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: -
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.