[RFI] GFI Problem

Ian White, G3SEK G3SEK at ifwtech.co.uk
Fri Feb 7 11:56:15 EST 2003

Earlier, I wrote:
>The three-wire filter cannot prevent RF current being induced into the 
>general mains wiring of the house, but that should be much smaller than 
>the directly-conducted component which the filter will stop.
A few afterthoughts...

1. Presumably in the USA you'd use a three-wire filter for a 115V AC 
feed (the modern type with a separate ground conductor) but you'd need a 
four-wire filter for the big feed to the amp?

2. I've said "filter" here, but an RF choke made by winding *all* the 
mains conductors on a large ferrite core would probably work just as 
well. (The ferrite core from a TV deflection yoke is large, free and 
satisfyingly ugly. 10 turns is about the right number for HF.)

3. In many systems, there is a 'ground loop' which starts at the RF 
ground at the antenna, continues along the feedline, crosses over in the 
shack to the ground conductor of the mains wiring, and then goes back to 
the separate mains ground. A mains filter or choke at the shack will 
break that loop, and will stop that component of the RF currents at all 
points around it.

73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'

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