> The important points are that the physical limitations of the test
> fixture due to its stray inductance and capacitance remain exactly the
> same; and above all, the need to prove the entire test setup by using a
> test load of the order of 10k.
The limiting factor with the bridge method is port isolation.
With nothing connected between the ports, you might see -70 dB for S21.
Errors will start to get significant above 15,000 ohms,
which is probably fine for balun testing. When doing VNA calibration,
it is typical to skip the port isolation step to save time, because it
usually doesn't matter in conventional measurements. However, it is
worth doing for bridge measurements. Your external fixturing can
also affect isolation. It is possible when measuring a very high
Q parallel resonant tank to have the stray transmission result in
a "negative" resistance reading. This is especially a problem with
common mode chokes using bifilar windings (vs coax).
Rick N6RK
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