The first time I ever saw a steel wool balun was in the ARRL Handbook. This was many years ago. This made no sense to me, so I tested some sample baluns. I found the steel wool actually decreased the impedance of the cable shield, exactly the opposite of the desired effect.
Recently the steel wool, brillo pad, or SOS pad balun appeared in an Internet forum. Also, a friend from down under now claims a steel bolt makes a good balun.
To settle this controversy (this really should never have been a controversy, because the idea is pretty silly) I decided to measure a steel wool balun and see how well it works. Since I'm fortunate enough to have a laboratory grade impedance test set in my workshop, I thought I would put the debate to rest with actual measurements of common mode impedance. The HP4191A can easily and accurately measure Z, R, X and many other parameters from 1 to 1000 MHz.
Test Setup
I calibrated the HP4191A from 1-30MHz with standard loads at the end of a test cable. This normalizes the instrument to the end of the cable. Anything attached to the end is read as if right at the impedance detector!
The above photo shows the reading with a short. It reads -3 milliohms and 7 milliohms reactance. -.003 R and .007 X is very close to a perfect dead short. We know the instrument is good at zero ohms.
Next we measure an open to confirm the high impedance end is calibrated.
As seen above, the open measures 80k ohms resistance and -80k ohms reactance, or above 130K ohms impedance. This is plenty good enough for this measurement so there is no need for recalibration.
Finally a 50 ohm precision load is tested.
It reads 49.97 ohms with .04 ohms of inductive reactance. This is an impedance of almost exactly 50 ohms. Once again, calibration is good enough for this measurement.
Steel Wool SOS pad Balun Measurements
The objective of installing a balun is to reduce feedline shield common mode current. A balun should cause the cable shield to have the highest possible impedance possible for any unwanted RF flowing on the outside of shield. We can measure that impedance quite easily with this test setup.
Notice the R X light is lit. A one foot loop of RG178 coax shield has a common mode impedance of .268 ohms resistance and 33.7 ohms reactance on 7MHz. This is an impedance of 37.8 ohms.
Next we wrap a super fine steel wool over the cable shield. This will show us how the impedance changes.
Steel wool is a mess around electronic equipment, but let's see if it works.
Next we measure R, X, and Z.
R has now increased from .268 to 1.067 ohms. X has decreased from 33.70 to 32.54 ohms. Despite being steel, the wool has actually lowered the inductance. This is because of Lentz's law. The magnetic field from the cable shield caused a circulating current in the steel wool. The magnetic flux from that circulating current opposes the magnetic field from the cable, and the inductance is reduced. This is why at radio frequencies we use powdered iron or other cores that suspend very tiny particles of iron in an insulating substrate. This is also why transformers and chokes are laminated, and the laminations are all insulated from each other. If metal touches metal, the eddy currents will cancel the magnetic properties at higher frequencies. The higher the frequency, the smaller the pieces of metal must be.
Physics cannot be ignored.
The impedance of the steel wool balun is 32.56 ohms (at 88.12 degrees phase angle). This is a DECREASE in common mode impedance from 37.8 ohms down to 32.56 ohms. In a balun or shield decoupling application the unwanted common mode current would likely increase! This is exactly the opposite effect from what we want. We have done a very bad thing by ignoring basic rules of physics.
Let's compare some real ferrite materials that are much smaller than the steel wool balun.
Two inches of #73 beads has a resistance of 136.5 ohms, and a reactance of 19.53 ohms. It is a dissipative resistance in series with a small inductive reactance. This is very good for RF suppression.
The impedance of the 73 beads is 137.9 ohms. In comparison the bare shield's impedance was 37.8 ohms, the steel wool was 32.56 ohms impedance. Steel wool was the poorest of the three systems tested, it was worse than nothing on the shield!
A different mix of a few less beads produced 86.45 ohms resistance and 46.15 ohms inductive reactance, over 100 ohms impedance. This was 10 beads of number 59 material.
Why do people claim steel wool works?
It's human nature. We like simple fast cheap solutions to everything. We also don't always know how to measure things and in many cases can't measure things. The human emotional side is what allows believing in something even when the science is meaningless or totally wrong. This is how all those rumors and folklore like the shielded magnetic loops or parasitics damaging amplifiers get started and why they are difficult to correct. It isn't that anyone has a bad intention, they generally just don't measure what they think they are measuring or they misinterpret the cause behind an effect they actually do observe.
Let's stop this myth about steel wool and steel bolt baluns.
since 8/18/06