[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.
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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