VNA screenshot. It’s all over the place.

Here’s a puzzle. This is a Diamond X510 antenna that should tune perfectly across 144-146 MHz. Here the dip is around 144.800 MHz, and it’s very narrow. SWR at 145.500 is already 1.8, and our TRXes are complaining.

I’ve seen this before, so there’s the answer: it’s been accidentally hooked up with a 75 Ohm cable.

You can spot this quickly by the wavey pattern of SWR - if the cable is terminated with a nice tasty 50 Ohm load, it will oscillate exactly between SWR of 1.0 and 2.0. With an antenna the pattern will be less exact.

To calculate the exact characteristic impedance of the cable:

  1. Connect a 50 Ohm VNA and a 50 Ohm load.
  2. Measure the impedance at the low and high SWR points (hills and valleys of the wavey pattern).
  3. Calculate the geometric mean: Z = √(Zlow × Zhigh).

If you only have antenna connected, not all is lost. If your antenna is resonant (or mostly resonant) across some range, measure the low and high impedance in that range. It won’t be as precise, but should be good enough.

PS. The best way to prevent this kind of mistake is with your eyes. 75 Ohm cables have a much thinner center conductor.