Re: Magnetic sheilding

hrhofmann_at_att.net
Date: 12/18/04 

Date: 18 Dec 2004 14:47:14 -0800

As I earned a living for many many years doing EMC, I think I am fairly
well-quailfied to comment. This will be a long response because it is
a complicated subject.

There are magnetic chields available for TVs and monitors, but they
cost almost as much as getting a new LCD type monitor/tv set.

First, you have to find out if the magnetic interference is coming from
the actual radiatior or from unbalances in the house wiring. Is the
heater one that uses a wound element that glows, like a giant toaster,
or is it some other type? Does it run on one 120 volt phase of a
3-phase system, or on 240 volts or on 120 volts? Anything that runs
on one phase of a 3-phase system causes unbalances in the circuit that
can cause the problems you are describing. Frequently, the problem
occurs if the power lines are run as Romex, rather than in steel
conduit which does provide some first-order shielding.

But, if there is a power transformer feeding the building that you are
in, and you are within 20 or so feet of the transformer, there might be
enough magnetic leakage from the power transformer to cause your
problem. These transformers are usually located in the basement of a
building due to their size and weight. It actually takes a very small
field to cause waviness in a crt display. The earths magnetic field
will even influence a CRT display. If you take a monitor with a fixed
display. and rotate it 90 degrees, you will see a noticeable
movement/displacement in the picture (typically about 1/8 inch). That
is due to the static (DC) earth's field.

If you can get your upstairs neighbor to cooperate, you could use
telephones or cellphones and arrange for them to turn on their various
heaters to see which one(s) cause the problem. Then, if the heater is
not quite close to your monitor, it is mostl likely wiring currents
unbalances that are your problem, and then your landlord might also be
concerned.

Let me know what you are able to find out, this process typically takes
several hours of on-site investigation to resolve, so via internet
should/will be even slower.

H. R. (Bob) Hofmann

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