With accurately controlled dimensions and materials, the cable has a uniform capacitance () and inductance ) per unit length and hence a predictable impedance and signal loss/attenuation characteristics. It also has a predictable signal propagation speed along its length.

From the standard engineering textbook: TRANSMISSION LINES AND NETWORKS. INTERNATIONAL STUDENTS EDITION by Walter C. Johnson, 1963, the speed of signal propagation in coax is given by:

m/s,

where is the distributed capacitance per unit length in pF and is the distributed inductance per unit length in µH.

Note that as long as the cable is in a state of inertial movement, i.e. not accelerated,

^{[1]}is independent of the orientation of the cable, or whether it is coiled, looped, etc. The exact values of the distributed and depend on the proportional radial dimensions of the cable and the materials used.

^{[2]}

The most common dielectric materials used in coax are solid polyethylene and polyethylene foam, giving a speed of signal propagation ~ 2e8 m/s, i.e. about 2/3 of the speed of light. Does this mean that the signal speed in coax depends on the speed of light? Well, not quite. The speed of light in a vacuum depends on the permittivity and permeability of free space:

m/s.

Since represent the absolutely lowest possible values of permittivity and permeability obtainable in nature, it means that represents the highest possible propagation speed.

The capacitance () and inductance () of coax depend on the permittivity and permeability of the materials in the coax and it gives a similar equation for the propagation speed in coax

m/s.

It is useful, but not required, to express the signal propagation speed of a specific type of coax as a fraction of the speed of light in vacuum. Is is the dimensionless velocity factor of the coax, i.e.

,

where and , the relative permittivity and permeability, each expressed as the ratio to the corresponding value for the vacuum.

Since the vacuum has the lowest possible values, it means the relative ratios must be larger than unity. The values for typical coax are and . The latter is so close to one because coax materials are generally non-magnetic.

The reason for hammering these relations is that there exists a false perception that the propagation speed in coax is a function of the speed of light. It is not - it only depends on the dimensions and electromagnetic properties if the coax materials. It can be calculated and measured without bringing the speed of light into the equation.

The salient point of this post is that the propagation speed in coax can be used to measure the one-way speed of light, without depending on the one-way speed of light. This issue has been discussed in the Physics thread: One-way Speed of Light a proven postulate? It was successfully concluded (before diverging into humorous banter), but there were some "dissident voices" that argued that "Lincoln's test" actually only measures the two-way speed of light.

For this to be true, it would require that the capacitance and/or the inductance per meter length of a coax cable must be changing with orientation in space. Or with how fast the cable is moving relative to some inertial frame. Or differently stated, for a straight cable, its propagation speed must be different in the two directions. Quite absurd, because it would mean that its capacitance/inductance must be different if measured from the two ends respectively.

Jorrie

[1] Acceleration may deform the dielectric and conductor shapes and hence change the cable's characteristics.

[2] http://www.ittc.ku.edu/~jstiles/220/handouts/Capacitance%20of%20a%20Coaxial%20Transmission%20Line.pdf

and https://en.wikipedia.org/wiki/Inductance#Inductance_of_a_coaxial_line