In this article, I will outline an evaluation of a ‘classic’ voltage balun, the 1:1 W2AU voltage balun, specified for 1.8-30MHz.
These were very popular at one time, but good voltage baluns achieve good current balance ONLY on very symmetric loads and so are not well suited to most wire antennas.
Above is W2AU’s illustration of the internals.
Mine barely saw service before it became obvious that it had an intermittent connection to the inner pin of the coax connector. That turned out to be a poor soldered joint, a problem that is apparently quite common and perhaps the result of not properly removing the wire enamel before soldering.
Having cut the enclosure to get at the innards and fix it (they were not intended to be repaired), I rebuilt it in a similar enclosure made from plumbing PVC pipe and caps, and took the opportunity to fit some different output terminals and an N type coax connector.
Above is the rebuilt balun which since that day has been reserved for test kit for evaluating the performance of a voltage balun in some scenario or another.
My rework did not attempt to duplicate the spark gap arrangement of the top terminals. It is doubtful that it is effective protection of an attached receiver.
InsertionVSWR is often an important parameter of nominally 1:1 baluns. So, let’s measure the balun’s InsertionVSWR by connecting it to Port 0 (Ch0) and connecting a good 50+j0Ω load to the output terminals.
Above is a sweep from 1-41MHz, Insertion VSWR looks pretty good above about 7MHz.
Let’s drill down on the low frequency performance.
Above is a Smith chart view of the sweep from 1-5MHz. If you are familiar with the Smith chart, you will recognise that the curve almost follows a circle of constant G (not drawn on this Smith chart unfortunately). That suggests that Yin is approximately 1/50+jB where B is frequency dependent.
nanoVNA MOD does not have an admittance chart (more’s the pity) but it does have a hammy substitute, the “parallel RLC” chart though it is actually Rp||Xp. Let’s sweep 1-5MHz to focus on the low frequency InsertionVSWR problem.
Above, the Rp||Xp presentation. Note that the Xp (blue line) is fairly straight and if you project it to frequency=0, Xp will be approximately 0 so \(Xp \propto f\). Recall that the reactance of an inductance X=2πfL, so Xp looks like it may be due to a constant parallel inductance, and the equivalent parallel inductance can be calculated. It can be, but no need as nanoVNA MOD conveniently displays the value in the cursor data, 12.9µH in this case. Note also that the value of Rp is approximately 50Ω independent of frequency.
The poor low frequency InsertionVSWR is due to the low equivalent parallel inductance of 12.9µH, the magnetising inductance as it happens.
So, what should it be?
Well that depends on how we might specify performance. If we wanted the balun to have an InsertionVSWR of less than 1.1 from 3.5MHz, then Xp needs to be greater than \(Xp>10Zo=500\Omega\) and therefore magnetising inductance \(Lp>\frac{10 Zo}{2 \pi f}=23µH\).
Increasing the magnetising inductance will typically degrade the high frequency performance, so finding a good design is a compromise between these and other factors.
So as voltage baluns go, this has quite good InsertionVSWR above 7MHz, but is a bit shabby below that.
The article has demonstrated how simple measurements made with the nanoVNA (or any other capable VNA or antenna analyser) can be used to evaluate not only the InsertionVSWR, but provide a likely explanation for its behaviour. Insufficient magnetising impedance is a common design flaw. You could use this approach to guide design of a DIY voltage balun.