Reprinted from October 1992 QST Lab Notes 
Copyright 1992 American Radio Relay League, Inc. 
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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.


To: ARRL Lab staff
de: Ed Hare - KA1CV

Re: RFI-resistant telephone

Lewis Cullen, an Alabama ATC (now TS) reports to me that Radio 
Shack ET-171, catalog #43-374A has adequate EMI suppression. He 
reports that it is quite immune to his 1200 watt station. Other 
telephones tried, notably the AT&T Trimline, do not exhibit this 
immunity. The telephone is also sold in other colors as catalog 
numbers 43-375A, 43-376A and 43-377A.


