> the secondary. This beast is heavy, and you need a fork
> lift to move it. The reason I specified two electrostatic
> screens is that in an American paper I read, on the
> internet, on feeding AC supplies to EMC/Faraday screened
> rooms, one or two isolation transformers were recommended,
> with two electrostatic screens per transformer, the
> reasoning being complex, so I just accepted it.....
The Internet is like Alice's resturant. We can find almost
anything we want on the Internet. A portion of it will be
accurate, much will not. Especially when it is something
designed to sell a certain product, or a clan or clique
thing like AudioPhool technical papers.
..I also had a proper ground system laid, by a firm of
lightning
> protection specialists...certified ground resistance now
> just 2.1ohms...the
What does that have to do with RF noise, which is what we
hear on receivers?
> incoming public supply is taken to the primary winding of
> the isolation transformer, the output, via a screened
> lead, to the shack. Both electrostatic sceens are taken
> back to the newly installed ground system, and there are
> now no connections between the public supply and my
> shack....the result, all the electrical noise has now
> gone, and apart from the radiated occasional radiated
> emissions, my radio background noise has dropped back to
> that of 40 years ago...
You might have revisited the shack equipment, antennas, and
connections before going through all those expensive
solutions.
The only noise that can "get into" our receivers is RF
noise. The only way in should be through the antenna
connections. Any other path should easily be down 100dB or
more.
I'd have looked at then installation of my feedlines and
their interface to antennas first.
>.In text books of that time, the electrostatic screen was
>described as a method of preventing the feed back of
>interference into the public supply, by the consumer,
>however, it was explained to me, by my local transformer
>manufacturer, that it was only looked upon as a safety
>measure, by the transformer manufacturers,
The manufacturers are correct. It primarily is a safety
device. So is bonding transformer laminations at ONE common
point and grounding them there, with all other areas of the
lamainations insulated.
>with the improved high termperature varnishes for the wires
>used in winding, they could save a small amount of money,
>by not including the copper electrostatic screens....which
>may be, why our received noise levels have been climbing in
>the past decades
Lack of shields in transformers has not increased noise. The
noise increase is because of the increase in mains voltages,
and because switching supplies and similar RF producing
devices have increased.
A primary shield is actually very little assurance noise
doesn't get through a system. It's a cheap additional
band-aid when initially winding a transformer, but the
problem is a lack of suitable line bypass capacitors and
small series chokes.
Anything you did with that big heavy expensive transformer
at radio frequencies, I can do better with something I can
hold in one hand....and at 10% of the cost.
You really ought to learn why your radio system is partially
using the power mains as an antenna. That's the real
question.
When radios were built on an unshielded wooden chassis, when
bypass caps were expensive and erratic radio frequency
impedances, before we had ferrites and other modern
components, and before good components at radio frequencies
were obtainable, things were done differently. Solutions
necessary back then are very often totally meaningless
today. The key is always understanding how things actually
work, and not relying on accidents to patch problems.
73 Tom
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