Somebody said -

The tricky part is measuring this correctly, because you
would need an SWR meter that is calibrated for the same Z as
Zo.

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No problem !

The fixed standard arm of the rho bridge (instead of a 50-ohms resistor) can
be just a very long length of transmission line of input impedance Zo =
Ro+jXo which, of course, varies with frequency in exactly the required
manner.

Or, as I often did 50 years back, make an artificial lumped-LCR line
simulating network to any required degree of accuracy.
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Reg