On Mon, 2 Jan 2006 17:36:24 -0500, Martin, AA6E wrote:
>When I have troubles, it's always with the external
>cabling. (Without chokes, external wiring can be an efficient
>radiator - comparable in length to a wavelength.)
Oh -- it's the fault of the cable that the computer puts the RF
current on the cable? :)
Let's be clear. ALL of that trash is coming from the electronics
-- either the computer, the monitor, the network hubs/switches,
etc. These electronic devices may radiate the trash directly if
they are unshielded, and they can shove it out on the cables if
they don't have sufficient common mode filtering (or if they have
pin 1 problems on those cables). And the trash radiated directly
by the wiring/circuit board of an unshielded box can vary widely
depending on how well (or how poorly) the designer minimized the
size of the current loops carrying RF currents, how well (or how
poorly) that trash was filtered and suppressed internally. Henry
Ott, Ralph Morrison, Howard Johnson, and Clayton Paul all cover
these issues quite nicely in their books and EMC lectures.
Those mechanisms are additive. When we stick ferrites on the
cables, we are minimizing (or attempting to minimize) the common
mode RF current that the electronics is trying to put on those
cables. The trash we hear will be directly related to the
magnitude of that current. But even if we manage to reduce that
current by 30 dB with really effective ferrite chokes, we will
still hear whatever the various electronic devices are radiating.
Jim Brown K9YC
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