Electronics > Beginners
Difference between coax cable and shielded cable
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tooki:

--- Quote from: soldar on October 16, 2019, 02:48:11 pm ---
--- Quote from: T3sl4co1l on October 16, 2019, 02:13:26 pm ---I've seen audio cables that are just, whatever.  A ground that's not woven, but all the strands wrapped around the same way, so they only provide a little electric shielding and no magnetic shielding (which is adequate for most audio purposes, where ambient electric fields are more offensive than magnetic; just don't run the cables by any transformers, motors or switches..).
--- End quote ---
Yup. Microphone cable is shielded but is not "coax" in the sense of built to offer a defined impedance at higher frequencies.

--- End quote ---
Microphone cable is actually not coaxial, it’s a twisted pair with shield. The question was about single-conductor wires with shields. Instrument cables (like guitar cables) fall into that category. So does the stuff used in RCA audio cables and the like.
T3sl4co1l:

--- Quote from: tooki on October 16, 2019, 02:53:59 pm ---Why would a nonwoven shield provide no EM shielding to speak of? Whether woven or coiled for flexibility, if the shield strands are all touching, wouldn’t they still form a largely continuous shield? It’s not litz wire where each strand is insulated, after all.
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I didn't say that, I said E, but poor M. :)  Strands touching is unlikely; it will happen, but not often enough to be significant.

Even (lazy) braiding doesn't do all that great -- RG-58 is a sieve. :P


--- Quote ---But doesn’t your reply pretty much confirm my answer to your question about who would make shielded single conductor without calling it coax? That they aren’t advertising it for good RF properties is hardly a mystery. Of course they’re fine for audio (or even some SD video), that’s what they’re made for!

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It's still coaxial in the sense that the outer conductor is cylindrical and on the same axis as the center conductor.  Just depends how generally the term "coax" is meant.  I was taking the more general, geometric meaning.

Tim
tooki:
We must not be thinking about the same kind of coiled shield. The ones I’m familiar with have enough strands that frequent contact is inevitable. (Like, obviously not every strand will be in direct contact with every other strand, but enough that cumulatively, a reasonably dense mesh covers every part.)

I understand that you were talking about geometry. You expressed skepticism that anyone would make such a cable with a coaxial construction and not market it as coax. I provided examples of exactly this.
tkamiya:
Actually, very cheap microphone cables are coaxial.  $10 microphone from RadioShack used to have this shielded cable with one center conductor with very little braid.  I think all pro stuff uses balanced transmission so they are twisted pair in shield.
T3sl4co1l:

--- Quote from: tooki on October 16, 2019, 03:54:55 pm ---We must not be thinking about the same kind of coiled shield. The ones I’m familiar with have enough strands that frequent contact is inevitable. (Like, obviously not every strand will be in direct contact with every other strand, but enough that cumulatively, a reasonably dense mesh covers every part.)
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We're talking quite low impedances to provide adequate magnetic shielding -- lateral and axial resistivity near that of solid copper!

Related subject -- stranded wire does in fact save some AC resistance over solid.  It's not as good as proper litz, but it's surprisingly not that much worse (given you may not have much choice over the strand size, of course).

Which is equivalent to saying: stranded wire is more transparent to magnetic fields than solid wire is.

The same will be true here, and is true of braid as well, though obviously braid does much better at acting solid than the swirl stuff, but again not as good of a shield as solid (semirigid).

Another way to put it: the contact resistance increases the cross-strand impedance, significantly increasing the skin depth in that direction, therefore allowing magnetic fields inside.



--- Quote ---I understand that you were talking about geometry. You expressed skepticism that anyone would make such a cable with a coaxial construction and not market it as coax. I provided examples of exactly this.

--- End quote ---

Yeah, foil cable came to mind first, didn't think of the spiral stuff at the time.

Tim
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