[RFI] Earth isolation

[Ian White, G3SEK]

Jim Brown wrote:
>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.

What you say is true, Jim... but having been picked up by accidental 
antennas, that RF current also needs somewhere to flow *to*. Whatever 
mother nature hasn't enlisted as an "antenna", she'll try to enlist as a 
"ground" or "counterpoise" instead.

There are some situations, particularly involving end-fed wires and 
inverted-L antennas, where large ground return currents are unavoidable. 
These return currents will divide among all possible ground-connected 
conductors, according to the series impedance that they see along each 
pathway. And if even a small percentage gets into the wrong places - 
especially the mains wiring - it can cause significant RFI.

I had that problem with the low-band antennas here. There was no 
possibility to change either the antennas or the feedline runs; both 
were in the only places they could be. Enough RF current was flowing 
back through the shack, and out again into the mains, to cause RFI to 
the neighbors. The cure was to place an RF block in the shack mains 
feed, using a three-wire filter which includes a ground-line choke. The 
unwanted RF current coming into the shack is greatly reduced, because it 
no longer has a low-impedance exit path into the mains.

These results have also been verified by measurements with a clip-on RF 
current meter.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek