ok on all Dave... However, what Jon shows in the video, the infamous UnUn is not an autotransformer...
In summary, the key difference between an autotransformer and a transformer is the presence of a single winding in the autotransformer that serves the function of both primary and secondary windings, whereas a conventional transformer has separate primary and secondary windings...
So he is using a conventional transformer and must control the coupling coefficient, k, very carefully (as close to unity as possible) to achieve the impedance transformation that he demos.
In the autotransformer, that turns out to be not so trivial!
