[Beowulf] propagation velocity in cables

Lux, Jim (337C) james.p.lux at jpl.nasa.gov
Wed Sep 5 16:18:08 PDT 2012


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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