You would do a bit better to cap the second port with the 50-ohm load.
S21 or S12 are not involved in what you are doing. Any reading is due to
crosstalk and lack of shielding and proper decoupling between the ports.
When I first encountered S-parameters long ago when I was long from
fossilizing, here is how I envisioned the S-parameter functions:
S11: This port is the source port and provides the RF energy to make
measurements. What is measured is the effect of what is connected to the
source port (the EUT) on the source port connection. This is a measure of
match to the impedance of the source port, only, of what ever is connected
to that port. In simple (and bending the definitions just a bit): The
effect on the source port (port 01) of what is connected to the source port
(the load), resulting in the 11 designation.
S22: The reverse of S11 measurement and represents the match to the
impedance what ever is connected to the sink port. By the same argument,
resulting in the 22 designation.
Both S11 and S22 require only single port calibration.
S21 and S12 are both transmission measurements. Both require a full OSLT
calibration.
S21: The impedance/gain/loss, ..... at the sink port measured at the sink
port with the EUT embedded between the source and sink port. Many times
referred to as the forward transfer coefficient. The 21 notation comes
from measuring the forward transfer coefficient at the sink port (the "2")
with the EUT connected between the source (the "1") port and the sink
port. Therefore, the 21 notation.
S12: The opposite of S21. The measurement at the source port with the EUT
embedded between the source and sink port. This is referred to as the
reverse transfer coefficient. Measures from the source port ("), the
effect of what is connected to the sink ("2") port.
The directionality will become clearer by thinking the "zero" port as the
source port and the "01" port as the sink port.
Hope this helps and not adds to confusion.
Dave - W?LEV
PS: If you ever deal with the Z, H, Y, or other matrix configurations, the
notation is the same. First row in the 2X2 matrix: 11, 12. Second row in
the 2x2 matrix: 21, 22.
Dave - W?LEV
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On Tue, Jun 2, 2020 at 12:52 PM <wsanders@...> wrote:
I am using the NanoVNA-F to tune a 75m mobile antenna (see picture.) There
is an interesting pattern on the S21 trace. The VNA has been calibrated
only using SOL (short-open-load). I have S21 "capped" with the "open"
calibration accessory.
What does the S21 reading mean in this context, if anything? Is this just
signal "leaking" back into S21 (for lack of a better term.)
This is a great tool! It made short work of telling me this antenna (a 75m
Shark hamstick-clone) was wound with a little too much inductance, and the
only way to match it was with a cap shunt across the base.
The F variant is pretty nice, it has an big touch screen, you don't really
need to use the wheel. The screen is very similar to my Arduino touch
screen shield. It has a monster battery (500maH allegedly), big enough that
the makers added a USB port solely for using the NanoVNA as a charging
source. I bought mine with a "demo kit" with various components to show how
real circuits respond to calibration errors, etc. One drawback so far is
that the USB port is a "C" port, and it doesn't speak the USB-C protocols
required for "To-Go" signaling, but I odn't really have a need for tha PC
or phone apps.
--
*Dave - W?LEV*
*Just Let Darwin Work*
CT2FZI