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How does Vector Network Analyzer (VNA) measures S parameters in reality?

Namely we have equations:

\$b_1=S_{11}a_1+S_{12}a_2\$

\$b_2=S_{12}a_1+S_{22}a_2 \$

In theory, if i wanted to calculate scattering parameters I would have to be able to make perfect matching on the source (\$a_1=0\$) and on the load side (\$a_2=0\$).

Unfortunately I do not understand how is this done inside the VNA. If I want to measure scattering parameters of a random network with input and output impedance different than 50ohm, how is in this case achieved \$a_1=0\$ and \$a_2=0\$?

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The HP8505 used directional couplers inside a special module located below the display/sweep/storage/compute unit.

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  • \$\begingroup\$ For what purpose did thy use them? \$\endgroup\$ – Nexy_sm Sep 1 '17 at 9:05

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