[TowerTalk] measuring cable loss with an MFJ 259B
jimlux at earthlink.net
Wed Jun 14 14:55:52 EDT 2006
At 08:03 AM 6/14/2006, 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
>-- 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?
Are these swings with the frequency held constant? Or wavyness as the cable
is swept in frequency.
Part of it might be that the bridge in the 259 has better accuracies at
some impedances than others. As you sweep the frequency, the impedance
being measured varies from a short to an open, with varying amounts of
reactance. It's not unexpected that there would be some calibration issues
at "oddball" impedances far from 50 ohm resistive. 0.5 dB is about 10%,
which is what I would expect for the overall accuracy of the 259B. The
A/Ds are 8 bit devices and have a precision around 0.5%, but if you write
out the equations that turn the A/D measurements into what shows up on the
display, there are cases where a small change in A/D results in a large
change in displayed value. [This is why there isn't a simple accuracy spec
on something like an Agilent network analyzer, but rather, a whole computer
program that tells you what the measurement uncertainty is for each
measurement, and that uncertainty is hardly constant across the band]
Tom Rauch has a page describing the inner workings of the 259B that might
Be aware that if you google "MFJ-259B accuracy", you'll find a lot of
stuff, not all of it exactly wonderful. (there is a difference between
precision, accuracy, uncertainty, etc., and for any sort of RF measuring
instrument, there's typically a fairly wide variation in these over the
operating range). Not to pick on any particular instance, but just because
the scale reads with a precision/resolution of 1 ohm does not mean that
the measurement is accurate to that level, any more than your $5 DMM with a
3 1/2 digit display is accurate to 1 millivolt reading the voltage of a
1.5V dry cell.
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