Tuning only the antenna does not make sense, since the feeder has an additional influence. Specifically on the Heltec Lora Tracker board, this short feeder shifts the frequency with the lowest SWR above 900 MHz. Moreover, these distortions in the radio path also depend on the bending of the feeder. When operating at a frequency of 868 MHz, the range of the device did not exceed 500 meters. When I resoldered the board and got rid of the feeder, the communication range in the forest was already 10 km.

The standing wave ratio (SWR), the ratio of the reflected power in relation to what is going out. If half of what you sent was reflected that would be a SWR of around 6 which is appalling, reflected power of around 10% would be a SWR better than 2 which would be acceptable in most cases. SWR can also be expressed as a Log in dB (as a Return Loss), A Return Loss of 10dB corresponding to an SWR below 2 and a return loss of 3dB correspond to an SWR around 6, or 50% of the power being reflected. Reflections occur when the characteristic impedance of the source device differs from the load (in your case the Aerial). Radio devices tends to have a characteristic impedance of either 50ohm or 75ohm in most cases (not all). Standing waves can blow up your transmitter because 1/ the power that should be going to the load (Aerial) must be absorbed by the source and 2/ that standing wave can produce high voltages at the transmit output when the transistors generating the power are basically current devices not accustomed to high voltages. The Short and Open circuit is used to calibrate your device, these conditions are effective worst case. The Termination is as close as possible a perfect match (should be a dot in the middle of your Smith Chart) and your device will give you a reading for it at good as it is capable. You will not make a measurement for SWR/ Return Loss better than that Termination. The Smith Chart is a little beyond this video but basically represents the impedance of your load showing its real and imaginary components. The real being resistance, and the imaginary relating to capacitance and inductance. Looking at your aerial, then the closer the trace to the center of the graph the better (the marker 1 being the frequency you earmarked with the higher and lower range of frequencies you are scanning represented by the rest of the line), the center of the smith chart representing the characteristic impedance of your source. Just a Radio Amateur guy shouting at the screen :-) A career in SatComs helped as well. The numbers Z=x+jy are J notation which is the Rectangular form to represent a complex impedance. As an Angle and a Magnitude being the other way (Polar form). Looking at, calculation, and manipulating impedance's/ matching using a Smith Chart is by far the easiest way to accomplish, this though when you are learning about Smith Chart's you would be forgiven not to think so 🙂

It's not just the SWR, you have to keep an eye on the impedance. Yours was at 28ohms, so matched a bit wrong.

Rule of thumb : if the SWR dip shows up on a higher frequency , then the antenna is too short. In this case you had to try lenghten the antenna. But nice to see you gain knowledge 🙂 (ignore my comment, hadn't watched the whole video yet).

Consider updating the firmware on your NanoVNA. There are a bunch of UKposts videos that will show you the process. Then install the PC software. Once you're connected to the PC the software will allow you to capture and compare results.

It would be nice for you to show the difference between stock and tuned antennas. Range test maybe? Great video!

Yeah yeah yeah...but will it play DOOM?

Hi can you plase share a link for the 3D files ?

8,6257 cm is the length for 868 MHz.