[sdiy] Re: Twisted

[Bob Weigel]

Indeed.  As to being able to 'saturate air'..well..that waits to be 
proven :-) When I get done building my talking arc I'll get right on 
that.  But for those who haven't studied it, here are a few useful 
tidbits. Materials used for favorable permeability in transformers and 
so on can have thousands to 10's of thousands of times the permeability 
of air.  (For instance...it seems like Iron is given a relative 
permeability of 200 compared to oxygen at .2 or like that and mumetal 
(the greek letter u or "mew" like the cat would say :-) ) has a rating 
of 20,000.  The stuff I bought for shielding of a monitor in my studio 
was even better as I recall. 
   The thought behind making the torroidal transformers we see in more 
and more euipment is that it's a symmetrical patternt hat promotes the 
most efficient 'containment' of the magnetic fields involved, hence 
lowering interference and..obviously at the same time losses.  But these 
materials have response times.  "hysteresis" is a form of 'magnetic 
memory' which occurs when you expose the material to a field in one 
direction then reverse it.   So whenever you for instance put an AC 
current through such a transformer there will not be a totally linear 
interaction and as frequencies are increased there will eventually be a 
drop in the perceived permeability of the material until the iron or 
whatever material being used is of no value or worse.
     This is why air core inductors are preffered in crossover coils for 
instance for PA speakers.  While it requires many more winds of wire to 
attain the same inductance, air is perfectly continuous near as we can 
tell in it's permeability.  No hysteresis.  No response lag.   Materials 
have been developed which work well through the audio range and are used 
in fine mixing boards for pre-amp isolation.  The idea of having a 
transformer which isolates microphones from the rest of what's going on 
electrostatically is nice provided the permeable material has a sweet 
quality for audio range. 
       The material used in power transformers is cheaper/slower and 
this has some advantage since it tends to reduce the transfer of RF 
noise for instance.  Devices known as ferro-resonant transformers have 
also been developed which tend to strongly attenuate everything but 60hz 
or 50 hz depending on who is building it.

harrybissell wrote:

>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.
>>>
>>>
>>>
>>>
>>>
>>>      
>>>
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