>From: Randy Lake
>To: rfi
>Subject: [RFI] Telephone RFI problem
To: <rfi@contesting.com>
>Date: Wednesday, February 11, 1998 10:15AM
>Hi all......
>During the NH qso party this past weekend
>my next door neighbor,who's house is abt
>200' from the tower, called because he needed
>to make a call to his mother and father. In
>the past he has heard me just a little bit on the
>phone but today he cud not hear anyone on
>the phone but me....So I turned off the amp
>until he left the house...
Sounds like something has changed. Did he buy a new phone, answering
machine, modem? Etc.
>Has anyone found a nice way to deal with this
>type of problem? He is very willing to try things!
>Filters? new phone?
In most cases, filters will do it. These are widely sold and advertised.
Radio Shack now sells telephone filters (the ARRL Lab helped hook them up
with a manufacturer of good filters!)
I first recommend you learn about the problem. ARRL has a
telephone-interference package available for download from our Web site,
http://www.arrl.org/. You may also want to get our RFI Pamphlet, available
for an SASE from our Technical Department Secretary, 225 Main St, Newington,
CT 06111. This pamphlet was written to explain interference to a
non-technical neighbor.
To summarize, interference to non-radio devices is not the fault of the
transmitter. The FCC states in their material that telephones that pick up
radio signals are improperly functioning as receivers. You may want to
help your neighbor find a solution, but you are in compliance with FCC regs.
First, simplify the problem. Disconnect all of the telephone devices. If
you have an "RFI proof" phone (TCE Labs, Pro Distributors), plug it in and
try it. If it works, you have determined that there is no problem with the
lines or telephone company equipment.
Now, start adding devices back to the lines one at a time. If you do have
interference, try to eliminate it with filters. All of the filters should
be mounted as close to the telephone as possible. First, try a telephone
line filter. You can buy a commercial product, or make one with about 10
turns onto an FT-140-43 ferrite core. If the interference is primarily from
40 m and below, an FT-140-75 core may work a bit better. You may also have
to filter the handset cord. K-Com sells handset filters. If the telephone
or telephone device has a connection to the AC line, usually through one of
the "wall cube" type supplies, you may need to try a common-mode choke on
the power lead to the phone (10 turns on an FT-140-43 or -74).
In some cases, a telephone-line imbalance may create a differential-mode
signal on the line. In this case, a 0.01 uF ceramic capacitor across the
telephone line may cure interference that doesn't respond to conventional
telephone (common-mode) filtering.
The ARRL book, Radio Frequency Interference: How to Find It and Fix It, has
a chapter on telephone interference.
73 from ARRL HQ,
Ed Hare, W1RFI
ARRL Lab
>Any help wud be appreciated..
>Randy N1KWF>>
>
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>
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