Julf
Facts are our friends
Thanks for getting involved. As to where the noise might be coming from, I'd guess it's mainly from normal stuff in the house as the nearest neighbour is a few fields away. However the mains quality in general may be poor due to milking machines and similar on neighbouring farms (however on our 11kV to 240v transformer there is just 1 other property - although he does use a fair number of machine tools).
All that stuff is better dealt with with a suppressor capacitor instead of grounding schemes.
As to straight copper, I guess you mean copper strip, and may well be correct - I remember we used to use similar at the radio station, and it's often used for lightening conductors where I guess high frequencies (fast pulses) are likely to be an issue. Do you know of any good sources for such material - I probably need around 25m?
A think enough normal copper cable (2.5mm) would be the best solution.