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I suspected the 50 Ohm calibration resistor and ordered two new ones.
After recalibration and measurement, the SWR is 1.26 at 172 MHz and 1.65 at 522 MHz.
Both measurements with SOL calibrated center frequencies 172 span 50 MHz and center 522 MHz span 50 MHz

that is much more plausible to me, i'm right?

I think that was the reason.

thanks for your support