I tried this connection with full through cal (excluded isolation) with the
HP 8753C. Shields were commoned at the junction of the coax from the two
ports. Choke was inserted between the S11 and S22 center conductors.
Readings were quite believable and reasonable in dB (log Mag). Among my
'collection' of CM chokes, I found one that didn't hit less than -30 dB
until 29 MHz and good to beyond 50 MHz with no resonances - happy camper.
It's now in my HF system between the output of the home brew L-Network and
the open wire feedline with considerably less RFI. I have not tried this
with the NANOVNA or the NANOVNA-F.
Dave - W?LEV
toggle quoted message
Show quoted text
On Mon, Feb 10, 2020 at 12:31 AM Kurt Poulsen <kurt@...> wrote:
Hi Igor and others
Just a small note
There is a "snag" regarding converting a S21 measurement to impedance
because the math used request that both the source and load impedance from
both of the two calibration planes, where the DUT is inserted in between,
must be pure 50ohm. That is not the case for a NanoVNA but for a VNA
running full 12term error correction.
Kind regards
Kurt
-----Oprindelig meddelelse-----
Fra: [email protected] <[email protected]> P? vegne af Igor
Lapko
Sendt: 9. februar 2020 10:13
Til: [email protected]
Emne: [nanovna-users] Measuring CMC and S21 readings
Hi,
Does anybody use this device to measure common mode choke impedance?
G3TXQ for instance suggested using |S21| and phase angle to recalculate X
and R of the DUT in the spreadsheet.
NANO VNA-F however has a bunch of formats for S21, including Resistance,
reactance, smith chart, etc. So it seems we can get both X and R directly
in traces without any external calculations
However, for instance, resistance and reactance readings of S21 are very
big. For instance, Port A and Port B shortened by kit's cable shows about
1.2 M of resistance and -500K of reactance at 7.2M frequency (CW). Both
values are not stable and actually are infinities...
While phase value looks OK (-0.15¡ã) and complex representation is OK as
well (1-j0).
Probably I miss some basics in understanding of S21....
Would you please (1) suggest the best way of getting common mode choke¡¯s
impedance with the device and (2) explain the S21 readings in different
formats...
--
*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*
