On Tue, 5 Dec 2006 09:29:58 EST, Telegrapher9@aol.com wrote:
>You must also take care
>to properly bond the filter case to earth ground
HUGE MISCONCEPTION! Current flows in loops, and the source of the noise
current
is the TV, not "earth ground." "EARTH GROUND" will not solve a noise problem --
the purpose of the bond of the filter case is to provide that current loop back
to
the chassis of the source of the interference, BY THE SHORTEST POSSIBLE PATH.
That
path carries noise current, and that current will create magnetic and
electromagnetic fields. Your antenna will hear those fields, just like any
other
signal. Now, that chassis, MUST, by code, be connected via the green wire of
the
power cord to a proper "ground" within the building. But a connection to EARTH
has
NOTHING to do with eliminating the noise. It has ONLY to do with SAFETY.
That "shortest possible path" is usually most easily obtained by breaking the
power cord and adding the filter in series, VERY close to the set. And we all
agree that we don't want to open the box (the neighbor's TV). ") Also, the
green
wire from the power outlet MUST be carried through from the wall plug to the
set.
>such that the connection cannot be removed from outside the box
This is a limitation placed upon manufacturers in the design of product, so
that
users cannot defeat safety or filtering.
>Let's say the AC line is not the point of egress. Connect the video input
>back. Let's say the RFI becomes strong now. You will need a few hundred ohms
>of
>common-mode attentuation at 1.8 MHz. A ferrite core and several turns of the
>coax will do it, given the correct core.
My experience is that FAR more than a few hundred ohms will be needed. Rather,
I
would aim for a few kOhms. See the measured data for ferrite chokes on my
website.
http://audiosystemsgroup.com/publish
73,
Jim Brown K9YC
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