A couple of points.
- You did not say how you connected your antenna to the NanoVNA. If you connected with a piece of coaxial cable and did NOT calibrate at the end of the cable you are NOT measuring the feedpoint impedance of your antenna. If you just calibrated the VNA at the SMA terminals and then connected the coax you are measuring the impedance at the input of the coaxial cable. The coaxial cable will be an "impedance transformer" and the impedance the NanoVNA measures will not represent the impedance at the antenna so reactance = 0 does not mean the antenna is resonant. However you will know the "approximate" resonant frequency of the antenna when the SWR is at a minimum.
- The dimensions of the antenna you gave will result in a resonance that is higher than 8 MHz. The resonant frequency will vary depending on the height off the ground and the ground conditions at your location. The feedpoint impedance will also vary considerably especially the closer you get to ground. I ran two simulations for you in EZNEC using 8M elements. One was 8m off the ground and the other 1M. Average ground conditions for both. Note that when reactance = 0 the antenna is resonant by definition.
Roger
Roger Need
