is
proportional to the number of turns on the core and the current in the wire
(ampere-turns which is the "B" field on the B-H curves). As you noticed the
higher permeability material seems to work better, that 's what I would have
expected. That's my 2 cents worth of advice.

--
Regards,
Leland C. Scott
KC8LDO

"K7ITM" wrote in message
...
I have an interest in making RF filters with low distortion (for
example, as preselection filters for receivers). I've found that very
often I need to use air-core coils to get the low distortion I want,
but it would be convenient if I could use powdered iron or ferrite
core toroids to get higher Q at low frequencies without having to make
the coils physically big. I've always assumed that powdered iron
would give me the best results, but someone suggested I try low-
permeability ferrite. I just did some experiments with that. I wound
three toroids, all 0.37 inch OD cores; one is on mix 2 powdered iron,
and the other two are on type 68 and type 67 ferrite. I put each
coil in a simple 1MHz high pass filter: two 2.2nF caps in series,
with the coil connected between them to ground. I tested for harmonic
distortion with a 1MHz input, and for the 2.42MHz intermod product
with 0.62MHz and 1.52Mhz inputs.

With about +16dBm coming out of the test filter at 1MHz, I measured
second harmonic at about -95dBc for the T37-2, -90dBc for the FT37-67
and -57 for the FT37-68; the third was at -74dBc, -61dBc and -51dBc
for the three coils in the same order.

The 2.42MHz intermod, with each tone at +18.5dBm out of the combiner,
measured -71dBc, -58dBc and -38dBc for the coils, again in the same
order.

My conclusion from this is that at least these two ferrites aren't
going to help me in my search for a better low-distortion core
material, but at the same time, it was interesting to note that the
higher permeability -67 performed better by far than the lower
permeability -68. It gives me some hope that if the ferrite materials
guys went looking for a formulation that would be specifically as low-
distortion as they could get, they might find something pretty good.

Before someone asks, no, I didn't run a T37-6 core coil along with
these. I expect it would be a little better than the T37-2. And I've
measured a few coils wound on (unknown formulation) powdered iron rods
that show significantly lower distortion than those on toroid cores;
but of course those have a large "air gap" and relatively high
external field.

Cheers,
Tom