Hi David,

You wrote:

Initially I want to characterise some passive components (caps etc) and I
also want to be able to measure the unput impedance of an active device.

When you measure a passive component, you may want to measure the
capacitor shunted on the transmission line. Consider, in your mind,
what will happen at DC and at infinite frequency when these two
different signals see this network. At DC, the capactitor is an open,
so we should have full power transfer, and at infinite freq, the
"ideal" capacitor is a short, so we should have zero transfer of
power. Obviously, this is not the case of a real capacitor. It acts
like a capacitor up until it hits it's self resonant frequency (SRF)
then it starts becoming more inductive as you increase in frequency.
You should see this on your log plot when you measure S21 insertion
loss with the capacitor shunted on a transmission line between ports 1
and 2.

FH