There is little doubt that the nanoVNA has made VNAs very popular in the ham community, possibly more so that any other device.

Eager owners are trying to apply them to solve lots of problems, often without sufficient knowledge or experience to properly inform the measurements.

An example that has a appeared a few times on online forums in the last weeks is measuring the matched line loss (MLL) of a section of RG6 coax… to inform a decision to discard it or keep it.

The common approach is to use a measurement of |s11| and to calculate Return Loss and infer the MLL.

## DUT

For discussion, lets consider an example of 30′ of Belden 1694A RG6 solved in Simsmith. We should note that unlike most RG6 in the market today, this uses a solid copper centre conductor.

## Short circuit termination

Some authors insist that the half return loss method is to be performed using a short circuit test section. Bird does this in their Bird 43 manual.

Above is a plot of calculated |s11| (-ReturnLoss) from 1 to 20MHz for the test section. The three plots are of |s11| wrt 50Ω, 75Ω and frequency dependent actual Zo (as calculated for the model). The cursor shows that the actual |s11| is -0.37474dB (ReturnLoss=0.37474dB). Using the half return loss method MLL=ReturnLoss/2=0.37474=0.187dB/m.

Now to the other two traces.

|s11|(50)=-0.4241dB, quite different from the correct value 0.37474dB (the red curve).

|s11|(75)=-0.4521dB, quite different from the correct value 0.37474dB (the red curve).

Note that at other frequencies the error is different.

## Open circuit termination

Above is a plot of calculated |s11| (-ReturnLoss) from 1 to 20MHz for the test section. The three plots are of |s11| wrt 50Ω, 75Ω and frequency dependent actual Zo (as calculated for the model). The cursor shows that the actual |s11| is -0.37474dB (ReturnLoss=0.37474dB). Using the half return loss method MLL=ReturnLoss/2=0.37474=0.187dB/m.

Now to the other two traces.

|s11|(50)=-0.2636dB, quite different from the correct value 0.37474dB (the red curve).

|s11|(75)=-0.2922dB, quite different from the correct value 0.37474dB (the red curve).

Note that at other frequencies the error is different.

## What about averaging?

If we average the |s11|(50) measurements for short and open short we get (-0.4241+-0.2636)/2=-0.3439 for MLL=0.1719dB, well below the correct value of 0.187dB.

If we average the |s11|(75) measurements for short and open short we get (-0.4521+-0.2922)/2=-0.3722 for MLL=0.186dB, very close to the correct value of 0.187dB.

## Conclusions

The method of inferring Matched Line Loss from measured Return Loss of an open circuit or short section line section is soundly based but often fails in the execution. The Return Loss (or -|s11|) used as the basis MUST be with reference to the actual Zo of the line, though a good approximation can be obtained by averaging the Return Loss measurements for S/C and O/C section when Zref is close to Zo.

The knowledge and experience important to exploitation of the nanoVNA does not come in the box.