[TowerTalk] Elementary feedline loss question

[Steve Hunt]

Here's a method I use:

Terminate the line with a short circuit. At the other end of the line 
use the analyser to find the frequencies where the line impedance is 
lowest resistance and zero reactance - those frequencies will be where 
the electrical length of the line is a half-wave or multiple. Note the 
resistance R.

Then for low losses:
Line Loss(dB) = 8.69 x R / Zo

Or, more generally:
Line Loss (dB) = 10 x Log[(Zo-R) / (Zo+R)]

If you need results at a few more frequencies, open circuit the line at 
the far end and repeat the process with line lengths of a quarter-wave 
and odd multiples.

If you're interested in the underlying maths, it's on my web site:
http://www.karinya.net/g3txq/wet_ll/tl_formulas.pdf

Steve G3TXQ


On 04/04/2014 14:53, Pete Smith N4ZR wrote:
> If I terminate a long 50-ohm coaxial cable with a 50-ohm dummy load, 
> and put an MFJ-259B on the other end, and it reads R=56, X=0 at a 
> given frequency, what is the mathematical relationship between the 
> measured R (leaving calibration out of it, for now) and the loss in dB?
>