Pete Smith wrote:
> A friend and I have been corresponding about seeming anomalies in
> results obtained when attempting to measure cable loss with the
> MFJ-259B.
>
> With the opposite end of a 50-ohm cable open, the manual says, and I
> would expect, a gradually increasing loss as the frequency increases.
> Instead, what we both see is variation above and below an increasing
> average value. With a 100' piece of RG-213, he is seeing swings of
> as much as .5 dB, which I assume rules out A/D converter jitter as
> the cause, so the questions are:
>
> -- what is going on?
>
> -- what value represents the real loss of the cable? Do you need to
> draw a loss curve that averages out the variations in order to
> compare your results with the cable spec?
Pete,
When sweeping feedlines at cell sites, we use (max+min)/2 (over the
frequency range of interest) for cable loss (cable terminated in a
precision short). This gives a number that is somewhat (20%-30%) better
than manufacturer's specs. The worst case (highest return loss) by
itself is usually slightly better than spec.
I've always assumed that the roller coaster curves were due to the
varying termination impedance. One would think that the best case
(lowest return loss) would occur at termination impedance peaks and
therefore be closest to reality, but that appears not to be the case.
Steve K8LX
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