ARRL Technical Information Service
EMI/RFI Package

Updated: January 27, 1995   File: rfigen.txt

Reprinted from February and March 1992 QST "Lab 
Notes." Copyright 1992 American Radio Relay League, 
Inc.  All rights reserved.

Interference Primer - Part 1. Derived from QST February 
1992 Lab  Notes 

This month Ed Hare, KA1CV, Senior Laboratory Engineer, 
grapples  with one of the most difficult problems in Amateur 
Radio:  electromagnetic interference, or EMI. EMI is a 
catch-all term for  TVI, RFI, BCI and any other form of 
electromagnetically generated  interference. Regardless of 
the terminology, the ultimate  translation is trouble!--Steve 
Ford, WB8IMY 

You may find this hard to believe, but nearly every person in 
the  technically-developed world has experienced EMI. At 
the ARRL  Laboratory we receive telephone calls and letters 
on the subject  of EMI every day. 

Let's start by defining the term interference. To some people, 
it  implies action and intent. The statement, "You are 
interfering  with my telephone," sounds like an outright 
accusation, doesn't  it? For our purposes, let's define 
interference as any unwanted  interaction between electronic 
systems--period. No fault. No  blame. It's just a condition. 

Each person involved in an interference problem has 
individual  needs, a unique perspective and a varying degree 
of understanding  of the technical and personal issues 
involved. On the other hand,  each of them may have certain 
responsibilities toward the other,  and should be prepared to 
address those responsibilities fairly. 

Whenever I host an EMI technical forum at ARRL 
conventions or  local club meetings, I always conduct an 
informal survey. I ask  the members of the audience to raise 
their hands if they've ever  had an interference problem. I can 
always count on a 30% response,  and most of the elevated 
hands belong to individuals who have  experienced some 
form of interference involving their ham  stations. 

After taking a quick tally, I carefully explain that 
interference  caused by electric motors, power lines, CATV 
leakage and spurious  emissions from consumer devices must 
be included in the  definition--as well as interference to the 
their own consumer  electronics equipment from all of these 
sources. When I see the  looks of enlightened surprise, I 
know it's time to repeat my  survey. Before I even finish 
asking the question nearly every hand  reaches for the sky! 

So, now that I've convinced you that you may have a 
problem (as if  you really needed convincing), let's move on 
to the questions and  answers. 

Q: My new neighbor, Sam, just knocked on my door said 
that my  signals are tearing up his TV and telephones. He 
threatened to  call the FCC if I didn't stop transmitting. Am I 
in serious  trouble? What should I do? 

A: Why don't we indulge in a bit of psychology first? Hams 
are  great communicators over the airwaves, but some of us 
need  practice when it comes to communicating face-to-face. 

You can't overestimate the importance of personal diplomacy 
when  you're confronted with an EMI problem. The way you 
behave when  your neighbor comes knocking sets the tone 
for everything that  follows. No matter what you think of 
your neighbor, you have to  remember that the best solutions 
are built on cooperation and  trust. Knowing all the technical 
tricks in the book won't do you a  bit of good if your 
neighbor won't let you through the door! 

Q: I don't know . . . he seems pretty angry. What can I tell 
him? 

A: It sounds like your neighbor has already branded you as 
the  villain. He's angry and fully expects you to respond with 
denials  and evasions. Don't do it! 

Begin by accepting the fact that he doesn't enjoy having his  
lifestyle hampered by EMI. Put yourself in his shoes. Admit 
that  EMI is highly annoying. (Both of you can quickly agree 
on that  point!) Calmly explain that you are responsible--by 
law--for the  proper operation of your station. Assure him 
that you'll check  your equipment right away and make any 
necessary corrections. 

With any luck, the situation will begin to calm down. Now is 
the  time to explain to your neighbor that the root cause of his  
problem could also be from a source other than your station.  
Perhaps his own equipment is to blame. Before he has a 
chance to  misunderstand the last point, tell him that you're 
willing to help  him solve the problem--even if it's not your 
fault. 

Until the problem is resolved, try making some goodwill 
gestures.  As a temporary measure, reduce your output 
power. (You may  discover that you didn't need all those 
watts, anyway!) If you  have a beam antenna, don't point it at 
your neighbor's house.  Above all, try to gain some 
prospective on the situation. Amateur  Radio may be your 
passion, but it doesn't mean a thing to your  neighbor. 
Attempting to justify EMI by saying "There was a rare DX  
station on 10 meters and I just had to work him" may just 
sound  like ham "lingo" to him and probably won't get you 
very far.

Q: Wait a minute! I've heard that interference is usually not 
the  fault of the amateur. If I haven't done anything wrong, 
why should  I feel obligated to help? 

A: Usually is the operative word. By helping him solve the  
problem, you'll be making a friend, not an enemy. What if,  
sometime in the future, you buy that new amplifier and start  
calling CQ when it's third down at the goal line with 30 
seconds  left in the game. If you blitz your neighbor's TV, he 
may be more  understanding and less likely to do something 
rash.

There are two basic things that can result in interference. 
Your  transmitter may be emitting low-level signals outside 
the amateur  bands. These signals, called harmonics or 
spurious emissions by  the FCC (or spurs in common 
parlance), can be the direct cause of  interference. FCC 
regulations are quite clear about spurious  emissions: they 
must not cause interference to other services. The  operator 
of a transmitter must take whatever steps are necessary  to 
eliminate interference from this cause. This almost always  
involves additional filtering, grounding or shielding of the  
transmitting equipment. 

On the other hand, the world is filled with radio signals. Any  
piece of consumer electronics equipment should be able to 
respond  only to signals it is designed to receive. The 
filtering and  shielding in your neighbor's TV (or other 
equipment), however, may  be inadequate to reject your 
strong fundamental signal. This  condition is commonly 
called fundamental overload. Interference  that results from 
fundamental overload is really a no-fault  situation. 

Q: No-fault? That sounds like my auto-insurance policy. 

A: Wow! There's a concept . . . EMI insurance! With all of 
the EMI  in the world today, can you imagine the steep 
premiums you'd have  to pay? Instead, let's consider the 
following scenario: 

You, as an amateur, have purchased a transmitter that meets 
all of  the FCC requirements for proper operation. You have 
installed it  in a well-engineered station with proper 
grounding and filtering.  You know your station is clean 
because you don't interfere with  your own equipment. You 
have done nothing wrong. 

The manufacturer of your neighbor's TV has designed and 
built the  best possible product, constructed to meet hundreds 
of regulations  set by dozens of federal regulatory agencies. 
The product has  probably met a few voluntary standards set 
by independent  associations as well. Within the constraints 
of the law, the  manufacturer has done nothing wrong. 

Your neighbor has gone to the electronics store and has 
purchased  a piece of equipment that has a fine reputation for 
quality and  service. He has every right to expect his 
equipment to function as  advertised. Clearly, your neighbor 
has done nothing wrong. Even  so, when he turns on his set 
and you go on the air, you both have  an interference 
problem. 

So who is at fault? It should be obvious that no single 
individual  is to blame. Everyone has done everything 
correctly, but the  system has failed! 

Q:  If the  system isn't working, then we all share the blame. 

A: That's right! EMI has been cited as one of the 
fundamental  threats to the Amateur Radio service. Complex 
electronic circuitry  is found in all sorts of devices used in the 
home. This results in  a vast interference potential that didn't 
exist in earlier,  simpler decades. One of our ARRL 
Laboratory Engineers recently  noticed an advertisement for 
a computer-controlled ac-power outlet  strip. He wryly 
observed that this wonderful new product would now  allow 
him to interfere with his neighbor's extension cord! 

It's always important to remember our place as Amateur 
Radio  operators in the overall scheme of things. Consider 
the fact that  national governments extend to amateurs the 
privilege to operate  in valuable portions of the radio 
spectrum. Amateurs have gained  these privileges because 
the world benefits from our existence. In  addition to the 
emergency communications services we provide, the  world 
gains a reservoir of self-trained radio operators, skilled  in 
operating practices and electronics technology. Solving you 
and  your neighbor's EMI problem is an excellent application 
of your  technical skills. 

Q: You implied that I had access to the technical resources to   
help my neighbor. What if I'm a new ham and I don't feel 
confident  enough to call myself an EMI expert? How can I 
get some help? 

A: Your ARRL Technical Coordinator (TC) is the first 
person to  contact. The TC often has a cadre of assistants 
(Technical  Specialists) available, and there may be one near 
you. If you have  local clubs with EMI or TVI committees, 
they usually coordinate  their activities through the TC. The 
TCs often have liaisons with  local utilities such as telephone 
and cable companies. Knowing the  right individual to 
contact may prevent a repair person from  pointing at your 
antenna and telling your neighbor, "It's all his  fault." 

Some people choose to call the TC only as a last resort, 
waiting  until all diplomatic and technical solutions have 
failed. This is  a bad idea! The TC is a volunteer and may 
choose not to  participate in a situation that has deteriorated 
badly. Most of  them prefer to be involved right from the 
start. They are often  skilled (read: practiced) in the art of 
EMI negotiations. 

Q: Okay, I'm convinced. How do I find my TC? 

A: The easiest way to find your TC is to ask your ARRL 
Section  Manager. Section Managers (SMs) are listed on 
page 8 of any recent  QST issue. Most SMs include their 
telephone numbers, but be  considerate. Call during the day 
or early evening. You can also  call ARRL Headquarters to 
ask for the name of your TC, but we do  not give out their 
telephone numbers to protect their privacy. 

Q: The TC, my neighbor and I all want to know the source of 
the  problem. What should we do next?

A: Offer to arrange a test. Ask your neighbor to invite a 
friend  to visit your shack during the test. In addition, ask you 
neighbor  if it would be possible for one of your friends to 
monitor the  test at your neighbor's home. Having impartial 
witnesses will make  you and your neighbor more 
comfortable with the outcome --  whatever it may be.

Be sure to choose your witness carefully. Select someone 
who is  known for diplomacy and tactfulness. (Your TC is a 
great candidate  for this role!)

Your test must be thorough. Transmit on each band and 
mode you  normally operate. If you have a beam antenna, 
aim it in different  directions while you are transmitting. Try 
various power levels,  too. Ask your friend to keep detailed 
notes of the results. A  radio or telephone link between you 
and your friend is almost a  necessity.

Even if your test proves that your station is not at fault, don't  
just drop the problem in your neighbor's lap and say "Good 
luck!"  Offer to help find a solution.

Q: My neighbor and I have agreed to stop blaming each 
other and  work together to find a solution. But now he's 
accused me of  causing interference during a football game 
that was televised  last Sunday. I don't think I was even near 
my shack while it was  on. What gives? 

A: Even though it's not a legal requirement, it's a good idea 
to  keep a detailed station log. Now that you're involved in an  
interference issue, it's a necessity! You should ask your 
neighbor  to keep notes, too. Ask him to identify which piece 
of equipment  experienced the interference, what channels or 
frequencies were  involved, the date and time the 
interference occurred and a  description of the interference 
and its severity.

If you're lucky, a comparison between your log and his log 
may  indicate that the interference isn't coming from your 
station. On  the other hand, if your signal is the source of the 
problem, your  neighbor is the lucky party--although he may 
not see it that way  at first. As an Amateur Radio operator, 
you have access to the  technical resources necessary to solve 
the problem (either from  your own knowledge and 
experience, or with the help of other hams  like your 
Technical Coordinator or local EMI expert). This is not  
necessarily true if the source of the problem is a 
business-band  or citizens-band transmitter, for examples.

Q: My neighbor's problem isn't limited to TVs. What about 
his  telephones and other audio devices? 

A: In almost all cases, interference to an audio device is 
caused  by detection of your fundamental signal, just like a 
crystal-detector radio receiver. Your detected signal gets 
amplified along  with the desired voice or music signal. 

This is clearly not the fault of the transmitting station. The 
FCC  Interference Handbook, available free of charge from 
the ARRL  Regulatory Information Branch here at 
Headquarters, states on page  18: 

"Telephones, stereos, computers, electronic organs and home  
intercom systems can receive interference from nearby radio  
transmitters. When this happens, the device improperly 
functions  as a radio receiver. Proper shielding or filtering 
can eliminate  such interference. The device receiving should 
be modified in your  home (the neighbor's home--Ed.) while 
it is being affected by the  interference. This will enable the 
service technician to determine  where the interfering signal 
is entering your device." 

The FCC clearly puts the responsibility for interference to 
audio  devices on the manufacturer. Most manufacturers 
respond  appropriately if contacted about consumer EMI. As 
discussed in the  following question, contact the EIA and 
they'll help you find the  right person to write or call. 

Q: Can I get any help from the consumer-equipment 
manufacturer?

A: The manufacturers also shoulder some responsibility for 
EMI  problems. Public Law 97-259, enacted in 1982, gave 
the FCC the  authority to regulate the susceptibility of 
consumer electronic  equipment sold in the United States. 
The FCC, working with  equipment manufacturers, decided 
to allow them to develop  standards for EMI immunity and 
implement their own voluntary  compliance programs. No 
system is perfect, especially a voluntary  system, but the 
ARRL Laboratory staff has noted that EMI involving  TVs, 
for example, seems to be decreasing. The manufacturers are  
making some real progress and we feel confident that they 
will  continue to do so. 

One prominent manufacturer program is a contact data base 
that's  maintained by the Electronic Industries Association 
(EIA), 2001  Pennsylvania Ave NW, Washington, DC 
20006, tel 202-457-4977. When  you have an interference 
problem with a piece of consumer  electronic equipment, call 
or write the EIA to determine who you  should contact for 
assistance. The EIA also keeps a record of each  report. (The 
EIA prefers that you write rather than call. The  details a 
problem can often be communicated more clearly in  written 
correspondence.) 

You may be surprised to know that the number of reported 
cases of  interference to consumer electronic equipment in 
recent years has  been very small. This is our fault! Amateurs 
are notorious for not  reporting EMI problems. Contact the 
EIA! Working with  manufacturers makes them aware of the 
need to continue to develop  better shielding and filtering 
methods. It also demonstrates to  your neighbor that the 
manufacturer should receive a little of his  anger and 
frustration too! 

Q: In spite of my efforts, and the diplomatic skills of the  
Technical Coordinator, my neighbor must have called the 
FCC; I  just got a letter from the local FCC Field Office. 
What now? 

A: Well, you could sell all of your equipment, cancel your 
license  and take up basket weaving . . . or you could sit 
down and answer  the notice! The exact FCC response to 
consumer-interference  complaints varies slightly among the 
different Field Offices, but  they use similar steps steps to 
resolve interference cases. 

You already have the first step in hand. Your letter from the 
FCC  Field Office is stating that you and your neighbor are 
involved in  a mutual problem. I hope both of you will get 
the message the FCC  is trying to deliver--that it's in your 
best interests for you to  find a solution that's acceptable to 
everyone. Years of experience  in interference resolution has 
taught the FCC that imposed  solutions are not the best 
solutions for local problems. You and  your neighbor will be 
happier if you're able to find a solution  and an understanding 
on your own. Your TC, acting as a third  party, may be able 
to help you with the technical and  interpersonal aspects of 
the problem. 

The first order of business is to answer the FCC letter as  
accurately as you can. If you've offered to cooperate with 
your  neighbor and were turned away, say so. If your TC has 
been helping  you solve the problem, explain what the TC 
has done and what  conclusions have been reached. The FCC 
is interested in hearing  that your station is grounded (keep in 
mind, however, that a  station ground is not a cure-all for 
EMI!), properly filtered and  that your station is 
well-designed. Tell the FCC whatever you  think is important 
to the proper resolution of the case. Try to  minimize 
emotional comments, extraneous data and fluff. 

If the FCC is satisfied with the answer, or if you and your  
neighbor find a solution, the case is closed. If not, the next  
steps are a bit more drastic. The FCC may inspect your 
station. In  extreme cases, quiet hours may be imposed, 
limiting the times of  day you are allowed to operate. The 
mere thought of quiet hours  should give you plenty of 
incentive to cooperate fully with the  FCC! 

Q: Well, I've found some local helpers and they're really 
making  progress. I'd like to know more about EMI. (I might 
want to offer  assistance to another unfortunate ham 
someday.) Where can I learn  more about EMI? 

A: Reading this column is a good start. We can't teach 
everything  about EMI in a few pages, but we'll provide 
some important  highlights. Several good books on the 
subject are readily  available. Information on ordering these 
books is found in  Appendix I at the end of this handout.

The best one is the ARRL's book -- Radio Frequency 
Interference: How To Find It and Fix  It. It was written by a 
number of authors ranging from ARRL  Technical 
Coordinators to EMC (electromagnetic compatibility)  
engineers. The book covers EMI fundamentals and 
troubleshooting as  they apply to transmitters, receivers, TVs 
(VCR and CATV),  telephones, computers, audio devices 
and automobiles. 

William (WA6FQG) Nelson's Interference Handbook is an 
excellent  volume to add to your collection. Nelson is a 
former RFI  investigator for Southern California Edison 
Electric Company. It  should be no surprise that his book is 
especially strong in the  area of power-line interference. 

Over the years, most Amateur Radio magazines have 
published  articles about EMI. These articles, including some 
classics from  the 1950s and 1960s, are informative reading. 
A bibliography of  QST EMI articles is available from the 
ARRL Technical Department  Secretary. QST back issues 
are available from our Publication  Sales Department. 
Photocopies of articles in League publication  articles are 
available from the Technical Department Secretary.  Contact 
ARRL Headquarters for information about these League  
services. American Radio Relay League, Administrative  
Headquarters, 225 Main St., Newington CT 06111 (203) 
666-1541.

Q: I've had enough of the preliminary stuff and I'm eager to 
learn  more about the technical side of EMI. When do we 
start? 

A: I don't know how to break this to you, but we've run out 
of  page space. As we said at the beginning, this is going to 
be a  two-parter! Instead of making you wait two months, 
however, we're  going to modify our format and continue this 
topic in the March  issue. (We were having so much fun, we 
decided to do the column  two months in a row!) So think 
about the important start we've  made here, talk about EMI 
on the air and wait until March to hear  the rest of the 
story.--Ed Hare, KA1CV, Senior Laboratory Engineer 



Interference Primer -- Part 2 Derived from Lab Notes QST 
3/92 

Copyright 1992 American Radio Relay League, Inc. All 
rights  reserved.

Last month we discussed electromagnetic interference (EMI) 
from a  legal, diplomatic and psychological point of view. 
Now it's time  to talk about specific solutions. Aided by the 
knowledge of many  people who have assisted him over the 
years, Ed Hare, KA1CV,  Senior Laboratory Engineer, will 
explore the fundamentals of  electromagnetic compatibility 
(EMC) and offer some tips to exorcise those stubborn 
electro-magnetic gremlins! --  WB8IMY 

It's impossible to discuss all the technical aspects of  
interference in two pages. Instead, we'll concentrate on some 
EMC  basics. Like anything else in life, once you understand 
the  basics, the rest follows easily. As I mentioned last 
month, there  are entire books devoted to EMC and EMI and 
I strongly recommend  that you read them. 

There are a few things to cover before we can get to specific  
cures. Several factors are present in any interference 
situation:  a source of electromagnetic energy, an affected 
piece of equipment  and a path from the interfering signal 
source to the affected  equipment. A clear understanding of 
these factors is important to  your overall grasp of the 
problem. Any EMI cure that is effected  is going to involve a 
change made to the source, the path and the  affected 
equipment. 

Q: Well . . . I'm the source, right? I mean, isn't my station  
always the source of interference? 

A: Not necessarily! Remember: your station is only one of 
many  possible interference sources. What about broadcast 
stations,  taxicabs and police and fire services? What about 
cable TV  leakage, unlicensed Part 15 devices (baby 
monitors, computers and  so on)? Add power lines and 
electric motors to the list as well.  They're all potential 
interference sources. Your neighbor's TV can  even interfere 
with you!

Q: Aha! So that's what I've been hearing every 15 kHz on 80  
meters!  I'm still a little fuzzy on the path concept. Can you  
explain? 

A: Interference can propagate via several possible paths. 
Take a  look at Fig 1. The easiest path to understand is the 
direct path.  In this case, the interfering signal is transmitted 
by your  antenna (or possibly by your feed line or ground 
leads) and  travels directly to the hapless TV, VCR or 
whatever. Interference  via this path is particularly difficult to 
control. 

Conducted interference travels from the source to the victim 
by  wires. For example, a vacuum-cleaner motor may 
introduce RF noise  into the ac-power system of your 
home--which conducts the noise  directly into your amateur 
receiver! 

In most instances, however, you'll be dealing with a signal 
that's  been induced into the external (or internal) wiring of 
the  victimized equipment. Its wiring acts as an antenna, 
funneling the  radiated signal to the location where it can 
generate the most  misery. 

Technically speaking, all interference begins and ends as a  
conducted signal--no matter what happens in between. 
Understanding  the subtle differences in signal paths is 
important, however. A  successful diagnosis depends on 
determining how the EMI gains  entry to the device. Armed 
with this vital knowledge, you're ready  to start 
troubleshooting. 

Q: Good! Where do I begin? 

A: We touched on this point last month, but I'll repeat it 
because  it's the first rule of EMI control: Make sure your 
own house is  EMI-free! Cure your own EMI (if any) first. If 
you're not  experiencing interference on your own 
equipment, it will go a long  way toward convincing your 
neighbor, and the FCC, that you're not  the cause of the 
interference.

Let's start in your shack. You need to be sure that your 
station  is not a source of out-of-band spurious 
emissions--particularly of  the VHF variety. The easiest way 
to reduce VHF spurs is to use a  low-pass filter. It should be 
installed after the linear amplifier  and any accessory 
equipment (SWR meter, TR switch and so on). A  50-ohm 
filter works best in a 50-ohm system, so you'll have to  install 
it before the antenna tuner, if you have one. 

While grounding is not a cure-all for transmitter EMI, you 
must  consider your ground system. If the FCC gets involved 
(let's hope  not!), they'll want to know that your station is 
properly  grounded. Improved grounding may provide a 
measure of EMI control  since it effectively rearranges the 
RF voltage and current  distribution, moving hot spots away 
from potential problem areas.

Another important troubleshooting step is to make sure that 
your  station is well engineered. Poorly soldered connectors, 
corrosion,  a rat's nest of wiring or an overdriven amplifier 
can all  contribute to or cause EMI. Neatness counts when it 
comes to  diplomacy, by the way. If your neighbor has a 
chance to visit your  station, its neatness will boost your 
credibility. 

Q: I've tested my station from top to bottom and everything 
looks  fine. When I checked around the house, I still found 
interference  to my VCR, video-game machine, stereo TV 
and video intrusion  monitor. All of these devices are 
connected to each other by an 8-way splitter! Where do I go 
from here? 

A: I can hardly imagine a worse case! (I say "hardly" 
because we  get some real strange telephone calls and letter 
here in the ARRL  Lab!) Now is the time to state the second 
rule of EMI control:  Simplify the problem! Connect the 
incoming CATV cable (or antenna  feed line) to only one TV 
(assuming you have more than one). For  the time being, 
completely disconnect the VCR and other video  goodies. 
The result will uncover an important clue. 

One troubleshooting technique is to try an EMI cure and see 
what  happens. This brings us to the third rule of EMI control 
(and all  other troubleshooting, for that matter): Always try 
the easy  things first! This rule applies to the susceptible 
equipment and  the suspected EMI source. Begin by 
installing the appropriate  filter on your TV. For an 
antenna-connected TV this is a high-pass  filter. (For a 
CATV-connected TV, a common-mode choke should be  
tried first.)

Q: Nope! It didn't work. What now? 

A: Hmm. . . we may be dealing with a very susceptible TV. 
If the  TV is of recent manufacture, however, that is less 
likely. In a  two-wire system (such as a coaxial cable) there 
are two modes of  propagation for conducted EMI: 
differential mode and common mode  (see Fig 2). 

In the differential mode, the signal travels down the center  
conductor and uses the shield (or other conductor) as its 
return  path. In the common mode, all wires in the system act 
as one wire,  with earth ground (usually through the ac 
wiring) forming the  return. The resulting circuit is just like 
an end-fed antenna  worked against earth ground. 

An in-line coaxial high-pass filter can be quite effective 
against  differential-mode EMI signals, but ineffective when 
common-mode  propagation is present. The high-pass filter 
blocks signals on the  center conductor, but passes everything 
on the shield! This is a  serious weakness because induced 
signals on antenna feed lines or  CATV cables are 
predominantly common-mode in nature. Most of the  
high-pass filters that are commercially available are  
differential-mode filters. Unfortunately, common-mode 
signals are  the ones most "commonly" seen.  

So let's try a different tactic. Leave the differential-mode 
high- pass filter and the ac-line filter in place. Now add a 
common-mode  choke to the antenna feed line (or CATV 
cable) and the ac line.  This places a high impedance in series 
with the incoming common- mode signal and the earth 
ground return. 

The EMI/RFI Resources Directory lists a company that sells 
a  common-mode choke. Or, you can make a common-mode 
choke by  wrapping 10 to 20 turns of the antenna feed line or 
CATV cable  through a ferrite toroid. Follow the same 
procedure with the ac  line. Use #75 (also known as "J"), #73 
or #77 material if the  interference is mainly from signals 
below 10 MHz. Use #43 ferrite  material for the higher bands 
or low VHF. The mis-application of  ferrites has led to a 
mis-conception that ferrites don't work for  EMI control, so 
always use material of known characteristics. The  
permeability or frequency range of junk box ferrites may be  
unsuitable. If this is a paper copy of our EMI/RFI package, 
the attached appendices list several sources of ferrite 
material. If you obtained this file eletronically, a separate file 
contains the sources for EMI/EMC components. Chapter 35 
of the ARRL Handbook also contains a complete  reference 
list of component suppliers, among which are ferrite  
suppliers.

If this doesn't eliminate the interference, you either have a  
spurious emission from your station (time to install that 
low-pass  filter!), or the TV circuitry is picking up the 
offending signal  directly. If the latter is the case, refer to last 
month's column  and contact the EIA to obtain assistance 
from the TV manufacturer. 

Q: I installed the chokes and the TV looks much better! 
What about  the other devices? 

A: If you've cured the EMI at the TV, start hooking up the 
other  devices one by one, eliminating any additional EMI as 
it appears.   If you're lucky, you'll eliminate all of the 
problems. If not, at  least you can point to one particular 
piece of equipment and say,  "That's the culprit!" 

As you put the system back together, do not create a tangle 
of  wires and cables. All cables should be connected 
properly, routed  neatly and no longer than necessary. An 
8-foot piece of cable  picks up a lot more RF energy than a 
1-foot piece!

Q: Your suggestion made a big difference! Even so, I still 
see a  trace of interference. What gives? 

A: EMI control is a complex business. The tiny bit of 
interference  could mean a lot of things. You may need a bit 
more attenuation of  the common-mode or differential-mode 
signal. In some cases, an  additional high-pass filter or 
common-mode choke may help. If you  add more filters, 
experiment with their placement if possible.  Sometimes a 
second filter works best when it's positioned a few  feet away 
from the first one. You may also be dealing with  
interference that results from more than one cause. 

Q: Well, my family is finally satisfied with the TV, but we 
still  can't use the telephones. What can I do? 

A: There is hope. Several companies manufacture telephone 
EMI/RFI  filters and most work quite well. Some of these 
manufacturers are  QST advertisers. Remember the three 
rules of EMI control and  follow them religiously as you 
install the filters. A few  companies also make 
interference-resistant telephones. Many of  these companies 
are listed in the appendix.

Inspect the telephone system. Corroded wiring (common in 
damp  basements) or a defective lightning protector 
(common in areas  where the protector has done its job!) can 
rectify the RF signal.  Unlike your TV problem, the resulting 
audio interference cannot be  filtered out. 

Rectification can also occur in telephones and other devices  
connected to the system. Before you begin connecting filters,  
disconnect all telephones and accessories except one. 
Remember to  use the systematic divide-and-conquer 
approach, beginning with one  device and working forward. 

Take a careful look at the wiring while you're investigating 
the  problem. Sometimes the twisted pair has been spliced 
with  nonstandard wiring, such as zip cord. This type of 
jury-rigging is  more prone to interference pickup. 

If you discover a problem with the lightning protector or 
outside  wiring, leave those items for the telephone company 
to fix or  replace. The responsibility for inside wiring may 
vary from one  area to another. Check with your phone 
company for guidance. 

For more information on telephone interference, read the 
October 1992 QST "Lab Notes" column on page 67. A 
reprint of this article is available for a 9"X12" SASE with 
two units of postage from the ARRL Technical Department 
Secretary, 225 Main St., Newington CT 06111, (203) 666- 
1541. Ask for the  "EMI/RFI - Telephone" Technical 
Information Service (TIS) handout.

Q: Everything is fine now. I think I'll buy a bunch of filters 
and  head on over to my neighbor's house!

A: Whoa! What kind of arrangement are you going to work 
out with  your neighbor? What if there are other neighbors in 
the area  experiencing similar problems? You may be setting 
yourself up to  spend a lot of money on filters! Other than 
problems that  originate from your station, you should 
consider yourself as an  advisor, not a service technician or 
parts supplier! 

You may be walking into murky legal waters, too. Some 
states  require you to hold a repair license to perform even 
the simplest  services--free or otherwise. Consider the future 
consequences of  your actions as well. I recently heard of a 
well-meaning amateur  who installed a high-pass filter on his 
neighbor's TV. When the  picture tube on the old clunker 
suddenly went bad, the neighbor  claimed that the filter 
caused the failure! 

This doesn't mean you should never offer a helping hand, but 
it  does mean that you should look before you leap. You are 
the best  judge of your neighborhood situation. Only you can 
decide what  kind of assistance and diplomacy is appropriate. 

Q: Thanks for the warning. By the way, I've found that EMI 
also  makes my stereo act up. Do you have a magic cure for 
that too? 

A: We're almost out of room, so I'll give you a short answer. 
Many  problems with stereos can be traced to common-mode 
propagation on  long speaker leads and interconnecting 
cables. You can often  effect a cure by keeping wire lengths 
to a minimum. If you can't  shorten the wires, use 
common-mode chokes. Low-value bypass  capacitors can be 
used on input leads (try 100-500 pF), but do not  use 
capacitors on speaker leads unless you check with the stereo  
manufacturer first. Adding capacitors to speaker wiring can 
cause  some amplifiers to launch into an ultrasonic, 
full-power  oscillation--often resulting in permanent damage. 
If you think you  had a problem before you destroyed the 
family stereo, wait until  you see what happens after you do! 

If you have any questions about ARRL EMI/EMC policies, 
or need help solving an EMI problem, contact the RFI Desk 
at ARRL Headquarters, 225 Main St., Newington, CT 06111 
(203) 666-1541,  Internet: ehare@arrl.org, MCI: 215-5052

(Figures are not available electronically.)

Fig 1 - Conducted and radiated EMI. 

Fig 2--(A) Differential-mode signals are conducted between 
two  wires of a pair. The signals are independent of earth 
ground. (B)  Common-mode signals are in phase on all 
wires, with all wires acting as if they were one wire, with the 
return path being earth ground.

*eof

