QRZ! Ham Radio 4/QRZ Ham Radio Callsign Database - Volume 4.iso/files/arrl/tis/rfigen.txt

Text File | 1994-11-27 | 38KB | 738 lines

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

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Subject: ARRL EMI/RFI Package - "RFI Tips"

Note: electronic mail readers should also retrieve the file
EMI-SOURCE from the ARRL Information Mail Server ([e-mail]).

Interference Primer - Part 1. Derived from QST February 1992
Lab Notes ***revised May 12, 1992***

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 0TC, 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 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
*** revised May 12, 1992 ***

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 electromagnetic 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. Chapter 35 of the ARRL Handbook 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 "Basic Steps
Toward Eliminating Telephone RFI" by Pete Krieger, WA8KZH, QST,
May 1991, pp 22-25. A reprint of this article is available free of
charge from the ARRL, 225 Main St., Newington CT 06111, (203) 666-
1541. Contact the Technical Department Secretary and ask for the
"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!

Figures - 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 that form the
conductor (this includes coaxial cable). All wires act as if they
are one wire. The ground connection forms the return path, as with
a long-wire antenna.

The members and HQ staff would like to thank the following people
for their contributions to this information file:

KA1CV, Ed Hare, [e-mail]

Send any additional information or changes to [e-mail].

73 from ARRL HQ.