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I'm having trouble understand why coaxial cables tend to have higher bandwidth compared to twisted pairs. I couldn't find any source explaining why, they just mention the fact.

For instance:

"In comparisons between coax vs twisted pair, coaxial cables are able to transmit over 80 times the amount of data as a twisted pair cable."

After doing some more research, it seems like what's receiving more consensus is that at the same bandwidth we'll be able to have a longer coaxial cable. Can we at least justify this fact?

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Why are coaxial cables typically faster/longer than twisted pairs?

The truth lies in the motivation for making twisted-pair (TP) cables: -

  • TP has closely magnetically coupled wires that naturally occupy a small cross sectional area.
  • If TP didn't have a small cross-sectional area then it has larger leakage inductance.
  • TP aims for low leakage inductance so that magnetic induction (from nearby sources of interference) is near-equal on both wires.
  • This means that noise sources induce a CM voltage and not a differential noise voltage
  • CM noise is quite easily removed with a good differential amplifier

So, if someone decided to make the gap between the pairs bigger then that misses the whole point of TP and, they become: -

  • Larger in cross section (not surprisingly)
  • Higher leakage inductance thus hence asymmetrical induction increases
  • Thus larger (and non-cancellable) differential noises are induced
  • Thus poorer performance

So, there's no motivation at all for making TP any bigger than it absolutely needs to be.

This isn't true of coaxial cable...

Coaxial cable has great ability to counter magnetic induction from noise sources irrespective of its dimensions. AND, importantly, the bigger/fatter you make coax, the better it is regarding bandwidth and losses per metre: -

enter image description here

Image from here.

So, just like with a "small" dimensioned coaxial cable, twisted-pair is restricted in bandwidth and has a worsening attenuation figure per metre. And, there's no point trying to make TP any fatter because that misses the point.

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    \$\begingroup\$ hm, not convinced about the bandwidth thing. Coax capable of transporting GHz bandwidths in the mmWave regime is tiny, whereas Coax for 30 MHz transmitters with a bandwidth of up to at the very most even theoretically 30 MHz is relatively large – see "cutoff frequency" for any given mode, e.g. the fundamental TE<sub>01</sub> \$\endgroup\$ Jan 7 at 13:36
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    \$\begingroup\$ You can get good BW with small dimension for coax but the losses will be quite bad per metre. You have to factor in losses and bandwidth when scaling the size of coax. 12.5 mm coax has good BW and good losses but half that size and something has to give out @MarcusMüller \$\endgroup\$
    – Andy aka
    Jan 7 at 14:02
  • \$\begingroup\$ true, good comparison. \$\endgroup\$ Jan 7 at 14:04
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    \$\begingroup\$ For losses, it's basically a matter of resistive losses in the center conductor, all other things such as materials being the same. Keep in mind that at RF frequencies, the signal/current flows along the surface of the conductor. To reduce the resistive loses, you need to make the center conductor bigger. And in order to keep the same nominal impedance of the coax, typically 50 ohms, you have to make the dielectric thicker. Therefore the entire cable gets bigger. \$\endgroup\$
    – SteveSh
    Jan 7 at 15:14
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    \$\begingroup\$ @Essam that's a little trickier than you probably thought. Regards scaling up you can use more copper and that immediately indicates lower losses. But also, with a thicker dielectric between inner and shield (due to scaling up), the electric field strength will be reduced and this improves high frequency losses. \$\endgroup\$
    – Andy aka
    Jan 7 at 15:16

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