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nected to the phone line—
answering machines,
modems, surge protectors,
and so on.
4. Listen for interference at
the control point.
5. Now reconnect one device
and listen for interference
at the control point again.
If the interference has
increased noticeably,
the device you just plugged
in is a source of, or at least
a contributor to, the
interference.
6. Unplug that device and
plug in another, then listen
again. Use the same meth-
od to check all the
equipment.
7. To complete the test, move
the original control-point
telephone to another phone
jack. Plug another phone
into the jack at the control
point. Listen at the new
phone for any change in
the interference.
If one or more phones or other
devices are causing most of the
interference, you’ve found your
problem.
Mapping Out Your System
Home telephone systems start
at the protector box, which is
installed on the outside of the
house, where the line comes in
from the telephone company.
(This box protects the home
telephone wiring from lightning.)
Next, the wiring goes inside the
house to the “service entry,”
sometimes called the “interface”
or “connector block.”
From here, the wiring can go
to the phone jacks in one of two
ways:
1. In “loop series” wiring, one
continuous cable feeds all
the telephone jacks in the
house. If you see only one
cable leaving the service
entry, you have loop-series
cabling.
2. “Parallel distribution” uses
a different cable for each
telephone jack. If you see
several cables leaving the
service entry—one for
each telephone jack in the
house—your phone system
is wired with parallel
distribution.
It’s also possible to have a
combination of the two—say, a
loop that serves the main house,
with separate wires for an
addition.
What Condition Is Your System In?
Telephone wiring is different
from one house to another, but
every wiring system should meet
your phone company’s specifica-
tions. If it doesn’t, it will be too
sensitive to RFI. That’s why inter-
ference can be worse at one
house than at another, even when
both houses are exposed to the
same source of interference.
So start by examining your
telephone wiring. Even exper-
ienced telephone personnel
sometimes overlook these prob-
lems that contribute to RFI: poor
system ground, system deterior-
ation, and installation errors.
Important Safety Note:
If you’re not familiar with tele-
phone wiring, don’t try to fix the
wiring yourself. Dangerous cur-
rents can flow through the wires,
especially when a ring signal
comes in.
1. System Ground
To keep your lines as free as
possible from interference, make
sure you’re using the highest-
quality system ground. Proper
installations establish system
ground either at the protector box
or at the service entry. You should
see a heavy-gauge ground wire
that goes either to the power-
company ground or to a cold-
water pipe that is known to be
conductive. Be suspicious of
connections to any other ground.
Your telephone company will
probably correct an improper
ground at its own expense.
Good grounding goes a long
way toward shielding your
telephone wiring from inter-
ference. Sometimes upgrading
the ground can eliminate the
interference altogether; other
times, upgrading the ground will
reduce the interference notice-
ably, so that you need fewer filters
to eliminate it.
2. System Deterioration
Your telephone system might
have many problems and still
keep working. When system
deterioration is slow, you might
not notice anything to be con-
cerned about until a nearby radio-
frequency field is present. Poor
connections in telephone wiring
make your phone system much
more susceptible to RFI.
If you’ve already determined
that the system ground is good,
look for signs of deterioration.
Check for corroded wires, poor
connections, moisture, decaying
insulation on wires and cables,
and stretched or damaged cables.
Examine as much of your system
as you can—from the protector
box to the service entry to every
visible cable run to every phone
jack. Fix whatever problems you
notice.
3. Installation Errors
When your house was wired
for telephones, did the installer
take shortcuts? You might not
notice—remember that many
problems won’t show up until a
nearby interference source
appears. Here are some install-
ation errors to look for:
• In loop-series installations,
a common oversight is inter-
ruption of continuous
ground. Telephone cable
leaving the service entry
includes the “pair” and the
ground wire. Correct wiring
brings the cable to the first
phone jack in the system,
connects the pair to the jack,
and then continues the pair
and ground to the following
jacks. Sometimes an install-
er will continue the pair but
not the ground. As you ex-
amine each telephone jack,
make sure the ground con-
ductor continues to the next
jack. Wiring will stop at the
last jack in the system.
• Beware of speaker wire,
thermostat wire, or any other
kind of wire that isn’t actual
telephone wire. Unqualified
installers often add new
telephone jacks without re-
gard for the proper type of
wire. Even if the wire is tele-
phone wire, the installer
might not have chosen a
matched pair of wires in the
cable.
• Staples driven through tele-
phone cables can create
real havoc. They can cause
wires to cross, resulting in
an imbalance that upsets the
interference rejection that’s
supposed to be designed
into the system. If an install-
er’s badly aimed staple had
severed one wire in the pair,
the telephone fed by that
cable wouldn’t work, and the
installer would have fixed
the error. But if it severed
only the ground wire, or if it
caused crossed wires, it
could easily go unnoticed.
Look carefully for staple
damage like that.
Unused Conductors and Cables
To upgrade your system’s RF
shielding, ground all unused wires
inside active cables. Unused
wires are commonly left hanging
at the service entry. Simply con-
nect them to system ground.
If your house is wired for
parallel distribution and there are
extra phone jacks you’re not
using, disconnect the cables
feeding them at the service entry.
The cables you disconnect don’t
need to be grounded, since
they’re no longer part of the
system.
Installing Filters
When you’ve checked out
your wiring system and you’re still
having RFI problems, use our fil-
ters to stop the interference.
Install an RF Line Filter at the
line-telephone’s cord input. It’s
important to locate the filter near
the telephone, because long line
cords can intercept radio fre-
quencies in the 14-to-30-MHz
range. Check the results by listen-
ing for interference.
