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

Text File | 1993-11-21 | 22KB | 444 lines

.rm66
Reprinted from October 1992 QST Lab Notes
Copyright 1992 American Radio Relay League, Inc.
All rights reserved.

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Lab Notes Interference Primer -- Part 3 Telephone Interference
Conducted By Steve Ford, WB81MY, Assistant Technical Editor and
the ARRL Laboratory Staff

Just when you thought it was safe to go back in the shack, Ed
Hare, KA1CV, ARRL Laboratory Supervisor, is at it again! This time
his topic is telephone interference -- the bane of every amateur
who has to go QRT whenever a telephone rings! -- WB81MY

The cures for telephone interference are usually fairly easy to
understand and implement, but there are often other issues that
need to be addressed. Technical issues are really the second half
of an interference problem. First and foremost you must deal with
personal and political matters. The February and March 1992 QST
Lab Notes columns (retrieve the file emi-gen from the ARRL
Information Mail Server) discussed the non-technical side of
interference. Read these columns again to refresh your memory.
They've been reprinted in the ARRL Technical Department "RFI
Package. (Note 2) The "RFI Package" also contains a copy of our
new RFI consumer pamphlet plus an up to date "ARRL RFI Resources
Directory." The Directory tells you where to buy filters,
identifies several EMI-resistant telephones and has a contact list
for telephone manufacturers. Much of the information in this file
is found in the emi-gen file -- those with an actual interference
problem should get the paper file from the Technical Department.
See note 2 at the end of this file.

Now, let's answer your questions!

Q: I was about two minutes into a QSO with a ham in Outer Elbonia
when my doorbell starting bonging like crazy. It was my neighbor
Sam, telling me that I was interfering with his new telephone. I
tried to tell Sam that it might be the fault of his telephone, but
he wasn't convinced. He insisted I was wrong because it's a brand-
new model! According to the salesperson, his telephone is the very
best one on the market. What's the deal, Mr. EMI wizard?

A: We'll have to break the bad news to Sam gently, but let's begin
by establishing the fact that interference to telephones is not
the fault of the transmitter. A quick quote from the FCC's
Interference Handbook (Note 3) should explain things clearly:
"Telephones, stereos, computers, electronic organs and home
intercom devices 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 FCC doesn't require that telephones include EMI protection and
they don't offer legal protection to users of telephones that are
susceptible to interference. If you show this to Sam, he should
realize that the interference is not caused by a problem on your
end. Fig I pretty much sums up telephone interference at a glance!

This doesn't mean you should leave Sam on his own, though. You
want to be on good terms with your neighbors; besides, maybe
you'll want to put up a tower someday. Keeping Sam on your side is
a good idea!

Q: Fine, but what can I do?

A: If you can reduce the strength of your signal at the telephone
equipment or cables, you may reduce the chance that any of your
neighbors will experience telephone interference. If practical,
try moving your antenna or feed line farther away from (or higher
than) his equipment. Using a common-mode choke on the feed line
may help. (See Fig 2).

Q: Well, I must confess that I've been interfering with the
telephones in my own home, too. I remember your advice to make
sure my own house is clean. Should I start here?

A: You betcha! There are several good reasons to start in your own
home. If you own your telephone wiring, you have the flexibility
to try many different troubleshooting steps easily. If there are
problems with Sam's telephone wiring system, however, you'll
probably want to call a professional (often your local telephone
company) to do the actual repairs. By performing repairs on Sam's
wiring, you may be held liable for any problems that occur -- even
if the problem is with his toaster.

If you lack experience, you may want to hire local experts for
your own wiring, too. Before you start troubleshooting your
system, check with your local regulators. There may be state or
local requirements that must be met.

There's another good reason to start with your own house: You'll
be able to demonstrate that EMI cures are not only effective,
they're harmless. The next time Sam complains about interference
to his telephone, invite him to come over and try your telephone
instead! By using your telephone when you're on the air, he'll
quickly understand that the problem must be on his end.

Q: Okay, I'm ready. Where do I begin?

A: Read the March 1992 "Lab Notes" column and review some of the
troubleshooting techniques. You can also learn quite a bit about
troubleshooting telephone EMI problems from the troubleshooting
and telephone chapters of the ARRL book Radio Frequency
Interference -- How to Find It and Fix It. (Note 4)

Rule number one: Simplify the problem! If your home telephone
system consists of eight two-line outlets with a telephone in each
room, two answering machines, a modem and a fax machine, it may
take quite a while to get it all straightened out. Simplifying the
problem saves lots of time and headaches!

Most cases of interference stem from more than one cause. If you
start with a systematic approach to troubleshooting -- debugging
and fixing the problems one at a time -- you can usually resolve
the interference quickly. Disconnect all but one telephone. Cure
the EMI bugs (usually with the proper application of filters),
then begin adding other devices one a time, curing each EMI
problem as you go. With any luck you'll complete the process with
an EMI-free system. If not, at least you'll identify the specific
equipment that's not immune.

In modern installations, telephone companies often install a phone
jack where the wiring enters the house. This jack makes it easy to
unplug the rest of the house wiring and substitute a telephone in
its place. If you have access to an EMI-resistant telephone (refer
to the ARRL "RFI Package"), you can try that phone at the service
entrance (drop). If the EMI-resistant telephone doesn't pick up
interference at that point, you've eliminated the telephone
company system as a possible culprit.

In many cases, the telephone company is responsible for their
lines only up to the point where they attach to the customer's
wiring and equipment.

Q: Uh, oh! I plugged in my EMI-resistant phone and I still hear
interference. What does this mean?

A: It sounds like this is a problem for the telephone company. For
example, the telephone-company lightning arrestor may be
defective. Defective arrestors can act like diodes, rectifying any
nearby RF energy (just like the crystal radios many of us built
when we were kids). Telephone-line amplifiers or other electronic
equipment may also be at fault. Leave the telephone company
equipment to the experts. There are important safety issues that
are the sole responsibility of the telephone company.

In your case, the EMI-resistant telephone is probably responding
to the audio that's present after the "diode" as a result of RF
rectification. Once the RF has been detected and turned into
audio, it cannot be filtered out because the interference is at
the, same frequency as the desired audio signal. This principle
applies whether rectification occurs in the telephone company
system, your house wiring or a defective telephone or answering
machine in your home.

Q: Well, I got the phone company repair personnel out here. They
found a defective lightning protector. After they replaced it, I
plugged my EMI-resistant telephone into the service entrance and
it sounded fine. As soon as I reconnected the house wiring, the
interference returned. What now

A: Replacing the arrestor wasn't a waste of time. Lightning
arrestors are notorious for contributing to telephone EMI. This is
especially true when the protector has done its job a few times!
The fact that it's rectifying RF is one indication that it needed
to be replaced anyway.

By plugging your EMI-resistant telephone in at the service
entrance and finding no problems, you've proven that the EMI-
resistant telephone and the phone system are clean. When the
problem reappears after you reconnect the house wiring, the house
wiring itself is the prime suspect.

A careful inspection of the wiring may reveal the source of the
problem. Years of exposure in damp basements, walls or crawl
spaces may have caused deterioration. Be suspicious of splices
that are corroded or badly discolored. Metal corrosion can form a
fairly efficient diode, especially if dissimilar metals make
contact. If you find this type of corrosion, correct the problem
by repairing the splices.

In many cases, homeowners have installed their own telephone
wiring, often using substandard wiring. If you find sections of
telephone wiring made from two-conductor zip cord, 300-ohm
television twin lead, or some other mystery cable, replace it with
standard twisted-pair wire. Radio Shack, among others, sells
several types of telephone wire. The best telephone wiring systems
use twisted-pair, balanced wiring to minimize pickup of external
electromagnetic fields. If this system becomes unbalanced in some
way, perhaps by one of the wires developing a short circuit to a
nearby conductor, the amount of RF picked up by the wiring will
increase. Keep your eyes open for this type of problem when
examining your existing twisted-pair wiring.

Don't forget to consider hidden, unexpected connections to the
telephone line. If your telephone line is hard-wired to your
automated alarm system, for example, you really haven't simplified
the problem completely -- there's still something hooked up to the
line!

Q: Bingo! I'd forgotten about the alarm system. When I disconnect
the telephone line from the alarm box, my EMI-resistant telephone
works fine in every house outlet. Why did the alarm system cause
the problem?

A: Well, I can't be sure without seeing the schematic. My guess is
that the RF was being conducted down the telephone line and
rectified by a solid-state device in the alarm system. The
resulting audio signal was then placed back on the line. This
effect is not uncommon with alarm systems, answering machines, fax
systems and modems.

Q: I have a service contract, so I called the alarm company. After
a bit of discussion (which included a lot of pointing at the FCC
Interference Handbook), the service installer agreed that he
should cure the problem. Neither of us knows where to begin,
though. Any ideas?

A: You'll sometimes find that a manufacturer or utility company is
willing to responsibly address an interference problem, but lacks
the experience and training necessary to apply the correct
solutions. You may need to apply your skills (and those of your
ARRL Section Technical Coordinator or local RFI committee) to help
their personnel understand the technical issues.

Start by installing a commercially available telephone EMI filter
on the alarm system line. Several companies advertise these
filters in QST. A list is also included in the Technical
Department "RFI Package." If the unwanted RF energy is being
conducted into the alarm system by the telephone wiring, this
filter will block the energy from the components that are
detecting the RF signal. As you can see in Fig 3, these modular
filters are attractive -- your family or neighbors probably won't
object to them on aesthetic grounds.

If the telephone filter doesn't work, try installing a common-mode
choke on the ac line cord that supplies power to the alarm system.
It may also be necessary to use a differential-mode ac line filter
(the commonly available "brute force" filter) on the ac-line cord
as well.

If all else fails, it may be necessary to modify the alarm system.
This is another job for the experts. Suggest that the repair
person contact the manufacturer.

These methods apply to any device connected to the telephone
system: your alarm, modem, answering machine and so on. Ac line-
connected devices are much more prone to interference. After all,
they offer two possible paths for the RF to enter: through the
phone line and the ac line. In extreme cases, check any other
wiring that connects to the susceptible device, including alarm
system wiring and computer cables.

Q: Great! The filters worked. I n plug in all my goodies and go
back to happy hamming!

A: Whoa! Don't jump to conclusions! In addition to the gremlins
you have uncovered so far, you may still have susceptibility
problems with some of your telephones or your answering machine.
Plug them in one at a time and see what happens.

Q: %%^%^%^%#@%^! The first two telephones worked, but my high-tech
bells-and-whistles telephone hears my 40-meter signal just fine!
It's my most expensive telephone! Why is it defective? And why
just 40 meters? What should I do?
***

A: There are two types of telephones that seem to be most
susceptible: the inexpensive ones that have virtually no shielding
or filtering, and the expensive ones that have many solid-state
devices rectifying RF signals. Of course, telephones that fall
between these two categories have problems, too !

There may be several reasons why your gee-whiz telephone is
particularly susceptible to interference from your 40-meter
signal. For example, internal resonances in the telephone
circuitry may allow more 40-meter RF energy to reach the
susceptible component inside the telephone. However, in this case,
it's more likely that the telephone wiring in your home is
resonant on 40 meters. The telephone wiring, its associated
grounds and any other equipment connected to the system form a
large antenna. Like any antenna, this system has high- and low-
impedance points. If the telephone happens to be located at one of
these nodes (either high- or low-impedance) it may be subject to
quite a bit of RF energy.

Q: Wow! What can I do about it?

A: Try a commercially available in-line modular telephone filter.
These should be installed on the telephone line, as close to the
telephone as possible, or on the handset. Telephone EMI filters
are made by several different manufacturers and come in several
varieties. The ARRL Laboratory Engineers have received reports
from the field that the effectiveness of these filters varies from
installation to installation, possibly dependent on the exact
nature of resonances, impedances and system reactances. Many of
these factors are hard to predict, so be prepared to experiment
with several different manufacturers' filters in difficult cases.

You may also need to break up the resonances in the telephone
wiring. Several manufacturers sell telephone EMI filters that can
be installed at strategic points to detune the resonances. This is
similar to the effect of using insulators to break up the guy
wires on a tower to prevent them from resonating at amateur
frequencies.

Q: I thought I was finished until I got to my answering machine.
When I hooked it back up, every telephone in the house had
interference again. It sounds awful! What should I do?

A: It's just a coincidence that your answering machine is doing
this. It could just as easily have been one of your telephones --
but it does give me a chance to give you two answers to one
question (heh, heh!). Yours is a situation in which one
susceptible device is detecting the RF energy and putting the
resultant audio signal back on the telephone line.

You should do the same things to the answering machine (or any
other ac-line connected telephone device) that you did to the
alarm system. Install a telephone line filter and don't forget to
add a common-mode choke and a brute force ac-line filter on the
answering machine line cord. This should clear up most cases of
interference to these devices.

Q: It sure did. . . but I need to bother you again! I have one
telephone that's a real bear to fix. I've tried several different
filters to no effect. Should r take it apart and put some filters
inside?

A: No! The manufacturer is responsible for the proper operation
and repair of the telephone. The ARRL encourages all people who
have an interference problem to contact the manufacturer of the
susceptible device through the Electronic Industries Association,
2001 Pennsylvania Ave NW, Washington DC 20006. This helps ensure
that the manufacturers (and their national association) are aware
of the interference problems their customers are experiencing.
Remember: interference that isn't reported of officially doesn't
exist! (Our "RFI Package" includes a report form -- please use
it!) Some manufacturers supply filters free of charge. We've even
heard reports of major telephone manufacturers giving refunds for
defective telephones.

Some consumer devices are so susceptible they can't be filtered
externally. If the manufacturer is unable to help, the best
solution might be to give the telephone to someone who lives far
away from your transmitter and purchase an EMI-resistant telephone
to replace it!

Q: What about my cordless telephone? It seems to pick up
interference no matter what I do. Is there anything that will cure
the problem?

A: The FCC does not intend Part 15 devices to be protected from
interference. These devices usually have receivers with very wide
front-end filtering, making them very susceptible to interference.
A cordless telephone is a Part 15 device. It's an unlicensed
transceiver that shares the spectrum with other users and services
on a non-interference basis. There should be a label on the
telephone, or a paragraph in the owner's manual, that explains
that the telephone must not cause interference to other services,
and must tolerate any interference caused to it.

It's worthwhile to try a telephone filter on the base unit and
properly filter its ac line cord. (You might get lucky!) The best
source of help is the manufacturer, but they may point out that
the Part 15 device is not protected from interference. These types
of problems are difficult to fix after the fact. The necessary
engineering should be done when the device is designed.

Q: Yeah, that did it. Now my family can use the telephone while
I'm on the air. My teen-agers thank you. I guess it 's time to pay
Sam a visit. Any last-minute words of wisdom?

A: Don't forget money issues. Telephone companies often charge to
come out and repair wiring. Repairing consumer electronic
equipment usually isn't free, and telephone EMI filters require a
bit of wallet-digging, too! You and Sam should discuss these
issues up front, before you order filters or call the service
department. You're not responsible for purchasing filters or
repairing defects in your neighbor's house wiring. If you want to
be neighborly and buy a filter for Sam, that's your choice.
However, doing so may set a precedent. If you live in an apartment
building where there are hundreds of telephones to contend with,
you may have to re-evaluate your generosity!

Telephone interference is like any other EMI problem -- it usually
can be cured by properly applying good troubleshooting techniques
and effective filters.

Notes

1 -- "Lab Notes," Interference Primer -- Part 1, QST Feb 1992 p
73. Also, "Lab Notes," Interference Primer -- Part 2, QST, March
1992, p 81.

2 -- The ARRL Technical Department "RFI Package' can be obtained
by sending a 9- x 12-inch SASE with three units of First-Class
postage to the ARRL Technical Department Secretary Include your
request for the "RFI Package'

3 -- A copy of the FCC's Interference Handbook can be obtained by
sending a 9- x 12-inch SASE with three units of First-Class
postage to the ARRL Regulatory Information Branch. Include your
request for the FCC Interference Handbook. 4Radio Frequency
Interference How to Find It and Fix It is available from your
local dealer or directly from ARRL HQ. See the ARRL Publications
Catalog elsewhere in this issue for ordering information.

We welcome your suggestions for topics to be discussed in Lao
Notes, but we are unable to answer individual questions. Please
send your comments or suggestions to: Lab Notes, ARRL, 225 Main
St, Newington, CT 06111.

Fig 1 -- A transparent telephone allows us to see the internal
circuitry. Not much shielding here. In fact if we could see our
phones with RF rather than visible light, this is what most
telephones would look like!

Fig 2 -- A common-mode choke for coaxial transmission lines. A
similar technique is used to wind common-mode chokes for telephone
wiring or ac-line cords. Use #75 ferrite material to suppress HF
signals and 43 to suppress VHF.

Fig 3 -- A typical modular telephone EMI filter.