This article walks through a simple verification of nominal 50Ω coax cable with connectors against manufacturer specifications using a NanoVNA.
The instrument used here is a NanoVNA-H4 v4.3 and NanoVNA-D firmware NanoVNA.H4.v1.2.33.
The DUT is a 10m length of budget RG58-A/U coax with crimped Kings BNC connectors, budget but reasonable quality. It was purchased around 1990 in the hey day of Thin Ethernet, and at a cost of around 10% of Belden 8259, it was good value.
The notable thing is it has less braid coverage than 8259 which measures around 95%.
The cable has BNC(M) connectors, and BNC(F)-UHF(M) + UHF(F)-SMA(M) are used at both ends to simulate the impedance transformation typical of a cable with UHF(M) connectors.
Step 1: calibrate and verify the VNA
Note that |s11|<-40dB, or ReturnLoss>40dB. You really want at least 30dB ReturnLoss for this test to be as simple as described here.
The Smith chart plot of s11 is a very small spiral in the centre of the chart… approaching a dot.
Step 2: measure
The measurement fixture removed a ~150mm patch cable used during through calibration, so 750ps e-delay is configured on Port 2 to replace it. (one could just leave that cable in the test path and no e-delay compensation is needed).
Note that recent NanoVNA-D allows specification of e-delay separately for each port, don’t do as I have done if you use other firmware, leave the through calibration cable in place.
Above is a screenshot of the measurement from 1-31MHz.
Step 3: analysis
|s11|
Before rushing to the |s21| curve, let’s look at s11.
The Smith plot shows a small fuzzy spiral right in close to the chart centre, that is good.
The |s11| curve shows a periodic wave shape, lower than about -25dB. Recalling that ReturnLoss=‑|s11|, we can say ReturnLoss is greater than about 25dB. That is ok given the use of UHF connectors.
That wavy shape is mainly due to small reflections at the discontinuity due to the UHF connectors. They typically look like about 30mm of VF=0.67 transmission line of Zo=30Ω, so they create a small reflection. At some frequencies, the reflection from the far end discontinuity arrives in opposite phase with the near end reflection and they almost cancel, at other frequencies they almost fully reinforce, and in between they are well, in between… and you get the response shown in the screenshot. The longer the DUT, the closer the minima and maxima, and if the DUT is less than 180° electrical length, you will not see a fully developed pattern.
If you see a Smith chart spiral that is large and / or not centred on the chart centre, the cause could be that Zo of the DUT is not all that close to 50Ω, a low grade cable, possibly with migration of centre conductor or bunching of loose weave braid.
If your |s11| plot shows higher values, investigate them before considering the |s21| measurement. Check connectors are clean and properly mated / tightened.
|s21|
Now if that is all good, we can proceed to |s21|.
We can calculate InsertionLoss=‑|s21|, see Measurement of various loss quantities with a VNA.
It is interesting to decompose InsertionLoss into its components, because we may be more interested in the (Transmission) Loss component, the one responsible for conversion of RF energy to heat.
InsertionLoss for commercial coax cables is often specified as frequency dependent Matched Line Loss (MLL) or Attenuation (measured with a matched termination).
If InsertionLoss and MismatchLoss are expressed in dB, we can calculate \(Loss_{dB}=InsertionLoss_{dB}-MismatchLoss_{dB}\)
Above is a chart showing MismatchLoss vs ReturnLoss. If ReturnLoss>20dB, MismatchLoss<0.05dB and can often be considered insignificant, ie ignored.
For example, the marker, ReturnLoss=26.05dB, so MismatchLoss is very small and can be ignored in this example and we can take that Loss≅InsertionLoss=‑|s21|.
We can calculate the MismatchLoss given |s11|:
As stated, at 0.011dB, it is much less than InsertionLoss so can be ignored.
So do we give this budget cable and connectors a pass?
At 15MHz, we can take MatchedLineLoss to be 6.8dB/100m which is a little higher than Belden 8259 datasheet’s 6.2dB/100m. That is not sufficient reason to FAIL the cable, we will give it a PASS.
Problem analysis
Let’s discuss a problem scenario to encourage some thinking.
Above is a through measurement after calibration.
What is wrong, shouldn’t |s11| be much smaller, shouldn’t the Smith chart plot be a dot in the centre?
Yes, they should.
If the calibration process was done completely and correctly, then this indicates that Zin of Port 2 is significantly different to the LOAD used for calibration. Note that they could BOTH be wrong. An approach to resolving this is to validate the LOAD, and when that is proven sufficiently accurate, remeasure Port 2 using a short through cable and check |s11|. If it is bad then perhaps a GOOD 10dB attenuator on Port 2 might improve things.
Now let’s ignore that problem and proceed to try the cable measurement (with that setup).
Above is a through measurement of an unknown 10m coax cable with 50Ω through connectors… no UHF connector issues here.
As before, look at |s11| first. The cyclic wavy nature hints that there is problem with one or more of LOAD, Port 2 Zin and DUT Zo. They should all be the same.
Note also the spiral Smith chart trace, if it is not centred on the centre of the chart, it hints that DUT Zo, LOAD and Port 2 Zin are significantly different.
So, to resolve this, one path is to validate the LOAD, and when that is proven sufficiently accurate, remeasure Port 2 using a short through cable and check |s11|. If it is bad then perhaps a GOOD 10dB attenuator on Port 2 might improve things. If these are both good, then the measurement suggests the DUT cable Zo is wrong.
The |s21| measurement is suspect until you resolve these issues.
Conclusions
Analysis of the |s11| measurement is important, good |s11| results are a prerequisite to analysing |s21|.
Comparison with manufacturers data gives a basis for PASS/FAIL.