[Beowulf] propagation velocity in cables

Vincent Diepeveen diep at xs4all.nl
Wed Sep 5 16:43:39 PDT 2012

hi Jim,

Thanks for a more technical explanation. Difficult to interpret for  
me though.
The 1/3 speed someone gave to me some years ago.

Litterally quote : "electrons move at 1/3 of the lightspeed through  

Maybe signal speed is not the same like electron speed?

Would be very interesting if he said that wrong, as he has a degree  
here, Phd cum laude in Natural Science and he
improved something or better disproved something that has to do with  
quantum mechanics or something, basically
he calculates everything about particles such as 'electrons' (my words),
so i didn't verify all this, yet assumed what he said.

The most important question: what is the effective speed of signals  
through ocean cables?

We can safely assume they use fastest speed for this.

When i drove past that financial industry bunker in Land's End it  
seemed pretty solid to me.

The interesting question is that big satellite disk on the roof as well.
Microwave is simply civil allowed frequency i was told. Around  
2800Mhz or so?
How fast is such communication?

Using a powerful laser would it somehow be possible to rival speeds  
of these cables?

On Sep 6, 2012, at 1:18 AM, Lux, Jim (337C) wrote:

> Propagation speed in copper is not 1/3 freespace.  More like 2/3 or  
> 80%, depending on the effective permittivity of the dielectric.  (c  
> = c0/sqrt(epsilonr))  Cat 5 is typically about 68% of free space.  
> Foam dielectric coaxial cable is typically 81%  LMR-400 coaxial  
> cable is 85%.  There are air filled coax which get up to 90-92%,  
> and open wire line (like old telephone cables on poles) is around 95%
> For single mode optical fiber, n=1.62 for the core (1.52 for  
> cladding), so propagation speed is c0/1.62 or about 62% free space.
> Note that copper is faster than optical.
> People are setting up freespace microwave links for just this reason.
> Jim Lux
> C = 300k kilometers an hour.
> 300k / 3.6 meters a second
> 300 / 3.6 meters a millisecond
> 0.3 / 3.6 meters a microsecond
> 12 meters a microsecond
> Now i don't know whether signals travel through fiber at the full  
> lightspeed, as that's pretty important.
> In Copper it's roughly 1/3 lightspeed or so, so that's out of the  
> question to use.
> So saving some distance knowing 12 meters is a microsecond, that's  
> worth it.

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