[sdiy] Re: Twisted

[Bob Weigel]

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.
>
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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.
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