> Good point, I had to use one here when I had a 1KW on
> 870KHZ in the
> next room from me, thank goodness its gone. I had to
> mount the filter
> on the copper strap that runs across the desk to ground,
> just putting it
> in line was not good enough,
Perhaps it is worth beating this horse a little.
I know there is a popular myth, partially reinforced by
improper instructions and articles, that filters need to be
"grounded".
Filters, if the cabling is good and the shield have
integrity, never have to be grounded.
Think about how the cable works and what the filter does in
a coaxial system. The filter adds a high series impedance
with the center conductor, and in the filter a shunt from
the center to the shield. We all know (or should know) the
only signal the receiver responds to is the differential
signal between the center and the shield. We are in very
deep trouble if the receiver responds to common mode, which
would be the signal outside the shield. So once the filter
does its job it does it exactly the same if the case is
grounded or not.
As a matter of fact all my working life I've tested filters
and traps without ever worrying about grounding the case. If
the case is grounded or not, they all work the same. They
have the same attenuation. Always.
The only exception to this is if there is a resistive or
open shield someplace in the system between the measurement
point or detector and the filter. Even an open on the source
side of the filter doesn't change the notch, although it can
cause ripple in the source because of the impedance
mismatch.
If you have a filter that is "improved" by grounding
someplace in that system there must be a poor connection
between the filter case and the receiver. That's the way it
has to work.
73 Tom
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