So i used my nanoVNA to see if using its VSWR function will show anything usefull.
It does show the classic dipole response, which is very useful for determining its electric length, but I think there might be a problem with the OSL calibration of your VNA.
I did not pay attention to it before, but now I will look into it.
First of all, the excitation loop must be connected to the VNA during all three stages of the calibration: open, short and load.
Because the DUT is located inside the excitation loop, only the conditions inside that loop must change during the three calibration stages
When the exciation loop is empty (i.e the Stalker's coil is absent from it), that must be the OPEN condition for the VNA. In that condition the calibrated VSWR should approach infinity and │S11│ - unity. Is that true after your calibration ?
You may ask why ? A: Because there is no current flowing at the midpoint of the Stalker coil when that coil is entirely removed from the loop (obviously). The excitation loop does have current flowing in it, but you are not interested in it because you are trying to measure the current flowing in the midpoint of the Stalker coil - not in the excitation loop.
The SHORT condition would be equivalent to entirely filling the excitation loop with a copper cylinder
*, because that simulates the lowest impedance that this loop can achieve. In that condition the calibrated VSWR should approach infinity and │S11│ - unity, as well.
Finally, the LOAD condition would be equivalent to filling the excitation loop with a slotted copper pipe
* that has a 50Ω resistor
** across that slot.
Sorry to complicate it so much, but these are the consequences of using an excitation loop and calibrating it properly. I only hope it makes sense.
*
or at least a copper pipe or disk or ring**
The impedance of an ideally matched dipole is 73Ω+43j so a 73Ω resistor would seem better, but Stalker's coil is closer to a helical antenna, which has a different impedance when matched. I do not know what that impedance is for a low pitch helix antenna like this. It would be interesting to determine that matching impedance by seeking the lowest VSWR when fed with 1/2 wavelength resonance frequency.
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Topic: Dally, Shark & Ruslan workbench (Read 222351 times)
