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Electric Wire vs Coax Cable

The only difference are the few other insulator of Coax... but they are both copper wire. What makes Coax good use for Audio/Video and not the regular electric wire?

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The thing about coaxial cable is .. it's coaxial.

That means there is a core conductor running inside a shield. The shield is usually connected to ground. The filler material inside the cable keeps the core a specific distance from the shield, and with a specific dielectric constant, k. That causes the cable to have a specific, low, well-defined impedance to high-frequency signals like video.

It also rejects interference from outside sources. External electric and magnetic fields can touch the shield, but they can't reach inside - the field inside a conductive hollow cylinder must be zero. This is the most important factor. If you run video over cable not designed for it it will collapse into noise after a short distance.

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  • \$\begingroup\$ Is the interference inside a hollow cylinder exactly 0? I was taught that it was in theory, but in my experience theory very rarely lines up perfectly with reality. \$\endgroup\$
    – anon
    Oct 20, 2016 at 14:34
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    \$\begingroup\$ @QPaysTaxes The copper isn't a perfect conductor, and the ends of the coax aren't shielded either, so in practice is not perfectly 0. It's pretty good though. I'm not sure exactly how you would calculate it, but skin depth seems like a good first approximation. \$\endgroup\$
    – mbrig
    Oct 20, 2016 at 15:55
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Small signals such as audio are easily interfered with - fact.

So, "transporting" a small signal needs to be done carefully. If you used ordinary wires you can form an unintentional loop antenna. This loop antenna can add interference into your otherwise clean signal.

However, if your individual wires are twisted together you get less interference: -

enter image description here

Here, the top picture portrays loosely laid individual wires forming an unintentional loop antenna and, in the presence of an interfering magnetic field, will add noise to the signal. The bottom picture shows the wires twisted together and this does two things: -

  1. Reduces the loop area significantly
  2. Successive loops cancel interference

That alone is a big deal but you can improve things by adding a shield like this: -

enter image description here

Not only does this cable type avoid magnetic interference (by twists) but it avoids electric field interference (by shielding).

You can regard coaxial cable as a variant of this - the outer shield (being 0V) protects the inner wire from electric field interference and, because the shield and the inner wire are co-axial there is no net external magnetic field susceptibility (theoretically).

Also, a cable that is not greatly susceptible to external interference is also good at not generating interference.

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A regular electric wire will not have two different layers like a coaxial cable does. You will have the center core that is used for the signal and the outer conducting layer (metallic shield) used for ground most of the time. The dielectric material between them will have a certain impedance that will match well with the high-frequency signal that video would require.

Coaxial cable is a type of cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield. Many coaxial cables also have an insulating outer sheath or jacket.

enter image description here

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  • \$\begingroup\$ Centre core is used for signal \$\endgroup\$
    – Andy aka
    Oct 20, 2016 at 12:12

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