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I'm asking this because of the comments in this answer, that imply that steel (mostly iron) has similar characteristics to ferrite.

Specifically, it is implied that inserting solid steel into a single-layer coil (as is commonly used in an antenna tuner) will significantly increase its inductance at HF, as some ferrite and powdered-iron rods do.

Is this true under any circumstances?

What are suitable materials for this purpose?

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  • $\begingroup$ See w8ji.com/steel_wool_balun.htm. $\endgroup$
    – Mike Waters
    May 21, 2019 at 20:39
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    $\begingroup$ nice measurements :) I actually prefer to be proven wrong – being right holds no opportunity to learn something! $\endgroup$
    – Marcus Müller
    May 21, 2019 at 20:58

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Any material with a relative permeability greater than 1 will increase inductance when inserted into a coil.

Note that permeability is a complex number and frequency dependent. The imaginary part of permeability contributes to loss and appears as a resistance, so the addition of some material in the coil may increase inductance and also add resistance. Generally, the real part of permeability of materials decreases with frequency, while the imaginary part increases. Thus at some frequency, the material becomes useless for constructing inductors.

Many magnetic materials are also non-linear. This is because they work by aligning internal magnetic domains within the material with the applied magnetic field of the core. At some magnetic flux density, the domains are all as aligned as they can be, and can become no more aligned, and thus can not respond to increasing magnetic flux density. This is called "core saturation".

Any ferromagnetic material, such as iron, nickel, or cobalt, has a high permeability. Paramagnetic materials, such as liquid oxygen work as well. Generally, anything that will stick to a permanent magnet indicates a high permeability at 0 frequency.

In practice, ordinary lumps of metal do not make good cores for inductors because their real permeability decreases rapidly with frequency, and inserting them into a RF inductor just makes them hot. A resistor is a much simpler and economical way to achieve the same electrical effect.

Powdered iron cores and ferrite cores are two common materials engineered to overcome these effects to produce useful inductors up to 10s or sometimes even 100s of MHz. Beyond those frequencies even these materials become ineffective. Fortunately higher frequencies also tend to require lower inductances, and thus air-core inductors become practical.

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  • $\begingroup$ @ZeissIkon It's as good an answer as is possible, especially since we have no physical specifications to date. See my comment here. $\endgroup$
    – Mike Waters
    May 22, 2019 at 20:26
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TL;DR:
It's NOT mild steel, brass, or aluminum.

I connected two different inductors to an antenna analyzer, and all of the above materials decreased the inductance as they were inserted.

The larger the diameter of the steel I inserted, the lower the inductance and the greater the loss. It was the worst material I tried.

When I inserted some powdered iron (a 2.4" OD T-200-2 toroid core that I happened to have on hand), the inductance increased. The greatest increase by far was when I inserted a short piece of 1/2" diameter ferrite (Amidon R61-050-750) which was left over from a grounded-grid amplifier cathode choke.

From https://www.w8ji.com/steel_wool_balun.htm:

For example, inserting a solid iron slug inside a small RF coil shows a behavior almost identical to using brass or aluminum slugs. Inserting a solid slug of iron might increases the magnetic field concentration and inductance near direct current frequencies, but at some higher frequency eddy currents and the inability of the core to follow field changes cause the flux concentration to decrease.....eventually reaching zero. At some frequency, the counter MMF takes over. Inductance is actually reduced by the core.

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  • $\begingroup$ "Mild steel" is what common, ordinary Grade 5 hex-head screws, bars, and bolts are made of. What your local hardware store and steel warehouse offers. Type 1018 or 1020, as my freshman high school machine shop teacher told us boys. $\endgroup$
    – Mike Waters
    May 21, 2019 at 22:39
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    $\begingroup$ Steel probably increases the inductance, but any increase is hugely overshadowed by losses caused by eddy currents in the solid metal piece. The point of powdered ferrite/iron is to increase the inductance while not presenting a large enough contiguous conductive piece of metal that can allow eddy currents. $\endgroup$
    – user10489
    May 22, 2019 at 5:06
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    $\begingroup$ Well done for taking actual measurements. I'm still itching after seeing the steel wool near the instruments. I'm sure there's a frequency at which a steel bolt does increase the inductance of a coil, but it's probably below 100 Hz. $\endgroup$
    – tomnexus
    May 22, 2019 at 11:54
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    $\begingroup$ This makes complete sense. I (the author of the question in the link) had been thinking steel would increase inductance, because it does at frequencies where I have experience (AC line power, 60 Hz in the USA). After reading the "steel wool balun" article, I now understand why this changes at higher frequencies -- same reason we don't see steel core RF transformers. $\endgroup$
    – Zeiss Ikon
    May 22, 2019 at 13:57
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    $\begingroup$ Hmm. I have a form with a fast-twist screw insert that's size just a little bigger than common ferrite rod. Looks like I might have to build one and test it after all. Time to order some ferrite rods. $\endgroup$
    – Zeiss Ikon
    May 23, 2019 at 11:14

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