Steel Wool Baluns, Steel Bolt Baluns, Steel Core Baluns

Bolt baluns, steel wool balun cores, and other solid iron
or steel or laminated cores are a HOAX. They are not baluns at all. The text
below explains why they are hoaxes.

It is pretty common to see claims that iron increases inductance, or find
claims that iron concentrates magnetic fields. Iron certainly does concentrate magnetic
fields, and that often, but not always, results in increased inductance. But not
always…..

A problem arises when magnetic fields are changing level.
A time-varying magnetic field will generate currents in any closed conductor
path, even iron. This current is called an “eddy current”. The amount of current
depends on the size of the iron particle(s) forming the conductor and the change
rate of the magnetic field. The larger the particles, the larger the conductor
area for eddy currents become. The larger the area, the lower the frequency
where eddy current start to cause problems.  

Eddy currents generate their own opposing magnetic field
to the incident field exciting the core. If the iron has large enough cross section eddy
currents and the resulting counter MMF from the eddy currents will push the
magnetic fields back out of the core. With increasing frequency, a given size
iron particle has a frequency where inductance starts to decrease. This effect
is from the “shorted turn” generating opposing flux. As frequency increases
further, inductance decreases. At some frequency the core is no longer able to
support the field, and because of eddy currents, actually reduces inductance.

 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. 

We see this effect when we look at effective permeability of iron RF
cores. In the graph below, permeability (“inductance”) μ’s   is
flat until 1MHz. Above 1MHz  μ’s decreases. μ’s reaches zero
someplace above 30 MHz. At that frequency, the core is no longer magnetic. This
is because the iron in the core is too large in size, it cannot follow the
magnetic field, and it supports eddy currents. A steel bolt would be thousands
of times worse, and not even work at audio frequencies. Steel wool might work at
audio, I have not tested it.

Notice that losses µ”s
steadily increase from 100 kHz, peaking at 2 MHz.

Also notice that permeability
(inductance) starts to fall at 1 MHz, and 
µ’s drops off the graph at 30 MHz. 

This is largely due to the insulated iron particles in the core becoming too
large for the frequency. Circulating currents in iron granules become
electrically large enough to support significant eddy currents,  increasing
loss and decreasing inductance.

Steel Wool Balun      

The steel wool balun is a good example of not understanding behavior of irons and magnetic fields
at radio frequencies. The steel wool balun actually made it into the ARRL Handbook,
which is one of the most reviewed amateur radio technical publications.

Even at first glance, the steel wool balun made no
sense to me. Recently the
steel wool, brillo
pad, or SOS pad
balun appeared in an
Internet forum. Additionally, a Ham operator from Australia published an article
claiming 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
feed line shield
common mode current. 
A balun should cause
the cable shield to
have the highest
possible impedance
for any
unwanted RF flowing
on the shield’s outside. 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, reducing inductance. 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
transformer and
choke cores 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, worse than
using nothing on the
shield! The effect is opposite the desired and claimed effect. 

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
simply misinterpret the
cause behind an
effect they actually
do observe.

Let’s stop this
myth about steel
wool and steel bolt baluns. Audio core materials are laminated or powdered for
good reason. RF cores are small insulated particles packed together for a
reason. A solid material does not behave like a smaller cross section material. 

since 8/18/06