I do still have my doubts if this last test really shows the standing wave resonance and not again the LC resonance.
It's good to have doubts. You should perform a self-educational experiment by taking a looooong piece of 50Ω coax and setting it up for a TDR measurement. Verify that the classic pulse reflection is visible when the coax is shorted and opened at the far end. Next, connect a parallel combination of a ferrite torroidal coil and a good cap instead of the open and short termination at the far end. Verify that the TDR experiment still works. The LC resonant frequency of the coil&cap does not matter as long as it is different from that round trip reflection in the coax. While you still measure the pulse reflections from the far end of the coax in the TDR manner, add another stimulating frequency * that will resonate the lumped LC circuit connected at the end of the coax. Can you discern the round trip reflection in the coax from the response of the LC circuit at its end? You can do all that using your FG and scope. Use my FG programming technique to make it output nanosecond pulses (I made my 1st video about it ...if you remember). * That can take a form of two different frequency sine waves added together, ...or a rectangular pulse train of specific duty cycle and pulse repetition frequency.
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