"Voltage transfer function" by itself is not completely
meaningful at high frequencies.
definition is Vout/Vin, when driven from a voltage source,
and driving into an open circuit. This can often be
approximated at low frequencies (audio), but even there,
you must be careful not to make your measurements
Both a pure voltage source and an
infinite impedance load are, at some point, impossible to
obtain and still be useful. For example, an active
oscilloscope probe with 0.5 pF input capacitance still has
an input impedance in the hundreds of Ohms at 1 GHz.
If you're using a VNA, you have a 50 Ohm source and a
50 Ohm load. You may be able to compute the voltage
transfer function mathematically - but I'm not sure which
constraints are important to you. The information is
"hiding" mostly in S21, but needs compensation for S11 and
S22, probably. Our host here at RFCafe has posted a
complete set of equations for various conversions to and
from S-parameters at
You'll need to identify what you really need, either
as a Z-parameter, H-parameter, or Y-parameter.