[sdiy] Re: Twisted

[harrybissell]

Just an added point...

how well the magneticlly coupled signals can get into the system has
a lot to do with the frequency involved.  The longer the wavelength the
longer a conductor you need to generate a significany induced voltage.
Air is rather poor as a magnetic core (well at least you can't saturate it but...
;^)

How well coupled the conductors are to the source is another issue. Its
easy (in some cases) to align the source and reciever (right angles ?)

I don't know what frequency magnetic coupling  stops working well and
electrostatic coupling takes over.

H^) harry

Bob Weigel wrote:

> JH. wrote:
>
> >>Everybody always obsesses about the shield too.  We should just
> >>remember what the shield is for.  It is a shield against stray
> >>electric fields, not magnetic fields.
> >>
> >>
> >
> >Are you sure? I thought a coax structure is good for electric *and* magnetic
> >shielding. But it's been a long time since I looked at that.
> >
> >
> >
>
> Magnetic fields are not interrupted by conventional shielding well at
> all.  Special metals exist which are super high permeability that are
> used to 'contain' magnetic fields.  A company like Fry Steel or Ed Fagan
> sells special alloys for this purpose..and they are pretty spendy.  70
> bucks for a sheet oh 3x2' or something as I recall last time I had to
> purchase.
>     Conceptually, an electric field is generated from the existance of
> an 'imbalance of charge' at some point in space.  The field is
> proportional to the amount of charge and inversely to the square of the
> distance.  A conductor held at a particular electric potential will
> result in a potential gradient between the charge aforementioned.  If
> suddently another charge is brought into place, the conductor's 'charge
> source' which is holding it at the given potential, will respond by
> distributing charge as needed to compensate for the change in electric
> field.  On the other side of say this conductive plane of infinite
> dimensions (or..say a sphere or coax) no change in the electric field
> will be seen from this event since the potential of the plate has never
> changed.  It only maintained it's potential by distributing available
> charge to compensate for the change in field on the other side.
>      Magnetic fields meanwhile can change and induce currents in the
> conductor in the example above.  And the conductor will
> again...redistrute charge to maintain it's electric potential so that no
> change in the electric field will be seen on the other side ideally.
> HOWEVER...there is no property of the shield which says 'magnetic fields
> will be maintained at such and such tesla '.  Rather, the magnetic field
> goes right through the conductor EXCEPT that the conductor also have
> magnetic permeability properties.  The better the material/dimensions
> the better the magnetic field will be 'absorbed' into that space and
> prevented from having influence over the space on the other side of the
> material.
>        This is why balanced lines are used.  The shielding takes out the
> electric field ok.  But magnetic fields are all about as well.  And if
> you put TWO wires carrying signal opposite directions and subtract them
> to get a double signal value...you also subtract AWAY 'common mode'
> signal.  Magnetic fields in the audio zone will produce essentially
> common mode signals with typical wire spacings in balanced lines.  -.  -Bob
>
> >>That's why we like to
> >>use balanced lines in studios.  Using a twisted pair that results in
> >>equal magnetic and electric coupling in both wires of the pair, and
> >>permits the receiving amp's common mode rejection (either by using a
> >>transformer or active balanced input) to cancel the coupled signals.
> >>Actually, the twist is far more important than the shield.
> >>
> >>
> >
> >Twisted pair inside a shield is certainly the best solution.
> >And balanced connection, of course.
> >
> >I think the problem with magnetic fields and unbalanced coax cables
> >is not that the coax won't shield the voltage between inner and outer
> >conductor (differential mode) against the influence of the magnetic field
> >(I think it does ...), But that the magnetic field will cause common mode
> >errors along the shield, from one end to the other, the dreaded
> >ground loops in non-balanced connections.
> >
> >Does this make sense?
> >
> >JH.
> >
> >
> >
> >
> >