4 ms·

Why is it slower in coax?

ColinWright 12y ago
Broadly speaking, as it goes down a conductor it has to build a magnetic field, and as it does so the increasing magnetic field creates an electric field that opposes the motion of the signal. Roughly.
marcosdumay 12y ago
30cm/ns is the speed of light in vacuum. Coax has a hight refraction index, thus electricity moves slower in it.
zackmorris 12y ago
One more way to think about it is the propagation delay of a transmission line, or actually the opposite of that, which is the velocity factor, proportional to 1/sqrt(LC): https://en.wikipedia.org/wiki/Wave_propagation_speed https://en.wikipedia.org/wiki/Wave_propagation_speed Coaxial cables are great for blocking interference because one conductor is inside the other, so external electric fields can't penetrate. But they have a high parasitic capacitance because the outside conductor has a large surface area. So if capacitance is causing too much propagation delay, you can use twisted pair. It also works to avoid interference, but because the wires constantly change position in relation to electric fields: https://en.wikipedia.org/wiki/Twisted_pair https://en.wikipedia.org/wiki/Twisted_pair The problem with coax for cable TV though is that for high frequencies, losses can be high because the capacitor begins to approximate a conductor (and TV uses really high frequencies!). So cable length becomes a limitation. Then you use fiber optic cables (I find total internal reflection somewhat miraculous, like an optical analog to superconductors). Also in computer chips, parasitic capacitance is a problem because capacitance grows with area and shrinks with distance. But the conductors are so close to one another that even a small area results in a large capacitance. That's why the latency to memory can be orders of magnitude higher than say, cache. Optical interconnects would help here as well. It's been 15 years since I studied this stuff though, so if I misremembered, somebody please correct me!
- jpmattia 12y ago
  > * But they have a high parasitic capacitance because the outside conductor has a large surface area. > So if capacitance is causing too much propagation delay, you can use twisted pair.* The capacitance is not really "parasitic", it is the circuit element representation of the electric field which is propagating. The dynamic electric field gives rise to a magnetic field, which is the inductive element in the expression 1/sqrt(LC) that you listed. Twisted pair is no different: There is a capacitive element and inductive element per unit length. As it turns out, the capacitance varies as the log of the distance between the wires, so you can twist the wires and not change the capacitance very much. There used to be lots of "twinax", which kept the two wires at a fixed distance, but the separation plastic was mostly a PITA. (The pics I can find on google don't show the old flat plastic twinax that folks used to use. You can still find that stuff hanging off old television antennas.)

Broadly speaking, as it goes down a conductor it has to build a magnetic field, and as it does so the increasing magnetic field creates an electric field that opposes the motion of the signal. Roughly.

marcosdumay 12y ago

30cm/ns is the speed of light in vacuum. Coax has a hight refraction index, thus electricity moves slower in it.

zackmorris 12y ago

One more way to think about it is the propagation delay of a transmission line, or actually the opposite of that, which is the velocity factor, proportional to 1/sqrt(LC): https://en.wikipedia.org/wiki/Wave_propagation_speed https://en.wikipedia.org/wiki/Wave_propagation_speed Coaxial cables are great for blocking interference because one conductor is inside the other, so external electric fields can't penetrate. But they have a high parasitic capacitance because the outside conductor has a large surface area. So if capacitance is causing too much propagation delay, you can use twisted pair. It also works to avoid interference, but because the wires constantly change position in relation to electric fields: https://en.wikipedia.org/wiki/Twisted_pair https://en.wikipedia.org/wiki/Twisted_pair The problem with coax for cable TV though is that for high frequencies, losses can be high because the capacitor begins to approximate a conductor (and TV uses really high frequencies!). So cable length becomes a limitation. Then you use fiber optic cables (I find total internal reflection somewhat miraculous, like an optical analog to superconductors). Also in computer chips, parasitic capacitance is a problem because capacitance grows with area and shrinks with distance. But the conductors are so close to one another that even a small area results in a large capacitance. That's why the latency to memory can be orders of magnitude higher than say, cache. Optical interconnects would help here as well. It's been 15 years since I studied this stuff though, so if I misremembered, somebody please correct me!

jpmattia 12y ago
> * But they have a high parasitic capacitance because the outside conductor has a large surface area. > So if capacitance is causing too much propagation delay, you can use twisted pair.* The capacitance is not really "parasitic", it is the circuit element representation of the electric field which is propagating. The dynamic electric field gives rise to a magnetic field, which is the inductive element in the expression 1/sqrt(LC) that you listed. Twisted pair is no different: There is a capacitive element and inductive element per unit length. As it turns out, the capacitance varies as the log of the distance between the wires, so you can twist the wires and not change the capacitance very much. There used to be lots of "twinax", which kept the two wires at a fixed distance, but the separation plastic was mostly a PITA. (The pics I can find on google don't show the old flat plastic twinax that folks used to use. You can still find that stuff hanging off old television antennas.)