Also see Power lines
Switching mode power supplies, light dimmers, computer networking systems,
poor connections that arc, and other “accidental transmitters” that
either switch or spark can create considerable RF energy on wiring. It is
helpful, when attempting to reduce such noise, that we understand how the noise
travels from the source into the receiving system.
Definition of a Source
Let’s consider the problematic device a “thing in a black-box”, and
call it the “source”. It doesn’t matter if it is a
computer, the spark in an electric fence, a light dimmer, or a doorbell
transformer. The smallest area we can isolate creating the unwanted energy will
be the source.
Radiation from a Source
There is very little radiation from a source. As a matter of fact, even a
very powerful single-terminal source can’t transmit at any distance. In order to
be an effective radiator, the source must have two or more connections to the
outside world. The multiple connections (two or more) could be the two (or
three) wires in a power cord, or a insertion point in a single wire. It could
also be a connection to two totally different sources. Let’s look at examples of
Consider a poor slice in a conductor carrying more than a few volts can cause
a RFI generating arc. Two of the most common examples are an electric fence or
The noise results when an insulated area with an electrical open circuit at
low voltages breaks down as sufficient voltage appears across the gap. This
ionizes air in the gap, and the resulting plasma conducts heavily. Such arcs can
be particularly noisy at radio frequencies, because resistance can fall faster
than voltage increases creating a negative-resistance. Negative resistances
created in spark plasma were actually used as the amplifying element in crude
oscillators. The plasma excited resonant circuits in the very first
“single-frequency” transmitters, and at times those transmitters
spanned distances of thousands of miles to very poor receivers using what were
very poor antennas by today’s standards!
Each conductor direction leaving the arc forms half of a large antenna. The
entire system, from the source out in both directions, radiates. There isn’t any
effective way to cure or substantially reduce the unwanted radiation except by
stopping the arc. Poor slices can become quieter in heavy rain, and can be
broken up by wind or wire movement.
This type of arc excites the conductor entering the splice from one direction
as one terminal, and the wire exiting the splice as the other terminal! The
coupling mechanism is like a dipole feedpoint, and is very efficient at
radiating over very wide frequency ranges.
Arcing to Another Conductor
A poor insulator or a close-spaced gap can break down and generate
Once again, the noise results when an insulated area with an electrical open
circuit at lower voltages breaks down when sufficient voltage appears across the
gap. This ionizes air in the gap, and the resulting plasma conducts heavily.
Such arcs can be particularly noisy at higher radio frequencies, because
resistance can fall faster than voltage increases creating a
Shunting-arcs generally have poor low-frequency energy, because the gaps are
almost always connected to a very high impedance at low frequencies. This makes
a very poor antenna connection for the arc, and reduces low-frequency response.
If the gap had a path to a low resistance, the plasma would form a dead-short in
the path. This would quickly cause a catastrophic system failure in systems able
to supply sufficient current.
Non-arcing RF Generators
Many devices contain high-speed switching systems. These devices include but
are not limited to televisions and monitors, computers, lighting systems with
dimmers, and low-voltage lighting systems using switch-mode power supplies.
These non-arcing sources are similar to arcing sources, in that they require
at least two terminal paths to become effective radiators. They generally are
frequency selective or frequency periodic, and produce a broad buzzing signal
that drifts around.
No matter what source is at work, we can be sure more than one conductor is
at work in coupling from the source. If we disturb that path by isolating one
conductor with a high RF impedance, we can reduce the interference. But by far
the most effective way to remove or reduce interference, other than by removing
the actual offending source, is by “shorting” the RF path with a
bypass capacitor. As we will see later, this is almost always much more
effective than adding series
Common Noise Sources
Arcing Powerline Insulator or Hardware
The bell-insulator in a power line is probably the most common noise source.
Older bell insulators has two interlocking but well-insulated metal posts. These
metal posts have considerable area, because of the required mechanical strength.
This results in considerable capacitance, in the order of a few dozen picofarads
or greater. The ends of each bell terminal are held by a loose-fitting pin
assembly. This pin assembly is the root of most powerline noise problems.
Bell insulators are often misused by power line crews. They are sometimes
used to terminate short spans between poles, where the correct insulator would
be a rigid post type or a polymer insulated fiberglass rod. The bell insulator,
used in a short span, often is installed without sufficient tension. The lack of
tension allows corrosion to set-up on the loose-fitting pins, and the
capacitance of the insulator creates a voltage divider. Since the pin capacitive
reactance is very high compared to the insulators leakage resistance and
capacitance, a very large voltage can appear across the pin. This is true even
when the pin is connected to grounded hardware! These loose short spans are
called “slack spans”, and bells should NEVER be used on slack spans!
Slack spans with bell insulators generally become quieter during damp weather or
in or just after rain, and can often be broken up by wire movement.
As the sine-wave peaks, the corrosion or oxides in the joint of the
loose-fitting pin break down and arc. This causes an unintentional electrical
noise to be generated, and the two terminals for the transmitter become the
power line conductor and the grounded pole or hardware on the pole. With a
wooden pole, the entire upper area of the can be excited with leakage currents
from leakage capacitance and leakage resistances. Even if an insulator isn’t
arcing, a loose metal staple over a grounded wire or other poorly connected
metal-to-metal joints will arc. Even a very tiny arc can excite the ground wire
running down the pole and the power line wires as two (or more) terminals for
To a much lesser extent transformers, lightning arrestors, fuse assemblies,
and disconnects can have internal arcs.
Trees and foliage may also contact HV power lines and “burn”
against the wire. Generally foliage noises are not extremely loud sources at low
frequencies because one terminal of the source is through the tree, but
occasionally they can be strong at lower frequencies.
As a general rule, but not always:
- Series-connection arcs from loose or corroded metallic hardware are
reduced by wet weather. Water, contaminated by the corroded metals, forms a
path around the poor connection and stops or reduces the arc.
- Shunt connection arcs from poor insulation are often reduced and may
disappear in dry weather. They are caused by dielectric (insulation)
- Corona noises are stronger at very high radio frequencies
- Hardware arcs and bells are generally strongest at higher radio
- Hard arcs like splices, clamps, transformer insulation, and switches are
generally worse at lower radio frequencies
- The arc can be a very long distance from the point where you hear it on
LOWER radio frequencies
Older electric fences had a few-second timed off-and-on buzz. Newer fences
have a “popping” noise, because they charge the fence in
“tics”. Fence problems are similar to power line problems, except of
course the hardware is much smaller! Most severe problems are related to poor
insulators, bad splices, or loose hardware.
Computers, TV’s, and Switching Systems
In older times, TV sets created problems from sweep circuits. The horizontal
sweep circuit operates with a saw tooth shaped waveform that was rich in
harmonics. Early video systems overpowered the horizontal sweep system, and used
the extra energy to supply high tension for the CRT. This resulted in many
point-to-point wires carrying high power harmonic-rich 15kHz RF energy. A
typical 25-inch television would use about 15 watts of the horizontal sweep
system power for sweep, and perhaps 30 watts for powering the CRT second anode!
Wasted energy from the 50-75 watt sweep system was often coupled to the
outside world a push-pull signal between the antenna leads and the power
Modern computers and TV’s mostly create noise-problems from internal
switching power supplies, rather than sweep systems. The sweep systems, if ever
even required, now generally only power the sweep. The HV often comes from
separate HV power supplies.
Switch mode power supplies, whether in a TV, computer, telephone, video, or
lighting system are rapidly becoming the most common source of modern RFI
problems. Most of this has to do with poor testing techniques, lack of good
standards, poor enforcement of the few poor standards we have, and a lack of
skill or knowledge in power supply and device design and installation.
Fortunately most problems can be corrected by external filtering, but it
generally needs to be done at the offending device.
Curing RF Egress or Ingress
Once the outside world is reached, the push-pull nature of the
source can appear as a common-mode signal following a group of wires for many
miles. We can think of this as one very long wire with the earth as the return
path. The signal either directly radiates into our antennas, or it excites our
antennas via common mode paths along cable shields.
While power lines and high-tension electric fences must be cured
by removing the arc at the source, lower voltage systems can almost always be
cured through proper external bypassing.
As a first response, we often like to throw a few ferrite beads
at the problem. In more sophisticated approach, we might use multiple-turn
series inductors. What these approaches miss is the great difficulty in
obtaining adequate series impedance. When we consider the series impedance has
to be totally ISOLATED for each conductor, and further consider core
saturation from low-frequency operating currents we quickly realize series
impedance is NOT the most effective method. It is helpful and may
reduce RFI to acceptable levels in mild cases, but it certainly is an
ineffective cure. Even if we are successful at one frequency range, it becomes
virtually impossible to have a high series impedance over very wide frequency
The best approach is to add some RF series impedance and to
augment it with excellent bypassing of all leads entering or leaving a device to
one common point. That common point generally is connected to a safety ground,
available in the USA at the third ground terminal of every modern-code
Coaxial or audio cables should all leave the device through a
bulkhead plate with all shields grounded. The power cord should pass through
that bulkhead, and be bypassed to the bulkhead with properly selected UL/CSA/VDE
approved line-bypass capacitors. Any chokes should be installed between the
source device and the bulkhead. When this happens, the device becomes an
isolated entity all by itself. It can not excite outside world conductors with
It is indeed fortunate that the very same things that cure RFI
often lead to greatly improved lightning protection.