Kurt wanted to see some pictures so here they are.
The first picture is the dB logmag of S11 of a 30cm semi rigid cable (medium) and a 1 meter RG58 cable (long), both Open and Short at the end, both from 0.5 MHz till 900MHz
When looking at the polar picture it is obvious the wobble of the logmag at lower frequencies is caused by a shift of the center of the open/short rotation as the lines do not cross (see third polar plot of the long cable 0-300MHz)
In the second picture a 106fF Open C0 correction is applied (as automatically calculated by Octave). This shifts the rotation just enough to remove the crossing of the Open/Short logmag
When I let Octave calculate a cable impedance correction (assuming a small mismatch between the cable impedance and Z0) I get the fourth picture shows almost no wobble. The impedance correction is a shift in the order of +0.025 + 0.5i ohm
For know I am running out of ideas on how to proceed as I do have no good model for the semi rigid cable
Anyone that knows how cables behave in a VNA?
Erik, PD0EK
