WIA 4:1 current balun – further measurements

4-101a

I mentioned in my article WIA 4:1 current balun that the use of a single toroidal core in the above graphic compromises the balun. This article presents some simple measurements and analysis that question whether the balun works as so many users think.

The popularity of the balun derives from the work of VK2DQ and is often known as the VK2DQ 4:1 current balun (though probably not his invention).

Analysis at the limits

Often, analysis of a network as frequency approaches zero or infinity can simplify the analysis whilst allowing a reasonable test of the sanity of the design.

Above is a conventional transformer schematic of the WIA 4:1 current balun on a perfectly symmetric (balanced) load. At frequencies where the electrical length of each winding is very short, we can assume negligible phase delay along or between windings, simplifying analysis greatly.

If we assume that at such a frequency, that the core characteristics are such that:

  1. conductor resistance is insignificant;
  2. core loss is insignificant;
  3. permeability is relatively high and so flux leakage is very low (ie almost all flux is contained in the toroidal core; and
  4. magnetising impedance of a single winding is relatively high.

You might consider that this approaches an ideal transformer, and it does.

Under these conditions, the input impedance (at the left hand terminals) of the transformer (even with no load, ie Z=∞), approaches zero. That is to say that as you improve this transformer towards ideal characteristics, Zin approaches a short circuit, and the output voltage would approach zero.

That sounds a warning sign, that the ‘better’ you make this device, the worse it performs.

Measurement

This balun is often pictured with a red core with rounded corners, looking very much like a powdered iron core, eg T200-2 with relatively low permeability of 10.

4-102

Nevertheless the text further down recommends a FT140-61 ferrite core (which will usually be unpainted and have sharper chamfered corners.

Ft14061balun

Above is a balun wound to the directions given in the article on a real FT140-61.

Magnetising impedance

Earlier, we discussed that the magnetising impedance would approach zero for an ideal transformer of this configuration. The core used is low to medium permeability, so there will be significant flux leakage… so lets measure it.

Above is measured magnetising impedance. The graph is windowed at ±500Ω as we would want that the magnitude magnetising impedance was OUTSIDE this range in a 50Ω system. It is outside ±500Ω from about 9-18MHz.

So the low magnetising impedance (caused by the series opposed windings) will compromise InsertionVSWR (and therefore nominal impedance transformation) on 80, 40, 15, 12 and 10m.

InsertionVSWR with asymmetric load

Above is the InsertionVSWR plot for the transformer terminated in an asymmetric 200+j0Ω load. The response is never good, particularly poor at the lower frequencies, but supporters will hold that it works real good.

Common mode impedance (Zcm)

Zcm gives a good indication of whether a current balun is likely to give good common mode current reduction in most scenarios.

Above is the Zcm response, it shows a quite sharp self resonance around 22MHz. The sharp response is quite expected with #61 material which is a relatively low loss material at these frequencies.

The graph is windowed at ±2000Ω as we would want that the magnitude of Zcm was OUTSIDE this range for good common mode current reduction. It is outside ±2000Ω from about 16-30MHz.

So the low Zcm (caused by the series opposed windings) will compromise common mode current reduction on 80, 40, 30, and 20m.

Is it for you?

Make up your own mind, it is clearly not a good HF 4:1 current balun, but hey, you might work some DX and prove that it works real good.

If you use the balun with an OCF dipole, your antenna asymmetry and poor current balun performance working against you, so expect higher local noise pickup and increased risk of electromagnetic compatibility. The cry of mediocrity often heard is any antenna is better than none at all.

My own view: I do not recommend this balun for any scenario.

References

Last update: 5th April, 2020, 6:23 PM