[RFI] Earth isolation

[Jim Brown]

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