From: olson@mydual.uucp (Kirtland H. Olson)
Subject: Re: EMF interfering with modem communications--Any remedies?
Keywords: modem, communications, RFI, EMI
Reply-To: olson%mydual.uucp@alliant.com
Organization: The Harvard Group, 01451-0667
Date: Fri, 6 Nov 1992 14:27:50 GMT

In article <1992Nov5.213042.10314@midway.uchicago.edu> spcberto@psyche.spc.uchicago.edu (Robert Osterlund) writes:
>Last weekend, we moved into a new home close by a local radio station.  I

>The phone
>lines are quite noisy, and if you listen closely you can make out the voices
>and music of the radio station's broadcast.  Some times are worse than
>others.  The phone company says there is nothing it can do.
>

>Bob Osterlund
>berto@clio.spc.uchicago.edu
>SSPPCC--University of Chicago
>

You can solve this problem, but expect to learn more than you ever
wanted to know about RF, balanced circuits, unbalanced circuits,
nonlinear behavior of devices, and "grounds>." 

If none of what follows makes any sense to you, save it and find someone
who understands RF.  Give it to them and ask for help.

Here's a quick explanation of what's happening.  The radio frequency
signal is so large (in amplitude) that small nonlinearities in telephone
components act as rectifiers (detectors) and create the audio that you
hear.  If the noise and program info are generated on the phone system
outside your property, you will need help from the phone company.  But
you will have to work with them because they will not know what to do or
what to look for.

Use a real AT&T phone--an old rotary dial type is best--connected
directly at the entry point to your house to listen to the line with all
the internal wiring completely disconnected.  If the noise is still
there, you need to get the outside stuff done first.

If the noise is not present, connect the internal wiring and check
again.  If the noise does not reappear, an RF filter at the entry point
should cure the problem.

If the noise reappears, remove devices from the line (phones,
answering machines, modems) and test after each is removed.  If the
problem persists with all devices removed, look for joints or
connections in the wiring.  

Every joint is suspect because copper oxide/copper makes a diode good
enough to detect a strong signal.  

If one device or joint accounts for the problem, it needs to be repaired
or replaced.  If the problem fades slowly as devices are removed, you
need to reduce the RF voltage on the wiring.  

You will need RF filters to reduce the voltage at the devices.  You may
need to reduce the voltage between the two phone wires, between the
phone wires and the neutral of your power system, or both.  

To get practical information on filters, I suggest the Radio Amateur's
Handbook or other publications for amateurs or technicians.  You might
also check the amateur radio newsgroups and the HAM radio shareware
files.  You can buy parts at HAM fleamarkets.  

One useful component is the lossy core, a toroidal or cylindrical
ferrite that dissipates RF energy, turning it to a low level of heat. 
You may have seen such cores on monitor cables, covered with plastic, or
on the PC Jr keyboard cable.  You use these by wrapping the wire through
the center and around the outside several times.

Solving problems in circuits big enough to walk around in (like a house)
does not reliably respond to "buy X and plug it in."  Near a radio
station, the voltage between two ends of a wire can be large enough to
affect electronics so measuring at one end does not suffice.  But if you
work methodically and test carefully, your problem can be reduced.

Good luck.




-- 
Kirtland H. Olson                     olson%mydual.uucp@alliant.com
Newsgroups: comp.sys.ibm.pc.hardware