On Sun, 8 Feb 2009 15:16:23 -0500, WA3GIN wrote:
> But I had to connect the case of the Corcom filter to those
>useless ground rods ;-) or it didn't work worth a damn!
Dave,
A filter is essentially a frequency-sensitive voltage divider, so
the bottom of the voltage divider has to be connected to SOMETHING
to "work worth a damn." The bottom of the voltage divider is tied
to the filter enclosure, so until you make the proper input and
output connections to the shield enclosure, you haven't finished
hooking up the filter. The "normal" connection point for that
enclosure is the shielding enclosure of the equipment that the
filter is filtering. A connection to EARTH is not necessary.
You connected that point to ground rods, and that worked, because
it gave the voltage divider current somewhere to go. It would also
have worked if you had connected it instead to a long piece of
wire, especially if the wire was an open circuit quarter wave. And
it would have worked if you had tied the shield of the filter to
the common or the shielding enclosure of the telephone, again with
no connection to the earth!
The earth connection would probably NOT have helped if you had
made it to the telephone instead of the filter, because you would
probably have increased the RF current flowing into the phone
(unless you also bonded the filter common to the telphone common).
>Sometimes EEs can get a headache trying to figure out why things
>don't work the way the text book says it should -- must be the
>magic of radio ;-)
I'm sorry, but I don't subscribe to the "know-nothing" school of
radio or electronics. EE or no EE, you've got to draw the circuit
and study where the current flows. Once you do that, you
understand, and there's no magic, just plain ordinary science.
73,
Jim Brown K9YC
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