"Ian White wrote
Yup. It measures the reflection coefficient of whatever impedance is
connected to the port on the opposite side from the transmitter.

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No, it doesn't.

It measures the MAGNITUDE of the reflection coefficient. It discards
the information which is contained in the phase angle of the
reflection coefficient. As a consequence the only use which can be
made of the magnitude is to calculate the SWR on an imaginary 50-ohm
line. The SWR can be used to calculate the magnitude of the
reflection coefficient.
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Reg.