> > >
> > > In shielding cables from EMI what is the principle behind it? I'm
> > > specifically thinking of midi where it is 3 to 5 strand wire with a
> > > ground/shield.
> >
> > > The idea is simply to provide a constant voltage between the "world"
> > > and the signals similar to a guard rail/trace used to isolate a part
> > > of a circuit from the effects of another through the pcb?
> >
> > > What is the math behind it? Do Maxwell's equations prevent a
> > > differential change when the voltage is constant on the surface which
> > > makes the inner voltage constant as see from the outside? Sort of like
> > > a Faraday cage or charging the "bucket pale"?
> >
> > Sure. A Faraday cage is a particular sort of shield.
> >
> > The practical weaknesses of stranded shielding wire are twofold - the
> > first, largely theoretical problem is that gaps between the shielding
> > wires do allow the external electrostatic field to penetrate to the
> > central wire being shielded. It's essentially a theoretic problem
> > because the gaps between the strands are a lot smaller than the gap
> > between the strands and the central wire, and the external field is
> > attenduated by a couple of order of magnitude in most practical cases.
>
> I've seen electro-static ~25kHz 'stuff' from overhead fluorescents
> leaking in through 'standard' coax. That's why they make shielded
> cable that has both braid and aluminized mylar. You often see pieces
> of aluminum foil wrapped about the low level signal line in some labs.
>
> George H.
> >
Yup, RG58 talks to everything in sight. Double-braid coax types are a lot better (e.g. RG223 and RG6), and the sort of semi-semirigid types made by soaking the braid in tin are a whole lot better than either.
I think it's partly the gaps and partly the poor contact between the wires in the braid.
Cheers
Phil Hobbs