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Re: [RFI] Earth isolation

To: "RFI List" <rfi@contesting.com>
Subject: Re: [RFI] Earth isolation
From: "Jim Brown" <jim@audiosystemsgroup.com>
Reply-to: Jim Brown <jim@audiosystemsgroup.com>
Date: Wed, 08 Sep 2004 14:24:58 -0500
List-post: <mailto:rfi@contesting.com>
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?


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 
receiving the interference. 

To understand RF interference, we need to realize that there are many antennas 
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 
So when we key our transmitter, the normal operation of the intentional antenna 
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 
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 
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 
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 
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 
connector mounted to the PC board, with no direct connection between the shell 
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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