Select a ferrite core material and sufficient primary turns for a low InsertionVSWR 50Ω broadband RF transformer – nanoVNA gave an explanation of how to use a nanoVNA or the like to select a suitable core and sufficient turns for a low InsertionVSWR broad band 50Ω transformer.

The discussion of an example worked up the loss components at 3.5MHz of the example configuration, a 4t winding on a FT240-43.

This article demonstrates that a graph of the loss components from the saved .s2p is possible.

Let’s review some meanings of terms (in the 50Ω matched VNA context):

- \(Loss=\frac{PowerIn}{PowerOut}=\frac{P_1}{P_2}\);
- \(s11=\frac{V_{1r}}{V_{1i}}\);
- \(MismatchLoss=\frac{P_{1i}}{P_{1}}=\frac1{(1-|s11|^2)}\);
- \(s21=\frac{V_{2i}}{V_{1i}}\); and
- \(InsertionLoss=\frac{P_{1}}{P_{2}} \approx \frac{P_{1i}}{P_{2i}} =\frac1{|s21|^2}\).

It is assumed that Zin of VNA Port 2 is 50+j0Ω, and that therefore P2r=0. Error in Zin of VNA Port 2 flows into the results. A 10dB attenuator is fitted to Port 2 prior to calibration to improve accuracy of Zin.

With the quantities expressed in dB, we can derive that \(Loss=-|s21|-MismatchLoss\).

The graph gives a wider perspective of the contribution of Mismatch Loss and (Transmission) Loss to Insertion Loss.

Loss is core and copper loss, mostly core loss in this case.

Try 2, 3, 4, 5 turns on a FT240-43, what does that say about all the articles on the ‘net using 2t primaries?

Try FT240-61, how many turns are sufficient, how does the Loss compare? Before you jump to the conclusion that #61 is superior, you need to measure the broad band performance which may be impacted adversely by the length of the windings… a story for another article.