[RFI] Quiet Shack PC Systems?

[Ian White, G3SEK]

Tom Rauch wrote:
>> But from the lightning and RFI viewpoints, it is highly *un*desirable to
>> cross-connect the mains and RF grounds at the shack. If you do, you are
>> creating an alternative pathway for lightning and RF currents - a
>> pathway that now goes right *through* the building wiring, and probably
>> through your electronics.
>
>Unless I'm not following something, I don't know why that would be true. I
>don't isolate or choke the ground leads outside equipment and would never do
>that.
>

Tom's station and mine are very, very different... and possibly each of 
us is over-generalizing based on our own individual situations.


>I bond the safety ground of all power mains to the shack grounds inside the
>shack where all wires enter.
>Of course I bond the main grounds to all building entrance grounds at the
>building entrance point also.
>
Tom's station is exactly how things *ought* to be. He has organized a 
common entry point where all his feedline grounds and the mains ground 
are connected together. Since the two ground conductors enter the 
building at the same point and at the same RF potential, there will be 
no significant potential difference when they arrive in the shack. Tom's 
excellent ground management also means he has very few problems due to 
"RF in the shack".  It is completely understandable why Tom doesn't need 
a mains filter, and why it doesn't matter that he connects his mains and 
RF grounds together at the shack.

My station is a long way from how things ought to be... it's how things 
*have* to be. The mains comes into the front of the house but all the 
antennas are out back, so it is impracticable to bring the feedlines in 
through the same entry point as the mains. To make matters worse, the 
inverted-L antenna induces RF currents onto its own coax feedline, and 
the shack is on the second floor so there's no true "ground potential" 
up there.

Please don't anybody tell me what is wrong with this setup. I *know*... 
but if here is where you have to live, it represents the very best that 
can practically be done.

In this situation, there are significant RF potential differences 
between the mains ground and the RF ground, especially on the LF bands. 
If I connect the two grounds directly together in the shack,  several 
hundred mA of RF current flows into the mains ground! This is coupled 
across to the live and neutral conductors, and then the power company 
distributes it to the neighbors up and down the road... who for some 
reason don't seem overly impressed.

With the three-choke mains filter, all those problems disappear. The RF 
currents in the mains wiring drop to only a few tens of mA. The induced 
current on the feedlines also drops to the same level, because its 
former pathway into and through the shack now dead-ends in a high 
impedance.

Before installing the choke, I had some "RF in the shack" problems. With 
the choke, those too have dropped to the same low level as Tom's.

Unfortunately my situation is all too typical for many urban and 
suburban hams in western Europe. Since I published the idea of the 
three-choke mains filter, a number of people have written to say that it 
worked for them too.


In closing, I should point out that there's nothing technically 
inconsistent between Tom's findings and my own. Both our stations obey 
the same laws of physics, and it's only our situations that are 
different. It is perfectly clear why each of us finds the different 
things we do.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)

http://www.ifwtech.co.uk/g3sek