idk how you have done that, maybe i have older version but for me this marker just reads CH0 SWR 1.00/(value)
you can pull up a smith chart, this will tell you whether impedance is too low or too high, since it’s still not matched at resonance
Comment on NanoVNA vs. Loop antenna SWR testing [Question]
einfach_orangensaft@sh.itjust.works 5 days agoThank you for your answer!
I think i found the root of my problem, the scale settings on the NanoVNA where wrong, to a point where the super slim SWR dip of the Loop was just not drawn on the graph for lack of data points at this point. I changed the scale and was able to see the dip, then tune the coax stub to the frequency i wanted. Now it claims to have a SWR of 3.8 near the frequency i want to use:
But i am still a bit confused, the yellow number left of the SWR reading, i assume that is the scale? or does it mean a SWR of 6.125:3.8?
fullsquare@awful.systems 5 days ago
LH0ezVT@sh.itjust.works 5 days ago
All good points, especially calibration. Without, it still gives you a general idea, but it’s more a “meh looks OK” than a serious value.
Also, as fullsquare said, take a picture of yhe smith chart / impedance display as a second check, if it shows something like alot + j toomuch, you know your SWR is actually bad, and you know what direction you need to go for (add more capacity, inductance, or some resistive transformation).
fullsquare@awful.systems 5 days ago
i’ll add that in a way SWR chart is more resistant to misuse, because if nanovna is calibrated with wrong length of 50 ohm feedline, or without feedline at all, then smith chart will be rotated by angle depending on difference in length of that feedline, while SWR chart should look the same. for example, if real part of impedance at resonance is too low (ex. 20 ohm), and feedline is quarter wavelength different from what nanovna was calibrated with, then impedance will be still real but too high (ex. 125 ohm), while SWR chart should look the same (1:2.5 SWR minimum) (barring losses in feedline). (this works the same way as quarterwave long feedline impedance matching scheme). for different feedline length differences (non-multiple quarterwave) impedance will be complex at antenna resonance. this problem is avoided by calibrating nanovna with feedline
einfach_orangensaft@sh.itjust.works 4 days ago
Here is the measurement with smith chart:
There is 2m of BNC RG58 coax between the antenna and the NanoVNA. I calibrated the NanoVNA without the coax line because i was under the impression the line to the antenna is considered part of the radiating system and hence should not be calibrated out. But here is the reading with the coax line to the antenna calibrated out:
LH0ezVT@sh.itjust.works 4 days ago
I’d argue that your capacitor and loop are OK (otherwise there would not be a dip at 27 MHz).
But my guess is that the match to 50 Ohm is bad. Try playing around with your feeding loop.
You can ignore the cable if 1) it is properly matched to 50 Ohms on both sides and b) there is no field outside of the cable, meaning the cable does not act as an antenna itself.
You are clearly working on the 1) part, and for the 2) part, it depends on how you feed the loop, unbalanced vs balanced. For coax you either need an unbalanced feed loop, or a balun (“balanced-unbalanced” converter) or a ferrite or similar “Mantelwellensperre” (don’t know the English word, sorry).
fullsquare@awful.systems 4 days ago
Yeah this lower one looks better but still probably your capacitor value in loop is way off, try to find frequency where impedance is real (purely resistive; green line on smith chart crosses horizontal line in the middle) and work from there, then you’ll know whether to increase or decrease it. resonance is narrow so you might miss it. there’s a reason why magloops are made with variable capacitors (sometimes retuning is required due to changes in ex. humidity)
LH0ezVT@sh.itjust.works 5 days ago
Second number should be SWR, first one scale (value change per horizontal box). So that would be 3.8 SWR, which corresponds to about half a box of 6.1 - 1, because the lowest you can get is 1.