Noise and Interference
by Rob Gregor in the
Outback of Australia
Practical examples of
reducing EMI/RFI in Stand Alone Power Systems
Despite some argument on technical
differences, the terms RFI (Radio Frequency Interference) and
EMI (Electro-Magnetic Interference) are commonly used to
describe the same phenomena, which is that of disruptive
electrical noise generated mostly by nearby items of
electrical equipment and infrequently by local thunderstorm
activity. This noise travels by radio waves in free space and
along the hard wiring that connects various items of
electrical equipment in your residence.
It varies in its intensity from a nuisance
level of background noise, as is commonly heard on a radio
receiver when attempting to locate a distant station, right
through to severe which is disruptive and blanks out all but
the strongest radio signals. In some instances, it can
seriously disrupt the operation of electrical equipment such
as Bluetooth and Wi-Fi devices.
In the case of its deliberate use as a
military weapon, a massive EMI pulse can cause permanent
damage to many items of unprotected electrical equipment,
including mains power networks, and apparently it can erase
the memory storage in computers over a wide area.
The reason for writing this article:
In our situation, my wife and I live in an
isolated coastal area in South Australia and we have had to
develop a good degree of self sufficiency i.e. we build and
maintain our own access roads, collect our own fuel for
heating and use stored rain water for drinking. We generate
our power from solar panels and wind generators and we need to
have back- up systems for most things.
Keeping abreast of local and international
news is important to us and we typically use a Satellite
TV/Radio system. However, that is occasionally subject to
periods of very poor reception from thunderstorm activity,
therefore we also use back up radio receivers for listening to
music and news broadcasts on the MW (Medium Wave) SW (Short
Wave) and FM (low VHF) Bands.
Two of these radios are vintage valve/tube
radio receivers, which have an impressive performance when
fitted with a permanent long wire/random wire antenna and we
also have a good quality portable transistor radio which has a
built in ferrite rod antenna.
Because all but a few of the radio stations
are distant from us by at least 400 Km (and on the SW Band)
are even located in other countries, the signals vary from
weak to medium strength only and they were being severely
disrupted by the RFI noise that was emitted from our own power
generation and distribution system.
When I began the process of exploring
options to reduce the level of RFI, I did not manage to easily
locate straight forward guides on "how to reduce RFI/EMI in
the domestic situation" and I had to rely mostly on articles
of a technical nature which tended to focus on such items as
"process controllers" or VHF/UHF Radio Communications and
within a commercial environment.
This has prompted me to write a simpler
guide that sets out the process I have used to successfully
minimize RFI from a "stand alone" power system and which also
provides some practical tips that may assist others who find
themselves in a similar situation.
Typical sources of RFI:
Inverters and transformers, solar
regulators, electrical motors (such as in water pumps,
freezers, refrigerators, washing machines) fluorescent lights
(especially compact FL’s) and mains power lines to your
residence are among the most common sources of strong RFI.
Also, in our situation, the RFI intensity
emitted by our sophisticated solar regulator varied depending
on how busy the regulator was at any time of the day as did
from other appliances that are "demand activated".
Less common sources of strong RFI are
intermittent shorts or poor connections in house wiring, wall
sockets or old electrical equipment and among the worst
offenders for RFI are some makes of plasma TV’s and commercial
welders such as Arc, TIG or MIG. These are often the most
difficult to track down if they are located in your
neighbourhood but not under your direct control.
Wind generators, the "power packs" for down
lights, some flood lights and even for powering PC’s or
charging batteries in various devices that we all use on a
regular basis, are common causes of medium to strong RFI.
Again, in our own situation, we found many
other everyday electrical appliances that emitted low levels
of RFI but as we could control their time of use, we did not
consider these to be other than nuisance value.
N.B. I believe that it will be helpful to
interested readers if they appreciate that the amount of RFI
emitted in their locality will vary in its intensity and in
its frequency spread, depending on who else is operating
equipment nearby and by whichever manufacturer has made the
equipment. It can even vary between the various models made by
the same manufacturer, as I have found when testing LED Bulbs.
With this being the case, then the nature
of the problem and the cures may differ from situation to
situation but the steps which I have outlined will still be
useful in the majority of domestic situations.
Tracking the source:
As I do not have sophisticated test
equipment, I used a portable radio that has an internal and
directional ferrite rod antenna plus my ears to track the
sources of RFI.
This involved walking from point to point
inside and around our house and power shed and scanning across
the broadcast frequencies, in order to roughly locate the
worst sources of RFI.
Then after I had taken measures to obtain
some reduction to the level of interference, I then went back
over the same routes and rescanned across the various
broadcast bands in order to check the remaining locations of
RFI noise reduction options:
The most commonly suggested options seem to
be as follows:
Replace those items which produce RFI
with ones that don’t (generally only practical with a few
items due to the expense involved)
Use a Farraday cage to shield equipment
from the sources of RFI
Bury the mains power cable**
Add ferrite cored RFI/EMI filters to
Use earthed metal shielding around
power leads (not very practical in our situation)
Relocate the main causes of RFI to a
more distant location (also not very practical in our
Use a directional antenna or relocate
the radio antenna further from the major source of RFI. As
we needed to upgrade our fixed long wire antenna, this was
done and it did improve the situation.
**N.B. Our main power cables are already at
a depth of 700 to 800 mm under the soil and they still produce
from low to medium levels of RFI, therefore I am not convinced
of burial as a good solution unless you can go deeper than
that and you also have "heavy" soil rather than light coarse
grained sand as we have.
Typical set-up of RFI filters on Power
As it seemed an easy thing to do, we began
by replacing the CFL’s in our house with LED light bulbs but I
found that the majority of the less expensive LED bulbs still
emitted strong levels of RFI, although it was generally at the
higher ends of the broadcast bands.
After further searching with the assistance
of a helpful wholesaler and a retailer of these products, I
found good quality cost effective LED bulbs that are made by
one of Taiwan’s oldest and largest manufacturers of lighting
However, this same manufacturers smallest 3
watt LED bulbs still produce low to medium levels of RFI while
their 9 watt models are almost silent!
We have a metal framed and metal clad power
shed with an internal security screen of heavy gauge welded
mesh, so it was a relatively straight forward task for me to
convert it into a Farraday cage which "captures" the RFI from
all of the equipment within the cage walls and it
directs that signal to earth, thereby reducing the level of
radiated RFI signal which is usually intercepted by a nearby
This was achieved by adding "bond wires" to
any hinged sections such as the main door and roof access
hatch, which then formed a complete (i.e. electrically
connected) enclosure around all RFI producing equipment (other
than the roof mounted solar panels and the main power leads to
the house itself).
The bonding wires were used to reduce risk
of leaving poorly connected areas that could leak RFI and the
completed "Farraday cage" was then connected to the main
system earth (as is common practice for 240 V House wiring***)
As the main path remaining for the RFI was
then via the DC and AC power lines which ran underground from
the power shed to our house, I then added 8 of the largest
"clip on" RFI filters to the 240 V AC lead as it exited the
24/240 volt inverter plus 3 smaller filters on each of the DC
leads from the battery bank which provides power to the 24
volt "soft start" compressor motor of our refrigerator.
RFI filters on Main Power AC lead
N.B. If building an all enclosing Farraday
cage is not practical, it may be possible to build individual
cages around key items of equipment that are causing serious
RFI, by using light gauge welded mesh or perforated metal
However, doing that generally requires a
reasonable level of skills in construction and it requires
appropriate expertise in understanding the electrical
equipment that you are dealing with.
For instance, any part of the cage that
touches the metal housings of the equipment will cancel out
its benefits and it may add serious risk of intermittent
"shorting" and even personal electrocution, if a wiring
fault should later occur within any 120/240 Volt
My own attempt at a Farraday cage did make
a noticeable and very useful reduction in RFI on some sections
of the MW and SW bands but not on other sections and annoying
levels of noise still remained in the main power/distribution
leads and from the pressure pump motor for our water supply
plus the refrigerator motor, which emitted high levels of RFI
whenever they operated.
Adding ferrite core filters seemed the next
most practical solution but again it was not an entirely
straight forward process as it was difficult to find detailed
specifications on which frequency they operated up to, unless
I purchased expensive items designed specifically to meet
Even then, I found that two different
commercial brands of filters that were rated the same but did
not perform equally well in practice, so I went back to
purchasing from salvagers of commercial materials, simply in
order to keep my expenses down and this turned out to be the
best approach for me.
In total I added in excess of 30 ferrite
filters throughout the power distribution system within our
house, including 8 to the leads on the refrigerator alone,
plus I shortened the radio’s earth leads as much as was
reasonable and fitted a second earth stake to the radio earth
(a useful tip gained from a HAM radio site).
These actions provided a further reduction
in RFI and it bought the overall level of interference to a
low but still audible level.
On again checking for the source of this
with my small portable radio, to my surprise, I discovered
that the noisiest area remaining was in fact the earth leads
throughout the house and this was evident in the radio sets
earths, even when they were switched off!!
***After further experimenting with options
to reduce this problem, I then placed a separate earth
stake on the Farraday cage/power shed and
disconnected the original lead from the frame to the power
systems main earth as it appeared that using the system earth
was allowing a portion of the supposedly grounded RFI to
travel back along the Earth wires and into the house.
This finally achieved the desired level of
success as the RFI is now just a soft whine on a few areas of
the MW and SW and even low powered signals are audible,
although surprisingly it is still at nuisance levels on the FM
N.B. my research suggests that the most
likely source of the remaining RFI on the FM (low VHF band) is
probably from the solar panels themselves but as we only have
one local FM station, I have not spent any further time in
checking if that is correct or not.
Types and placement of RFI/EMI filters:
I have use both solid and "snap on" ferrite
core filters and while the solid filters seem to be slightly
more effective, I found that using "snap on" types
significantly reduced my work time and the risks associated
with disconnecting and reconnecting high voltage wiring, which
is often required when fitting solid core filters.
I found that it was most effective if I
added them closest to the source of RFI (see Photo 4 for
typical example on a radio power lead) and less effective when
added at the equipment operating at the end of the lead. The
most effective location was to add the RFI Filters to the
mains lead which exited from our inverter and from our battery
bank within the power shed.
To determine what spacing was best between
the RFI Filters, I experimented by placing 3 together on a
short extension lead then slowly moved them apart to see if I
could detect any change in the level of RFI on my radio. In
our situation, I found them to be best when placed at
approximately 25 mm spacing.
I used this same process to determine how
many filters were needed on any one piece of equipment and I
never found a situation where less than 3 filters were
required in order to gain a good reduction in RFI. I also
found that filters which closely fitted the power cable were
more effective than loose fitting ones.
Other persons have reported using different
spacing or none at all and I suspect that it may depend on
both the composition of materials in the ferrite within the
filter and on what section of the radio frequency is being
targeted, therefore the above test may also be useful in
determining what works best in your own situation.
Although other types of RFI/EMI filters and
methods of dealing with it are available and have not been
included in my article, I am well satisfied with what I have
achieved and believe that I have covered most of the practical
and cost effective methods.
I hope that my work will reduce the amount
of time and effort that others will need, if they set out to
reduce interference from RFI/EMI to their own equipment.
As I do not have sophisticated equipment
available with which to add objective support to my
conclusions and I have tested only a limited number of RFI/EMI
filters and LED light bulbs, I haven’t felt it appropriate to
publically name some manufacturers as either "good or bad"
Good luck with hunting your own RFI.