3
Cordless Telephones
The most common causes of
RFI on cordless phones are:
1. RF on the incoming phone
line;
2. RF on the power-supply line
cord;
3. Interference with the
phone’s radio circuitry.
If your cordless phone has an
intercom feature, it’s easy to find
out whether the RFI is entering
through the phone line. Discon-
nect the telephone line cord from
the phone. Use the intercom to
activate the phone so you can
listen. If you don’t hear the inter-
ference anymore, you’ve con-
firmed that there is RFI on the
phone line, and you can take care
of it the same way you would on
any ordinary phone.
If you still get interference
with the line cord disconnected,
the problem is in the phone itself.
You can try these tests to isolate
it:
1. You can sometimes find out
whether the interference is
on the power cord by mov-
ing the cord. If the inter-
ference changes as the
cord changes positions, the
power cord is your culprit.
Try moving the base unit to
another location; some-
times moving the base to
another AC outlet will solve
the problem. If not, try using
a ferrite choke on the pow-
er cord.
2. Interference in the phone’s
radio circuitry is probably
caused by sloppy design or
manufacturing. Try moving
the base unit to another
location, where a reduction
in the RF energy field could
make a difference. If that
doesn’t work, you might
have to replace the phone
with another that’s less
susceptible to RF. Because
manufacturing is not ab-
solutely precise, you might
find that one phone is less
RF-sensitive than another
of the exact same make
and model. As a general
rule, cordless phones that
operate in the 900-MHz
range seem to be less
prone to interference than
phones that operate at
40 MHz.
When One Filter Isn’t Enough
Sometimes it’s not enough just
to install a filter where the line
cord comes into the phone.
If the interference is reduced
but not completely eliminated, you
can try installing another filter
right behind the first one. Doubling
filters won’t cause any problems
on your phone lines.
If you’ve tried all of these
suggestions and you’re still
getting interference even with
one RF filter installed, you might
need filters in other locations in
your phone system. Call Black
Box Technical Support at (724)
746-5500; we might be able to get
them for you as a special order.
If the Problem Still Won’t Go
Away…
If filters have no effect, if the
interference is equally loud on
every phone in the house, and if
the interference sounds like a raw
AC hum, then the condition is not
RFI but “foreign EMF,” which
can’t be corrected with filters. You
might need your telephone com-
pany’s help to find and correct
this problem.
Choosing the Right Filter
Our RF Filters come in models
for one-line or two-line phones.
They’re available in four different
frequency ranges:
How do you pick the one that’s
right for you? Listen to the inter-
ference. If you’re picking up a
commercial radio station, for ex-
ample, you might be able to tell
which station you’re picking up.
Or do you hear truckers talking on
the CB radio? If you can tell
where the interference is coming
from, you can tell what frequency
you need to filter.
LF models (MC140A and
MC144A) are the best general-
purpose filters. They filter fre-
quencies between 0.5 and 3 MHz,
which includes the entire
AM-radio broadcast band and
160-meter amateur radio. AM
radio broadcasts are the most
common noise sources in cities or
near known AM radio stations.
HF models (MC141A and
MC145A) filter frequencies be-
tween 3 and 30 MHz, which
handles most amateur and side-
band radio frequencies. Some-
times an especially long phone
line in your system can act as a
resonant antenna and pick up
frequencies in the 14-to-30-MHz
range; these filters will take care
of that interference.
VHF models (MC142A and
MC146A) are recommended for
interference from CB radios.
They filter frequencies from 26 to
60 MHz.
UHF models (MC143A and
MC147A) filter the 100-MHz band;
they’re recommended for FM ra-
dio interference (including inter-
ference from devices like wireless
LANs or cordless phones that use
the FM band).
Number of Lines:
MC140A–MC143A: 1;
MC144A–MC147A: 2
Frequency Range:
MC140A,
MC144A: 0.5 to 3 MHz;
MC141A, MC145A: 3 to 30 MHz;
MC142A, MC146A: 26 to 60 MHz;
MC143A, MC147A: 100 MHz
Attenuation (Nominal):
50 dB
Connectors:
MC140A–MC143A:
(1) RJ-11 male, (1) RJ-11
female;
MC144A–MC147A: (1) RJ45 male,
(1) RJ-45 female
Power:
None required
Size:
0.8"H x 2.3"W x 0.8"D
(2 x 5.8 x 2 cm)
Weight:
0.9 oz. (26 g)
Specifications
Ordering Information
ITEM
CODE
RF Line Filters
RJ-11 (1 Line)
LF (0.5–3 MHz).....................................................MC140A
HF (3–30 MHz).....................................................MC141A
VHF (26–60 MHz) ................................................MC142A
UHF (100 MHz) ....................................................MC143A
RJ-45 (2 Lines)
LF (0.5–3 MHz).....................................................MC144A
HF (3–30 MHz).....................................................MC145A
VHF (26–60 MHz) ................................................MC146A
UHF (100 MHz) ....................................................MC147A
