Measure MLL using the half ReturnLoss method
Again in the theme of measuring something known, let us determine the matched line loss (or normally quoted attenuation) of our cable at 3.5MHz.
To make the measurement, just connect the two test line sections used in the earlier articles in this series in cascade with a joiner, and one end on the instrument, other end open circuit, and measure ReturnLoss.
Most analyser manuals and lots of helpful articles in journals and handbooks will tell you that MLL=RL/(2*length) where RL is the ReturnLoss of an open circuit or short circuit line section (the only requirement is that the ρ=1 at the line end).
Wow, that is so low, and using the traditional formula:
Of course we are measuring way low in the instrument’s capability and there is some considerable uncertainty… but when we consult a good transmission line loss calculator, we expect around 0.029dB/m… that is 12 times what we measured.
Ok, so let’s try the same measurement with a short circuit at the end of the line. I have rolled up a little ball of aluminium cooking foil and pressed it into the end of the connector to short the line.
Wow again, what a different result, and using the traditional formula:
Again, we expect around 0.029dB/m. so we might condemn this line section as below spec, except for the previous measurements that indicated it was unbelievably better than spec.
What is unbelievable is the traditional formula… yet it survives, and the purveyors of these instruments recommend the methods to unsuspecting buyers.
It turns out that averaging the two measurements gives a pretty good estimate of MLL, not exact, but a pretty good estimate.
The result is close to the spec value of 0.029dB/m so the cable looks good.
More on MLL measurement at: On Witt’s calculation of Matched Line Loss from Return Loss.