How have you terminated the coaxial cable length under test? Just
measuring S11, I would not recommend using Port 02 as a termination, but,
rather, the 50-ohm SMA cal standard. The impedance of Port 02 over
frequency is not necessarily known. It might also be instructive to
include an open and short test of the cable under test. Certainly, the
'174 should show additional loss over the 'good' cable lengths.
Just another thought: Above 300 MHz, the noise is very noticeable
greater. This shows especially in the tests other than the cal setup,
picture "0". The FW within the NanoVNA may not have the dynamic range to
pull out good data for your tests. An open test is especially good at
detecting loss.
The position of the peaks and valleys is only a reflection of differing
lengths of cable and, therefore, phase differences as measured against the
cal. standard.
Dave - W?LEV
On Sat, Nov 23, 2019 at 1:46 PM Norbert Kohns <norbert.kohns@...>
wrote:
Hi all,
today I performed a NanoVNA calibration test to find out if the coax cable
length and quality has any negative influence specially on the S11 return
loss.
The calibration setup is 10kHz-900MHz at 808 points.
For the test I used high quality coax cable (M17/152) and standard coax
cable (RG174U).
The reference test was done directly on the NanoVNA port connectors. For
the thru test a 5 cm long semi rigid coax cable was used (picture 1).
Next, a 20 cm long M17/152 coax cable was tested (picture 2).
Then, a 93 cm long M17/152 coax cable was tested (picture 3).
And last, a 70 cm long RG174U coax cable was tested (picture 4).
Picture 5 shows the test result for the RG174U coax cable for S21.
All test conditions have been repeated. The differences between the two
tests were negligibly.
One can see, that the coax cable length has some negative impact on S11,
specially with the 20 cm long coax cable.
The quality of the coax cable plays obviously a minor roll.
Do the experts among us have an explanation why this is as such?
Best regards
Norbert, DG1KPN
--
*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*
