nanoVNA – evaluation of a voltage balun – RAK BL-50A

In this article, I will outline an evaluation of a ‘classic' voltage balun, the 1:1 RAK BL-50A 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 a pic of the balun with load on test. It is not the greatest test fixture, but good enough to evaluate this balun over HF.

Mine has survived, but many users report the moulding cracking and rusted  / loose terminal screws, and signs of internal cracks in the ferrite ring.

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 and up to about 20MHz.

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, 11.9µH 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 11.9µH at low frequencies, 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.

If we look more widely at the Rp||Xp response, we see a self resonance around 22MHz,  and above that, progressively a lower and lower shunt Xc. So, just as low equivalent shunt Xl degraded low end performance, low equivalent shunt Xc degrades high end performance which is the main contribution to increasing InsertionVSWR above 22MHz.

So as voltage baluns go, this has moderately good InsertionVSWR from 7-20MHz, but is a bit shabby above and below that range.

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.

Further reading: Voltage symmetry of practical Ruthroff 1:1 baluns discusses voltage symmetry of the BL-50A.