Is it possible that your coax is not really RG-58A/U?
I created an EZNEC model of a free space 340 MHz inverted vee. The element lengths and angle between them were optimized for a 50+j0 ohm feedpoint impedance at 340 MHz. The complex feedpoint impedance between 200 and 500 KHz was then calculated over 101 steps and exported to SimSmith.
In SimSmith, 15 ft of various types of coax were added. The results are interesting. (This could have been done in EZNEC, but I find it easier to use SimSmith for this type of thing.)
With 15 ft of 75-ohm RG-59, the FREQ vs SWR plot looks almost identical to yours. The lowest SWR is at 352 MHz which is very close to the 353 MHz point you observed. My plot is attached.
Here's a quick and easy way to tell whether your coax is 50 ohm or 75 ohm. Calibrate the NanoVNA at the port as you originally did. Now connect the antenna with your 15 ft of coax. Sweep from 200 to 500 MHz. Now display the Smith chart instead of the SWR plot. If overall, the plot circles are roughly centered on 75 ohms, the coax is 75 ohms. (The 75-ohm point will be to the right of the chart center by about 10% of the chart width.) If the plot circles are roughly centered on 50 ohms (at the center of the chart), the coax is 50 ohms. Two example plots are attached.
When you calibrate at the end of the coax, you remove its effect, and you'll get the same results whether you calibrated with 50 ohm or 75 ohm coax. The SWR plot will then look like the one you got just as it did with my modeled version. Plot attached.
73, Dave
NU8A
