# Need assistance with transmission lines formulas

#### dannyj999

Joined Jun 7, 2015
1
Working frequency of the cable = 1.75Khz
Attenuation = 0.40 Np/km
Sending end voltage = 10V rms
Sending end current = 20mA rms

What I need to determine is the voltage and current in polar form at a particular point down the line, for example 20km.

I've been trying to figure this out for hours but keep coming to a dead end.

The formula I have for the current is;
Ir = Is e^-na

What I can't figure out is, is "n" the legnth? so for this example would it be 20? and "a" is the attenuation which I'm given?
so would this be;

Ir = (20x10-3)e^-(20)(0.40)

This equals a very small number, which is why I'm thinking its incorrect.

Or am I looking at this the complete wrong way? I thought originally I'd need to work out the characteristic impedance Zo but I'm not given the values for R L G and C ?

So yeah! Any help would be greatly appreciated.

#### MrAl

Joined Jun 17, 2014
8,494
Hi,

I've never done it this way, but that looks right.
'a' would be the attenuation factor 'alpha' and that would make 'n' the length in km.
The voltage formula should look similar.

For example, for a cable with a=0.8 Np/km there would be an attenuation of about 48.6db at a distance of 7km. Keep in mind that we are usually working in the range of thousands of meters here so there could be a lot of attenuation.