Gregory's Servermost likely it is a difference in dielectric constant, slight dimensional differences, and the coaxial cable interacting with the VNA calibration. i calibrated to the end of the blue cable but this doesn't account for the thin coax. the antenna doesn't have a matching network either which will have an impact.
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  figured out the problem! I neglected to include the thin coax in my calibration. it's much better now, I've even restored it nearly to the original length, and the impedance match looks better as well. open and short at the end of the coax was easy but for the 50 ohm load I had to solder this tiny smd resistor. another way is to calibrate to the end of the SMA cable and use the "electrical delay" setting. this didnt work well for me. I'm used to larger, more professional VNAs that let you adjust it with a knob and watch the Smith chart in real time. the NanoVNA makes you play a game of Guess-The-Number... @tubetime you should be able to measure the electrical length with the far end open (or shorted) by measuring the phase and halving it. Would be nice if the tool could do it for you though! I have some QoL ideas like this I keep meaning to try implementing in my nanoVNA after using it at work a bit @Darius yes it might actually be good enough to do it that way. i'll have to experiment some more. @dtelder it's 50 ohms but it is longer, so the calibration plane needs to move. @tubetime @dtelder i'm not 100% sure myself about why it caused a frequency shift, but you may be right about it being part of the antenna. the ferrite beads along the thin coax are supposed to prevent RF currents from flowing along it, but it's not perfect. |
advanced mode.