Measuring a 1/4 wave balanced line – nanoVNA

A question was asked recently online:

I am about to measure a 1/4 wave of 450 ohm windowed twinlead for the 2m band using my NanoVNA. My question is, since I will be making an unbalanced to balanced connection, should I use a common mode choke, balun or add ferrites to the coax side to make the connection, or does it really matter at 2m frequencies? The coax lead from my VNA to the twinlead will be about 6″ to 12″ long. I will probably terminate the coax in two short wires to connect to the twinlead.

It is a common enough question and includes some related issues that are worthy of discussion.

I must say I found the collective advice of the assembled online experts wanting, let’s explore the subject.

The fixture

Let’s deal with the measurement fixture first, failure to get that right produces confusing and incorrect results.

Calibration of a VNA establishes a correction regime based on a certain place known as the reference plane. The beauty of the VNA is that you can make this reference plane wherever you like (within reason) by choosing the point at which you connect the calibration parts during the process.

If you want to measure a length of transmission line (DUT), then making the reference plane the connection point of the DUT makes for simpler interpretation of the measurement data.

It is ok to use a short piece of coax, but the calibration should be done at the place where the DUT will be connected, that becomes the reference plane.

Connection of a symmetric DUT to the asymmetric VNA may have problems.

If you connect a line such as that mentioned directly to the coax port of a VNA or similar antenna analyser, you drive the line (DUT) with both common mode and differential drive, and your instrument makes measurement of the combined effect. If you want to measure only differential effects (which would usually be the case, and is the case in this example), then you must ensure that common mode drive is insignificant at the frequencies of interest.

A bad fixture for this DUT

Above is an example of a fixture and the calibration parts suited to measuring small components. I see the twisted transmission line has untwisted one turn with handling, it does not affect the results significantly, but to be thorough, the measurements below were redone with that line twisted uniformly. I might mention that the turned pin sockets I use are not particularly robust, the female part requires replacement from time to time.

Can you see the kinks in the green Smith chart spiral where markers M1 and M2 are located? That should not happen, it is not an attribute of the differential mode of the transmission line, but an aberration caused by common mode drive. The departure is easier to see on the plot of |s11dB| in yellow.

The departure is easier to see on the above plot of |s11|. The problem is that common mode drive is significant, and altering the load seen by the VNA port, most notably around 25MHz and its third harmonic.

The common mode loading also shows up as kinks in the impedance plot, so for instance if you were trying to find the frequency where X=0, you might get an inaccurate result.

The coax extension idea

The article Antenna analyser – what if the device under test does not have a coax plug on it? discusses some possible solutions to connection, and it is feasible to use a short coax extension to the reference plane (ie OSL calibrated at the end of the extension), but this does not address the common mode drive problem.

A better fixture for this DUT

Above is a better fixture and the calibration parts, all of which connected to pin sockets under the end of the PCB. Again, the untwisted end of the line was corrected for all measurements below.

This fixture is described in detail at A 1:1 RF transformer for measurements – based on noelec 1:9 balun assembly.

I have tried a number of different fixtures for two wire line sections for the range 1-100MHz, and this one (which is a voltage balun) is the best that I have tried.

You might think that this is clearly an application for a current balun, but keep in mind that good voltage baluns deliver good current balance on symmetric loads… and this load is symmetric.

So, what to we measure?

This time, the plot of |s11| looks more like expected, no local glitches.

Above is a plot of R,X (and the hammy |Z|… why do they insist on adding that) looking into the line. As the line section is open circuit, the first resonance (X=0) by interpolation is about 41MHz, accuracy could be improved by narrowing the scan to the neighborhood of 41MHz.

The velocity factor can be calculated as \( \frac{length}{FreeSpaceWavelength/4}=\frac{1370}{1828}=0.76\). Again, that could be improved by narrowing the sweep. That is probably good enough for most purposes, but if you want to reduce errors due to the end terminations, see Velocity factor solver.

We can approximate Zo as \(Zo=|X_{\lambda / 8}|=33 \; \Omega\). (You might have heard that it is not possible to make a twisted pair line of such low Zo, more ham myth!)

You cannot do these things accurately if the measurements are disturbed by common mode loading.

Conclusions

  • Common mode drive disturbs the thing being measured.
  • You might have experience of having made some measurements that appeared correct, and that is quite possible, but if you want to make reliable measurements, deal with the common mode drive problem.