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On Sat, Jan 14, 2023 at 08:16 AM, KESAHZ wrote:

I must admit I am not sure what these diagrams show.

Simply put, the dips in the green curves indicate the frequency at which the antenna is well matched.
The frequency of the yellow curve is optimal where it is as close as possible to the centre of the circle (i.e. 50?Ω).
In principle, two different indications for one phenomenon.