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Chapter 13
EMI/Direction
Finding
THE SCOPE OF THE PROBLEM
As our lives become filled with tech-
nology, the likelihood of electronic
interference increases. Every lamp dim-
mer, garage-door opener or other new
technical “toy” contributes to the electri-
cal noise around us. Many of these de-
vices also “listen” to that growing noise
and may react unpredictably to their elec-
tronic neighbors.
Sooner or later, nearly every Amateur
Radio operator will have a problem with
interference. Most cases of interference
can be cured! The proper use of “diplo-
macy” skills and standard cures will usu-
ally solve the problem.
This section of Chapter 13, by Ed Hare,
W1RFI, is only an overview.
The ARRL
RFI Book
contains detailed information on
the causes of and cures for nearly every
type of interference problem.
1
Decibel (dB)
— a logarithmic unit of
relative power measurement that ex-
presses the ratio of two power levels.
Differential-mode signals
—
Signals
that arrive on two or more conductors
such that there is a 180° phase difference
between the signals on some of the con-
ductors.
Electromagnetic compatibility (EMC)
— the ability of electronic equipment to
be operated in its intended electromag-
netic environment without either causing
interference to other equipment or sys-
tems, or suffering interference from other
equipment or systems.
Electromagnetic interference (EMI)
— any electrical disturbance that inter-
feres with the normal operation of elec-
tronic equipment.
Emission
— electromagnetic energy
propagated from a source by radiation.
Filter
— a network of resistors, induc-
tors and/or capacitors that offer little
resistance to certain frequencies while
blocking or attenuating other frequen-
cies.
Fundamental overload
— interference
resulting from the fundamental signal of a
radio transmitter.
Ground
— a low-impedance electrical
connection to the earth. Also, a common
reference point in electronic circuits.
Harmonics
— signals at exact integral
multiples of the operating (or
fundamen-
tal
)
frequency.
High-pass filter
— a filter designed to
pass all frequencies above a cutoff fre-
quency, while rejecting frequencies below
the cutoff frequency.
Immunity
— the ability of electronic
equipment to reject interference from ex-
ternal sources of electromagnetic energy.
This is the conjugate of the term “suscep-
tibility” and is the term typically used in
the commercial world.
Induction
— the transfer of electrical
signals via magnetic coupling.
Interference
— the unwanted interac-
tion between electronic systems.
Intermodulation
— the undesired mix-
ing of two or more frequencies in a nonlin-
ear device, which produces additional
frequencies.
Low-pass filter
— a filter designed to
pass all frequencies below a cutoff fre-
quency, while rejecting frequencies above
the cutoff frequency.
Noise
— any signal that interferes with
the desired signal in electronic communi-
cations or systems.
Nonlinear
— having an output that is
not in linear proportion to the input.
Notch filter
— a filter that rejects or
suppresses a narrow band of frequencies
within a wider band of frequencies.
Passband
— the band of frequencies
that a filter conducts with essentially no
attenuation.
Radiated emission
— radio-frequency
energy that is coupled between two sys-
tems by electromagnetic fields.
Radio-frequency interference (RFI)
— interference caused by a source of
radio-frequency signals. This is a subclass
of EMI.
Spurious emission
— An emission, on
frequencies outside the necessary band-
width of a transmission, the level of which
may be reduced without affecting the in-
formation being transmitted.
Susceptibility
— the characteristic of
electronic equipment that permits undes-
ired responses when subjected to electro-
magnetic energy.
TVI
— interference to television sys-
tems.
Pieces of the Problem
Every interference problem has two
components — the equipment that is in-
volved and the people who use it. A solu-
tion requires that we deal with both the
equipment and the people effectively.
First, define the term “interference” with-
out emotion. The ARRL recommends that
the hams and their neighbors cooperate to
find solutions. The FCC shares this view.
Important Terms
Bypass capacitor
— a capacitor used
to provide a low-impedance radio-fre-
quency path around a circuit element.
Common-mode signals
— signals that
are in phase on both (or several) conduc-
tors in a system.
Conducted signals
— signals that
travel by electron flow in a wire or other
conductor.
EMI/Direction Finding
13.1
Responsibility
When an interference problem occurs,
we may ask “Who is to blame?” The ham
and the neighbor often have different opin-
ions. It is almost natural (but unproduc-
tive) to fix blame instead of the problem.
No amount of wishful thinking (or de-
mands for the “other guy” to solve the
problem) will result in a cure for interfer-
ence. Each individual has a unique per-
spective on the situation, and a different
degree of understanding of the personal
and technical issues involved. On the other
hand, each person has certain responsibili-
ties to the other and should be prepared to
address those responsibilities fairly.
problem, or call channel 8 to see if the sta-
tion has a problem. If channel 8 was oper-
ating properly, you would likely decide that
your TV set is broken. Now, if you tune
your TV to channel 3, and see your local
shortwave radio station (quite possibly
Amateur Radio), don’t blame the short-
wave station without some investigation.
In fact, many televisions respond to strong
signals outside the television bands. They
may be working as designed, but require
added filters and/or shields to work prop-
erly near a strong, local RF signal.
Your neighbor will probably feel much
better if you explain that you will help
find
a solution, even if the interference is
not
your fault. This offer can change your
image from neighborhood villain to hero,
especially if the interference is not caused
by your station. (This is often the case.)
you are not causing the interference! This
is also a valuable troubleshooting tool: If
you know your station is clean, you have
cut the size of the problem in half! If the
FCC ever gets involved, you can demon-
strate that you are not interfering with your
own equipment.
Apply EMI cures to your own consumer
electronics equipment. When your neigh-
bor sees your equipment working well, it
demonstrates that filters work and cause
no harm.
To clean up your station, clean up the
mess! A rat’s nest of wires, unsoldered
connections and so on in your station can
contribute to EMI. To help build a better
relationship, you may want to show your
station to your neighbor. A clean station
looks professional; it inspires confidence
in your ability to solve the EMI problem.
Install a transmit filter (low-pass or
band-pass) and a reasonable station
ground. (If the FCC becomes involved,
they will ask you about both items.) Show
your neighbor that you have installed the
necessary filter on your transmitter and
explain that if there is still interference, it
is necessary to try filters on the neighbor’s
equipment, too.
Operating practices and station-design
considerations can affect EMI. Don’t
overdrive a transmitter or amplifier; that
can increase its harmonic output. You can
take steps to reduce the strength of your
signal at the victim equipment. This might
include reducing transmit power. Locate
the antenna as far as possible from suscep-
tible equipment or its wiring (ac line, tele-
phone, cable TV). Antenna orientation
may be important. For example, if your
HF dipole at 30 ft is coupling into the
neighbor’s overhead cable-TV drop, that
coupling could be reduced 20 dB by
changing to a vertical antenna — even
more by orienting the antenna so that the
drop is off its end. Try different modes;
CW or FM usually do not generate nearly
as much telephone interference as AM or
SSB, for example.
FCC Regulations
A radio operator is responsible for the
proper operation of the radio station. This
responsibility is spelled out clearly in Part
97 of the FCC regulations. If interference
is caused by a spurious emission from your
station, you
must
correct the problem
there.
Fortunately, most cases of interference
are
not
the fault of the transmitting sta-
tion. Most interference problems involve
some kind of electrical noise or fundamen-
tal overload.
PREPARE YOURSELF
Learn About EMI
In order to troubleshoot and cure EMI,
you need to learn more than just the ba-
sics. This is especially important when
dealing with your neighbor. If you visit
your neighbor’s house and try a few dozen
things that don’t work (or make things
worse), your neighbor may lose confi-
dence in your ability to help cure the prob-
lem. If that happens, you may be asked to
leave.
Personal Diplomacy
What happens when you first talk to
your neighbor sets the tone for all that
follows. Any technical solutions cannot
help if you are not allowed in your
neighbor’s house to explain them! If the
interference is not caused by spurious
emissions from your station, however, you
should be a locator of solutions, not a pro-
vider of solutions.
Your neighbor will probably
not
under-
stand all of the technical issues — at least
not at first. Understand that, regardless of
fault, an interference problem is annoying
to your neighbor. Let your neighbor know
that you want to help find a solution and
that you want to begin by talking things
over.
Talk about some of the more important
technical issues, in non-technical terms.
Interference can be caused by unwanted
signals from your transmitter. Assure your
neighbor that you will check your station
thoroughly and correct any problems. You
should also discuss the possible sus-
ceptibility of consumer equipment. You
may want to print a copy of the RFI
information found on
ARRLWeb
at:
www.
arrl.org/news/rfi/neighbors.html
.
Here is a good analogy: If you tune your
TV to channel 3, and see channel 8 instead,
would you blame channel 8? No. You might
check another set to see if it has the same
Local Help
If you are not an expert (and even ex-
perts can use moral support), you should
find some local help. Fortunately, such
help is often available from your Section
Technical Coordinator (TC). The TC
knows of any local RFI committees and
may have valuable contacts in the local
utility companies. Even an expert can ben-
efit from a TC’s help.
The easiest way to find your TC is
through your ARRL Section Manager
(SM). There is a list of SMs on page 16 of
any recent
QST
. He or she can quickly put
you in contact with the best source of local
help.
Even if you can’t secure the help of a
local expert, a second ham can be a valu-
able asset. Often a second party can help
defuse any hostility. It is also helpful to
have someone to operate your station
while you and your neighbor run through
troubleshooting steps and try various
cures.
Call Your Neighbor
Now that you have learned more about
EMI, located some local help (we’ll as-
sume it’s the TC) and done all of your
homework, make contact with your
neighbor. First, arrange an appointment
convenient for you, the TC and your
neighbor. After you introduce the TC,
allow him or her to explain the issues to
your neighbor. Your TC will be able to
answer most questions, but be prepared
to assist with support and additional in-
formation as required.
Invite the neighbor to visit your station.
Show your neighbor some of the things
Prepare Your Home
The first step toward curing an interfer-
ence problem is to make sure your own
signal is clean. You must eliminate all
interference in your own house to be sure
13.2
Chapter 13
you do with your radio equipment. Point
out any test equipment you use to keep
your station in good working order. Of
course, you want to show the filters you
have installed on your transmitter.
Next, have the TC operate your station
on several different bands. Show your
neighbor that your home electronics
equipment is working properly while your
station is in operation. Point out the filters
you have installed to correct any suscepti-
bility problems.
At this point, tell your neighbor that the
next step is to try some of these cures on
his or her equipment. This is a good time
to emphasize that the problem is probably
not your fault, but that you and the TC will
try to help find a solution anyway.
Table 13.1
is a list of the things needed
to troubleshoot and solve most EMI prob-
lems. Decide ahead of time which of these
items are needed and take them with you.
Table 13.1
EMI Survival Kit
Filters:
(2) 300-
Ω
high-pass filter (different brands recommended)
(2) 75-
Ω
high-pass filter (different brands recommended)
(2) Commercially available common-mode chokes (optional)
(12) Assorted ferrite cores: 43, 63 and 75 material, FT-140 and FT-240 size
(3) Telephone RFI filters (different brands recommended)
(2) Brute-force ac line filters
(6) 0.01-
µ
F ceramic capacitors
(6) 0.001-
µ
F ceramic capacitors
(2) Speaker-lead filters (optional)
Miscellaneous:
• Hand tools, assorted screwdrivers, wire cutters, pliers
• Hookup wire
• Electrical tape
• Soldering iron and solder (use with caution!)
• Assorted lengths 75-
Ω
coaxial cable with connectors
• Spare F connectors, male
• F-connector female-female “barrel”
• Alligator clips
• Notebook and pencil
• Portable multimeter
At Your Neighbor’s Home
You and the TC should now visit the
neighbor’s home. Inspect the equipment
installation and ask when the interference
occurs, what equipment is involved and
what frequencies or channels are affected.
The answers are valuable clues. Next,
either you or the TC should operate your
station while the other observes the ef-
fects. Try all bands and modes that you
use. Ask the neighbor to demonstrate the
problem.
The tests may show that your station
isn’t involved at all. You may immediately
recognize electrical noise or some kind of
equipment malfunction. If so, explain
your findings to the neighbor and suggest
that he or she contact appropriate service
personnel.
EMC Fundamentals
Knowledge is one of the most valuable
tools for solving EMI problems. A suc-
cessful EMI cure usually requires famil-
iarity with the relevant technology and
troubleshooting procedures.
radiated by the source and picked up by a
conductor attached to the victim (or di-
rectly by the victim’s circuitry) and is then
conducted into the victim. EMI from in-
duction is rare.
filter that passes desired signals and
shunts unwanted signals to the return line.
Most
desired
signals, such as the TV sig-
nal inside a coaxial cable are differential-
mode signals.
In a common-mode circuit, many wires
of a multiwire system act as if they were a
single wire. The result can be a good
antenna, either as a radiator or as a recep-
tor of unwanted energy. The return path is
usually earth ground. Since the source and
return conductors are usually well sepa-
rated, there is no reliable phase difference
between the conductors and no convenient
place to shunt unwanted signals. Toroid
chokes are the answer to common-mode
interference. (The following explanation
applies to rod cores as well as toroids, but
since rod cores may couple into nearby
circuits, use them only as a last resort.)
Toroids work differently, but equally
well, with coaxial cable and paired conduc-
tors. A common-mode signal on a coaxial
cable is usually a signal that is present on
the
outside
of the cable
shield
. When we
wrap the cable around a ferrite-toroid core,
the choke appears as a reactance in series
with the outside of the shield, but it has no
SOURCE-PATH-VICTIM
All cases of EMI involve a
source
of
electromagnetic energy, a device that re-
sponds to this electromagnetic energy
(
victim
) and a transmission path that
allows energy to flow from the source to
the victim. Sources include radio transmit-
ters, receiver local oscillators, computing
devices, electrical noise, lightning and
other natural sources.
There are three ways that EMI can travel
from the source to the victim: radiation,
conduction and induction. Radiated EMI
propagates by electromagnetic radiation
from the source, through space to the vic-
tim. A conducted signal travels over wires
connected to the source and the victim.
Induction occurs when two circuits are
magnetically (and in some cases, electri-
cally) coupled. Most EMI occurs via con-
duction, or some combination of radiation
and conduction. For example, a signal is
DIFFERENTIAL VS
COMMON-MODE
It is important to understand the differ-
ences between differential-mode and
common-mode conducted signals (see
Fig 13.1
). Each of these conduction modes
requires different EMI cures. Differential-
mode cures, (the typical high-pass filter,
for example) do not attenuate common-
mode signals. On the other hand, a typical
common-mode choke does not affect in-
terference resulting from a differential-
mode signal.
Differential-mode currents usually
have two easily identified conductors. In a
two-wire transmission line, for example,
the signal leaves the generator on one line
and returns on the other. When the two
conductors are in close proximity, they
form a transmission line and there is a 180°
phase difference between their respective
signals. It’s relatively simple to build a
EMI/Direction Finding
13.3
source. When the distance from the source
doubles, the strength of the electromag-
netic field decreases to one-fourth of its
strength at the original distance from the
source. This characteristic can often be
used to help solve EMI cases. You can
often make a significant improvement by
moving the victim equipment and the
antenna farther away from each other.
External Noise
Most cases of interference reported to
the FCC involve some sort of external
noise source. The most common of these
noise sources are electrical. External
“noise” can also come from transmitters
or from unlicensed RF sources such as
computers, video games, electronic mice
repellers and the like. Typically, such de-
vices are legal under Part 15 of the FCC’s
rules.
Electrical noise is fairly easy to identify
by looking at the picture of a susceptible
TV or listening on an HF receiver. A photo
of electrical noise on a TV screen is shown
in the TVI section of this chapter. On a
receiver, it usually sounds like a buzz,
sometimes changing in intensity as the arc
or spark sputters a bit. If you determine
the problem to be caused by external noise,
it must be cured at the source. Refer to the
Electrical Noise section of this chapter and
The ARRL RFI Book
.
Fig 13.1 — A shows a differential-mode, while B shows a common-mode signal.
The two kinds of signals are described in the text.
A strong fundamental signal can enter
equipment in several different ways. Most
commonly, it is conducted into the equip-
ment by wires connected to it. Possible
conductors include antennas and feed
lines, interconnecting cables, power lines
and ground wires. TV antennas and feed
lines, telephone or speaker wiring and ac
power leads are the most common points
of entry.
The effect of an interfering signal is
directly related to its strength. The
strength of a radiated signal diminishes
with the square of the distance from the
effect on signals inside the cable because
their field is (ideally) confined inside the
shield. With paired conductors such as
zip-cord, signals with opposite phase set
up magnetic fluxes of opposite phase in the
core. These “differential” fluxes cancel
each other, and there is no net reactance for
the differential signal. To common-mode
signals, however, the choke appears as a
reactance in series with the line.
Toroid chokes work less well with
single-conductor leads. Because there is no
return current to set up a canceling flux, the
choke appears as a reactance in series with
both
the desired and undesired signals.
Spurious Emissions
All transmitters generate some (hope-
fully few) RF signals that are outside their
allocated frequency bands. These out-of-
band signals are called spurious emis-
sions, or
spurs
. Spurious emissions can be
SOURCES OF EMI
The basic causes of EMI can be grouped
into several categories:
• Fundamental overload effects
• External noise
• Spurious emissions from a transmitter
• Intermodulation distortion or other
external spurious signals
As an EMI troubleshooter, you must
determine which of these are involved in
your interference problem. Once you do,
it is easy to select the necessary cure.
Fundamental Overload
Most cases of interference are caused
by fundamental overload. The world is
filled with RF signals. Properly designed
equipment should be able to select the
desired signal, while rejecting all others.
Unfortunately, because of design defi-
ciencies such as inadequate shields or fil-
ters, some equipment is unable to reject
strong out-of-band signals.
Fig 13.2 — The spectral output of a typical amateur transmitter. The fundamental
is at 7 MHz. There are visible harmonics at 14, 21 and 28 MHz. Unlabeled lines are
non-harmonic spurious emissions. This transmitter complies with the stringent
FCC spectral-purity regulations regarding amateur transmitters with less than 5 W
of RF output.
13.4
Chapter 13
discrete signals or wideband noise. Har-
monics, the most common spurious emis-
sions, are signals at exact multiples of the
operating (or
fundamental
)
frequency.
Other discrete spurious signals are usu-
ally caused by the superheterodyne mix-
ing process used in most modern
transmitters.
Fig 13.2
shows the spectral
output of a transmitter, including harmon-
ics and mixing products.
Transmitters may also produce broad-
band noise and/or “parasitic” oscillations.
(Parasitic oscillations are discussed in the
RF Power Amplifiers
chapter.) If these
unwanted signals cause interference to
another radio service, FCC regulations
require the owner to correct the problem.
millions. You probably will not see your
exact problem and cure listed in this book or
any other. You must diagnose the problem!
Troubleshooting an EMI problem is a
three-step process, and all three steps are
equally important:
• Identify the problem
• Diagnose the problem
• Cure the problem.
weather? You should do the same when-
ever you operate. If you can readily repro-
duce the problem with your station, you
can start to troubleshoot the problem.
Diagnose the Problem
Look Around
— Aside from the brain,
eyes are a troubleshooter’s best tool. Look
around. Installation defects contribute to
many EMI problems. Look for loose con-
nections, shield breaks in a cable-TV in-
stallation or corroded contacts in a
telephone installation. Fix these first.
Problems that occur only on harmonics
of the fundamental signal usually indicate
the transmitter. Harmonics can also be
generated in nearby semiconductors, such
as an unpowered VHF receiver left con-
nected to an antenna, or a corroded con-
nection in a tower guy wire. Harmonics
can also be generated in the front-end
components of the TV or radio experienc-
ing interference.
Is the wiring connected to the victim
equipment resonant on one or more ama-
teur bands? If so, a common-mode choke
placed at the middle of the wiring may be
an easy cure.
These are only a few of the questions
you might need to ask. Any information
you gain about the systems involved will
help find the EMI cause and cure.
Identify the Problem
Is It Really EMI
?
—
Before trying to
solve a suspected case of EMI, verify that
the symptoms actually result from exter-
nal causes. A variety of equipment mal-
functions or external noise can look like
interference. “Your” EMI problem might
be caused by another ham or a radio trans-
mitter of another radio service, such as a
local CB or police transmitter.
Is It Your Station?
— If it appears that
your station is involved, operate your sta-
tion on each band, mode and power level
that you use. Note all conditions that pro-
duce interference. If no transmissions pro-
duce the problem, your station
may
not be
the cause. (Although some contributing
factor may have been missing in the test.)
Have your neighbor keep notes of when
and how the interference appears: what
time of day, what station, what other ap-
pliances were in use, what was the
TROUBLESHOOTING EMI
Most EMI cases are complex. They in-
volve a source, a path and a victim. Each of
these main components has a number of
variables: Is the problem caused by harmon-
ics, fundamental overload, conducted emis-
sions, radiated emissions or a combination
of all of these factors? Should it be fixed
with a low-pass filter, high-pass filter, com-
mon-mode chokes or ac-line filter? How
about shielding, isolation transformers, a
different ground or antenna configuration?
By the time you finish with these ques-
tions, the possibilities could number in the
Cures
At Your Station
— Make sure that your
own station and consumer equipment are
clean. This cuts the size of the problem in
half! Once this is done, you won’t need to
diagnose or troubleshoot your station
later. Also, any cures successful at your
house may work at your neighbor’s as
well. If you do have problems in your own
house, refer first to the Transmitter sec-
tion of this chapter, or continue through
the troubleshooting steps and specific
cures and take care of your own problem
first.
Simplify the Problem
— Don’t tackle a
complex system — such as a telephone
system in which there are two lines run-
ning to 14 rooms — all at once. You could
spend the rest of your life running in
circles and never find the true cause of the
problem.
There’s a better way. In our hypotheti-
cal telephone system, first locate the tele-
phone jack closest to the telephone service
entrance. Disconnect the lines to more
remote jacks and connect one EMI-resis-
tant telephone at the remaining jack. If the
interference remains, try cures until the
problem is solved, then start adding lines
and equipment back one at a time, fixing
the problems as you go along. If you are
lucky, you will solve all of the problems in
one pass. If not, at least you can point to
one piece of equipment as the source of
the problem.
Multiple Causes
— Many EMI prob-
lems have multiple causes. These are usu-
ally the ones that give new EMI
troubleshooters the most trouble. If, for
example, a TVI problem is caused by har-
monics from the transmitter, an arc in the
transmitting antenna, an overloaded TV
preamp, differential-mode fundamental
overload generating harmonics in the TV
tuner, induced and conducted RF in the
ac-power system and a common-mode sig-
nal picked up on the shield of the TV’s
coaxial feed line, you would never find a
cure by trying only one at a time!
In this case, the solution requires that
you apply all of the cures at the same time.
When troubleshooting, if you try a cure,
leave it in place. When you finally try a
cure that really works, start removing the
“temporary” attempts one at a time. If the
interference returns, you know that there
were multiple causes.
OVERVIEW OF TECHNIQUES
Shields
Shields are used to set boundaries for
radiated energy. Thin conductive films,
copper braid and sheet metal are the most
common shield materials. Maximum
shield effectiveness usually requires solid
sheet metal that completely encloses the
source or susceptible circuitry or equip-
ment. Small discontinuities, such as holes
or seams, decrease shield effectiveness. In
addition, mating surfaces between differ-
ent parts of a shield must be conductive.
To ensure conductivity, file or sand off
paint or other nonconductive coatings on
mating surfaces.
Filters
A major means of separating signals
relies on their frequency differences. Fil-
ters offer little opposition to certain fre-
quencies while blocking others. Filters
vary in attenuation characteristics, fre-
EMI/Direction Finding
13.5
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