# Transmission Line Question

1. Sep 8, 2008

### americanforest

How is signal transmission speed determined in a transmission line? For example, what determines how long it takes a single pulse to go from one end of a line to the other end. I would assume it depends directly on the conductivity of the material from which the line is made but I'm not sure about that. Can someone refer me to a source which gives me a detailed explanation?

2. Sep 8, 2008

### cabraham

speed u = sqrt(1/LC); L=inductance per unit length, C=capacitance per unit length

3. Sep 9, 2008

### f95toli

The exact relation depends on the transmission line.
However, the speed of propagation is not related conductivity of the metal (the conductivity is only related to the losses in the line) but to the dielectric constant of the dielectric (the insulator); the speed of propagation is roughly $1/\sqrt\epsilon$.
Remember that the wave in a transmission line is not actually traveling in the metal (at high frequencies the skin depth is very small) but in the dielectric.

4. Sep 9, 2008

### vanesch

Staff Emeritus
To tune in with the previous poster: in a 2-conductor transmission line, what determines essentially the propagation speed are two elements:
1) the inductance per unit of length of the conductors
2) the capacitance per unit of length between the conductors.

5. Sep 9, 2008

### americanforest

Can anyone refer me to a book or website with a discussion concerning this subject?

6. Sep 9, 2008

### rbj

isn't there also the resistance per unit length and possibly a leakage conductance (between the two conductors) per unit length?

well (looking at the wiki page), maybe you're completely right, but i thought that R and G played a role in determining wave velocity in a lossy transmission line. but, it appears (not solving the diff eq myself) that it's just C and L.

maybe start here with the Telegrapher's_equations. the book or books i have on the topic are soooo old (35 years) that i can't recommend since they must certainly be out of print.

7. Sep 11, 2008

### sha_virgo

hi,, the transmission speed is propably depends on the line capacitance and line inductance,both components are reciprocal to speed.

8. Sep 16, 2008

### Pumblechook

There are only really three types of dielectric (insulator) used in coax cable. Air where the inner conductor is supported at regular intervals, PE (Polyethylene) or Teflon (PTFE). PE is by far the most common.

The velocity factor for PE is about 66% of the speed of light. PTFE it is about 70%. Some coax cables use foam or semi-air spaced PE and the velocity factor will be higher.

http://www.nr6ca.org/vf.html

Foam or air are used mainly to allow a larger centre conductor to be used for the same impedance in order to reduce loss. Loss per length increases with frequency due to the skin effect. The current travels in a thin skin near the surface of the conductor and the depth of this skin reduces as you go up in frequency.

9. Sep 16, 2008

### mheslep

There are two different topics in discussion here.
1. Velocity factor, the speed of electromagnetic radiation through a medium is dependent on the dielectric constant of the medium, as discussed in posts #3, #8.
2. Pulse propagation per the OP is about the movement of charge; that depends on the circuit characteristics distributed over the transmission line, namely inductance and capacitance, as discussed in the rest of the posts.

Thus, if you would like to know how long it takes a pulse entering a transmission line to reach its final peak amplitude at the other end you need only know the distributed L,C of the line.

10. Nov 22, 2008

### americanforest

Thanks for clarifying this, it was very confusing