On Wed, 8 Sep 2004 19:29:12 +0100, George Shaw wrote:
>Has anyone any ideas or suggestions on preventing interference from the
>earth connection in the shack? Is there a way of isolating the earth
>(probably a contradiction in terms) to prevent RFI getting back up the
>earth, or isolating the power earth? Or am I talking MDS?
MDS?
Is there some particular problem you are having? Tom Rauch has observed, and I
generally agree, that most "RF in the shack" problems are really antenna
problems. I
would extend that statement to say that most "RF in the shack" problems are a
combination of antenna problems and problems with the interface to the
equipment
receiving the interference.
To understand RF interference, we need to realize that there are many antennas
in
every ham shack. Everyone thinks of antennas as those wires we put up to
radiate and
receive RF, but every piece of wire in our ham shack, and every wire in the
house, is
ALSO an antenna! You may call it a "mic cable," but mother nature calls it an
antenna.
So when we key our transmitter, the normal operation of the intentional antenna
will
cause current to flow in all of those other antennas. The key to not having RFI
is keeping
that current on outside of the various electronics in our homes and shacks.
The most common interface problem is one that in the pro audio world we call
"the pin 1
problem," so called because pin 1 is the shield connection on the XL connector
commonly used for pro audio circuits. The CORRECT connection for the shield of
ALL
external wiring is to the equipment enclosure (i.e., the chassis). When this
connection is
made, any RF current flowing the shield goes to the enclosure and from there
harmlessly to "ground." The "problem" connection is one that ties the shield to
the
ciircuit board (often called "audio ground" or "control ground" or "signal
ground"). This
connection eventually gets to the chassis, but it wanders around the circuit
board for a
while, and that connection has inductance. The RF current flowing on the shield
will
create voltage drops as it flows along the circuit traces, and those voltage
drops will be
impressed at various points in the circuitry inside the box. This is "the pin
1 problem,"
and it is THE major cause of RF interference to ham gear, audio gear, computer
gear,
and other electronics. Sure, RF gets in other ways, but this is the most common
and is
usually the strongest path.
Most ham gear I've looked at has pin 1 problems. One of the most common is an
RCA
connector mounted to the PC board, with no direct connection between the shell
and
the enclosure. You'll see similar problems at the mic connector, at the
headphone jack,
and at various remote connectors.
Jim Brown K9YC
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