On testing coax cable loss with an analyser / VNA – part 2

On testing coax cable loss with an analyser / VNA – part 1 questioned a common method of measuring Matched Line Loss (MLL) of a section of an open circuit transmission line section, posing the questions:

The example gives MLL’ (based on half ReturnLoss) of about two thirds cable MLL.

• Why is that?

• What does it say for the measurement technique?

Recall that MLL’ calculated as half the RL(50) figure did not reconcile with the MLL calculated from line parameters.

Above is the complete output from TLLC for the same line section with short circuit termination, we are interested in the highlighted quantities. Again, $$MLL^\prime = \frac{RL(50)}{2}$$ quantity does not reconcile with MLL, but is considerably higher.

In fact, it turns out that in this case, and in general, that the average of  the two is a fairly good approximation, $$MLL^\prime \approx \frac{RL_{OC}+RL_{SC}}{4}$$.

Why is that, I asked?

Above is the same report with some magenta highlights. Note that Zo is not exactly 50+j0Ω, so ReturnLoss calculated / measured wrt 50+j0Ω (as is usually the case with an antenna analyser) is not that for the actual DUT. The report does contain ReturnLoss wrt the actual Zo, it is the second magenta highlighted quantity, RL at the load end, and it is 0.211dB, half of that reconciles with the MLL of 0.105dB.

It is a trap for the inexperienced, and it appears in places that might surprise, like the Bird 43 user manual.

(Bird 2004) gives the following advice.

Line loss using open circuit calibration: The high directivity of elements can be exploited in line loss measurements, because of the equality of forward and reflected power with the load connector open or short circuited. In this state the forward and reflected waves have equal power, so that φ = 100% and ρ = ∞.
Open circuit testing is preferred to short circuit, because a high quality open circuit is easier to create than a high quality short. To measure insertion loss, use a high quality open circuit to check forward and reverse power equality, then connect an open-circuited, unknown line to the wattmeter. The measured φ is the attenuation for two passes along the line (down and back). The attenuation can then be compared with published data for line type and length (remember to halve Ndb or double the line length to account for the measurement technique).

This also contains the hoary old chestnut that a good o/c termination is hard to achieve, but this author’s experience of measurement with modern VNAs is not consistent with Bird’s assertion.

Conclusions

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

It is a trap for the inexperienced, and it appears in places that might surprise, like the Bird 43 user manual.

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!

Links / References

• Bird Electronic Corporation. 2004. RF directional thruline wattmeter model 43 – instruction book. Cleveland: Bird Electronic Corporation. p14.