I had to actually look ho this connector style. Here it is on YouTube.

https://youtu.be/eplckCGbRPc?si=A1AG6APIZZSqB0d_

Wikipedia says it is not a well controlled impedance, and there are no standards for this type of connector

https://en.wikipedia.org/wiki/UHF_connector

Maybe a TDR measurement could shed some light on performance?

Steve

Steve's TDR suggestion is the right starting point, and I would add why it works here.

PL-259 / SO-239 was designed for HF and never had an impedance spec - the nominal 50 Ω is really an average, not a controlled value. The interface geometry puts a fair amount of series inductance in the shield path plus a capacitive step at the mating plane. At 30 MHz that is invisible. By 400 MHz you can see the mismatch on a VNA.

A TDR makes this concrete because you get impedance vs. length, not just vs. frequency - you can see the inductive region at the connector and the capacitive step at the mating interface, and how long each one is. From there you can estimate what fraction of a wavelength each discontinuity becomes at your frequency of interest, which is really the question that matters.

Practical notes if you measure it:
- Keep the launch coax short and use the same connector type on both ports, so you are characterizing one interface rather than two in series.
- Set the reference plane right at the connector, not at the cable end - otherwise the cable loss and delay blur the connector response.
- Expect the ham-radio data you found to be all over the place; most of it was taken without controlled launches, which is probably the methodology issue you sensed.