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my goodness. Please read the posts more slowly..

You need to learn the topic a little more..

this stuff:

CJ:""What is a "free space velocity of the dielectric"? Where can I find your back-up explanation "that if a coax is made with vacuum as the dielectric" in your above statement ????????
How can you expect anybody to understand what you meant w/o your
back-up explanation?

First off, a coax always comes with a dielectric. Your "IF a coax is made with vacuum" does not exist & your reasoning is moot.

I already stated clearly an ideal wire is a bare wire & there will be very little propagation delay in free air. In vacuum, the wave will travel in light speed along that wire. The idea is exactly the same as you justed posted.

However, your statement: "if a COAX is made with vacuum as the dielectric, the prop velocity of that coax will be the speed of light" is very questionable.

What is a coax? It comes with an overal surrounding shield ideally equi-distant to the centre conductor from all side. This is a capactor by itself due to its construction, regardless the dielectric is a realworld material or a theoretical vacuum as you suggested.

Its capacitance is given by the formula: c=7.36x(e) divided by Log10 D/d where e is the dielectric constant & D is the outside diameter of the coaxial cable (includes the shield) & d is the diameter of the centre conductor. Unit is pF/ft

With capactance involved, how can you expect light speed transfer of a signal along a coax even with theoretically vacuum as dielectric?""

Wow..please find a text, you are rather confused.

1. Free space velocity of a dielectric is the speed of propagation of an e/m wave in that dielectric.

2. The speed of travel of a signal in a coax is the same speed as that of free space.

3. Vacuum filled coaxial cables are a dime a dozen in the world of high energy physics.

4. A signal does not travel along a "wire". It travels along two wires, both of them together form both a capacitor and an inductor. Or, in free space, as a result of e/m wave propagation.

5. A coax forms a capacitor and it forms an inductor.

6. The velocity of propagation along a coax is directly related to the inductance per unit length and the capacitance per unit length. Do the math...calculate the inductance per foot, and the capacitance per foot, then the prop speed. You will find that vacuum coax (which is used in some parts of the world of science), and even nitrogen filled coax (which is a staple for high power rf coax), has prop speeds approaching C.

I'll touch on the rest of your somewhat haphazard post tomorrow..in the meantime, please pick up a good book on the subject, Jackson maybe, or Becker..

I can only but show you the door..

Cheers, John


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  • my goodness. Please read the posts more slowly.. - jneutron 14:06:41 04/04/06 (0)
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