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Old 18th Aug 2018, 3:58 am   #2
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: Long Wave interference problem solved.

It sounds as though your house wiring is pretty much alive with the interference. So when you operate a set there, the set is surrounded with it maybe as an electric field, maybe as a magnetic field. You're well within a wavelength of the radiating elements so a mature electromagnetic wave will not have formed so the field could be quite a lot stronger in one form than in the other. In the near field, the field strength of an unbalanced wave pair falls more rapidly with distance than you'd expect from the inverse square law applicable to balanced (377 volts per metre E-field for every ampere per metre of H field) mature electromagnetic waves. So the interference in the shed was mostly exported down the mains connection from the house. This would be why your new mains conditioner works better in the outbuilding than in the house. It shields your radio from mains-bourne interference, but not at all from radiated stuff.

Interference down the L and N wires of a mains connection either could be differential mode where L and N bounce around together, or it could be common mode where both bounce around together with respect to earth. And it could be a mixture of the two modes.

Your experiment with an X-class capacitor across L-N showed that the differential mode wasn't the worst culprit. That leaves the common mode as the offender.

Class-Y2 capacitors from L-E and N-E work on the common mode but can do a limited amount on their own. They also tend to simply couple the muck onto the earth wire as well.

Where Y capacitors come into their own is in conjunction with a common mode choke - the large wound toroid seen in the window of the new filtered socket strip. This choke has windings carrying both the L and N connections to the load. They are phased so that the load's normal operating currents creates opposing fields which cancel. This means that the power consumption of the stuff being powered doesn't act to saturate the core while the choke provides helpful impedance to interference driving L and N in parallel. Helpful enough to make the wanted improvement in this case.

I've sometimes seen large industrial filter units at radio rallies and you might find something adequately rated to handle the mains input to the outbuilding, or you could add one with mains wirie L and N wound together on a suitably large ferrite core. This would filter the supply to the whole outbuilding, not just the sockets on one strip. Or you could just get enough filtered strips to feed everything via one.

One disadvantage to having a lot of Y-mode capacitors is that their reactive current is sensed by RCDs. Have enough total microfarads and you start to get nuisance tripping. Of course, when an RCD trips your first thought is that there is a fault

The lab at work has just had to be re-wired with twice as many, smaller rings, each with a separate RCD, not because of the total power consumption, but because of the amount of L-E leakage in filter Y capacitors of a large number of pieces of low power gear.

David
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