#### sh-eda

##### Member level 1

I wonder if someone could help with a question I have.

I am investigating standing waves on signal (100KHz-200Khz) sent over on long cables, over 1km -3Km.

Ultimately with a known load and source impedance and the details of the cable, I need to be able to predict what the signal level will be at various points along a fixed length cable, and especially where along the cable we are going to see lowest signals levels, dues to standing waves (It is very unlikely that the cable will ever be properly matched.)

I have done some testing on shorter lengths of cables and done some simulations (using Multisim), with mixed results.

I am partly using the following formulas (from https://en.wikipedia.org/wiki/Standing_wave_ratio)

and I have plotted this in Excel to give me an idea.

Reflective Coefficient = (Z-Zo)/(Z+Zo)

VSWR = (1+p)/(1-p)=Vr/Vf

Vmot = A*sqrt(4*p*(coskx)^2+(1-p)^2)

Where

x=is the distance along the cable from the source

p= (rho) is magnitude of the voltage standing wave ratio.

A=voltage amplitude

K is the wave number (ie. = radial freqeuncy / speed of the wave)

I have a couple questions about this.

1. I am surprised that the distance from the source to the terminating load is not taken in to account.

Surely more signal will be reflected back If it is stituated in a peak, less signal will be reflected back if it is in a trough

2. In my case resistance of the cable at 2-3km could more than the source impedance or the load impedance.

Surely the loss in such a cable has to be taken into account when considering the amount of signal reflected back.

Maybe its's when calculating the reflective coefficient Vr/Vf

Hope someone can help me clear this up for me.

Thanks