I gladly defer to Tom, who has a great deal more experience with and
knowledge of measurement devices and techniques than I. My point was
that you need to know magnitude and phase of the measured quantity if
you're going to determine impedance. If all you were measuring was
power, you wouldn't have this information. So you must measure the
voltage (or current).

Roy Lewallen, W7EL

K7ITM wrote:
I'm going to take slight issue with what Roy wrote about measurement
using a directional coupler. Though it's true that the receivers could
be considered to be measuring voltage, they are measuring it across a
well known load (very close to 50 ohms). When we set up a system to
test the performance of units we build, it's done very carefully to
calibrate them for input _power_, not voltage. The usual directional
coupler in a VNA is indeed a resistive bridge, with some care taken to
make the known legs and the source driving it and the load observing
the imbalance all 50 ohms (in a 50 ohm system). Generally a bridge
whose impedance is near the impedance you want to measure is a good
choice for a starting point for making accurate measurements, but it's
for sure not easy to build a 10k ohm bridge at 10GHz, for example...
. . .