On testing coax cable loss with an analyser / VNA – part 3

On testing coax cable loss with an analyser / VNA – part 2 gave a method of approximating the matched line loss (MLL) of a section of transmission line based on measurements of ReturnLoss with the section terminated in both an open circuit and short circuit. The article demonstrated the method using TLLC to provide expected measurement values.

So, does it work in practice?

Let’s measure a 10m length of Belden 8267 (RG-213) fitted with N connectors using a Rigexpert AA-600 and an instrument grade N(F) short circuit.

ReturnLoss @ 3.5MHz is 0.15dB.

ReturnLoss @ 3.5MHz is 0.29dB.

In fact, it turns out that in this case, and in general, that the average of  the two is a fairly good approximation, \(MLL^\prime \approx \frac{RL_{OC}+RL_{SC}}{4}=\frac{0.15+0.29}{4}=0.110\) which reconciles with the expected MLL from TLLC.

Zooming in

The measurement graphs above are zoomed in a much as permitted in Antscope… it clearly is pretty disabled for this type of application. Let’s export the data and look at both data sets in VNWA software.

Above is a plot of measured ReturnLoss wrt 50+j0Ω (RL(50)) for open (OC) and short circuit (SC) terminations. The data is a little noisy (jaggy), but quite acceptable for the purpose.

So contrary to the common belief, RL(50) is not the same for OC and SC terminations, and relying upon just one of them to estimate ReturnLoss wrt actual Zo, and then MLL wrt actual Zo, is actual MLL, is prone to error

Summary

Measurement of a real cable with the AA-600 provided a calculated MLL which reconciled with TLLC and which reconciles with interpolation from the Belden datasheet.

Conclusions

What does it say for the simple half ReturnLoss measurement technique?

It is a trap for the inexperienced.

Because it works sometimes (and it was pretty good in the example shown in the video mentioned) does not mean that it always works, it does not mean that it is reliable.

Be wary of what you read (or see, eg Youtube), think!