QRZ! Ham Radio 1/QRZ Ham Radio Callsign Database - December 1993.iso/arrl/emi_gen

Text File | 1993-11-21 | 37KB | 704 lines

ARRL Technical Information Service
EMI/RFI Package

Updated: January 19, 1993  File: rfigen.txt

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

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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: This is beginning to sound like a conspiracy theory. 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.

In the ARRL Laboratory, we jokingly suggest that hams who install
highly-visible towers or antennas should not hook up the coaxial cable
until they take care of all EMI complaints. New antennas in the
neighborhood have a funny way of causing interference even when no RF
signal is applied!

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 [phone]. 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 [phone].

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 [phone], Internet: [e-mail],
MCI: 215-5052

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.