# [TowerTalk] Elementary feedline loss question

Steve Hunt steve at karinya.net
Fri Apr 4 17:02:10 EDT 2014

```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?
>

```

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