Transformer loss from half return loss

The popularity of the nanovna with new users has led to a lot of ‘beginners' seeking advice of the online experts.

A recent question and the responses exposes some common misunderstanding / woolly thinking.

A beginner in search of wisdom asked if there an easy way to measure balun loss?

One of the responses was…

Measure the return loss of the balun with the balun shorted. The return loss should be about twice the balun loss. This is similar to measuring the loss of a shorted or open piece of cable.

This was expanded by others, basically supporting the concept.

Let's put it to the test with an example transformer in Simsmith.

Above is a model of a 1:1 ferrite cored transformer where the flux leakage reactance is represented by L1 and L3, the magnetising impedance is represented by parallel equivalent components L2 and R1. L2 has a reactance of 500Ω at the model frequency.

We can see from the screenshot by observing the power in the load that InsertionLoss is 0.46dB.

Now let's follow the advice quoted above and explore the model with a short circuit at the load L and find half the return loss which it was stated should be about equal to the InsertionLoss.

Above is the short circuit scenario as recommended. The calculated value hrl at lower right is half return loss.

In the short circuit scenario, half return loss is 0.001748dB, nothing like the correct InsertionLoss of 0.46dB shown in the original model.

So the quoted advice is clearly nonsense in this instance. There might be scenarios where the half return loss of the short circuited network looks believable, but being correct is a whole lot more than simply looking believable.

This is application of basic linear circuit theory, and it takes only one case to disprove the expert's method.

There are variations on this method sometimes described, but they tend to fail under some scenarios which means they are not soundly based.