On the possibility of steel nails as EMI filters

I thought steel has a very high iron loss especially at high frequencies. So is it possible at least theoretically to use hard steel nails to attenuate high frequency noise in say power supplies?

No, not really. A ferrite bead (for instance) relies on the outer ferrite material (not the through-going wire) being both a poor electrical conductor at low frequencies but, at high frequencies, becoming a lossy capacitor and capable of turning EMI into heat. Here are a few examples from Murata: -

As you should be able to see, FBs are designed to target a specific range of frequencies and, different values in the same model range can be chosen to give better attenuation at certain parts of the spectrum whilst maintaining reasonably low losses for signals that should not be significantly attenuated.

I live in a place where ferrite beads for emi suppression is hard to get

A nail doesn't have one of the vital characteristics of ferrite that make it very useful as an attenuator namely; that it acts as a lossy capacitor as frequency gets higher and therefore resonates with the parallel inductance of the through-going wire.

There is a good document from Analog Devices that explains things in more detail and that document shows the developed model for a Tyco Electronics BMB2A1000LN2: -

R1 and C1 represent the lossy dielectric of the ferrite material and you just won't get that with a regular piece of iron or a nail. In case anyone notices the typo in the ADI picture above (L1 = 1.208 uF) should read 1.208 uH thus producing a peak resonance at around 112 MHz.

People used to make lossy inductors (chokes) by scramble-winding 100 turns around a 100 ohm resistor; have the choke and resistor in parallel.

The scramble/random winding was to minimize the coherent capacitance between layers of the wire, thus reducing risk of any resonances.

If this RFC Radio Frequency Choke were in the plate of a Class C amplifier, you'd probably use a 2_watt 1,000 ohm AllenBradley resistor; the large resistor was needed for surviving the high voltage.

Read some construction articles in old ARRL (ham) manuals. Or QST magazines.

The full theory here is very messy. You have skin effect and hysteresis loss in play. So, I thought I'd do an experiment. I close-wound a coil of #32 magnet wire on a 10D 3" galvanized steel nail. Here's the coil:

Note that the ruler is centimeters.

I measured the attenuation with the coil placed in series between a sinusoidal function generator and a scope with a 50 ohm terminator across its input. From that, I calculated the impedance versus frequency:

Not so different from a ferrite choke.

Edit:

For more detail, I took more measurements with a tightened-up test setup. I give raw measurements for your computational enjoyment. Input voltage was 1V RMS sine waves: I readjusted the function generator output for each frequency. The time delay through the setup with the coil shorted was 29 ns, measured with a square wave. I have not corrected the delay below for this.

• MHz Volts Delay(ns)
• 1.0 0.36 100
• 1.4 0.34 71
• 2.0 0.28 59
• 2.8 0.25 48
• 4.0 0.21 38
• 5.6 0.19 32
• 8.0 0.16 28
• 11.0 0.15 24
• 16.0 0.16 21
• 22.0 0.22 20

The "resonance" is extremely broad.