Next I measured the insertion loss of a few cables. Their lengths were also noted along with the connector brand (if known).
At 1GHz its 14.7dB for 100' or 0.91dB for our 74inches. Which is very close to what we measured with the added loss of our adapters and connectors.
So it seems like the liteVNA can produce some usable quantitative results.
I tried to reproduce your insertion loss measurement with my test setup. The liteVNA was warmed up one hour, and then calibrated with the supplied cables and standards from 100k-6.3G, 1kHz IFBW, ideal model. I did not go all the way up to 9G, because I stumbled upon a strange calibration artifact, which I will try to show later. Then, I connected one of my cheap SMA-M to BNC-F adapters to the double SMA-F adapter at the cable on port 1 and a SMA-F to BNC-F adapter to the cable at port 2. The DUT cables then went in-between the BNC-F adapters.
As cables, I had:
RG58 C/U MIL C17F, 107 cmDatasheet: 1GHz 51.8 dB Attenuation dB/100m -> for 107 cm : 0.55426 dB
Belden H155, 91.7 cmDatasheet: 1GHz 29.6 dB Attenuation dB/100m -> for 91.7 cm : 0.271432 dB
This is what I got:
The image shows two measurements for each cable in the 100k to 1G range. One measured with 801 points from 100k to 6.3G and one with 801 points from 100k to 1G. The one with the denser sampling showed a large peak at the beginning going up to about 5.2, a strange small disturbance at 100MHz (which stays there, even when varying the sweep parameters) and some periodic superimposed oscillation. Unfortunately, the measured insertion loss values are not even close to the theoretical values I listed above. Maybe I messed up something with my measurement setup or my SMA to BNC adapters are too bad. At the moment, I do not have sufficient materials to test the insertion loss of only the adapters.
I also got myself some PCB mount BNC-M connectors to experiment with building BNC calibration standards, like described
here. It seems those are only available from china manufacturers and no known quality manufacturer makes those. The first try was a complete and perfect failure.
The springy outer conductor of the connectors is too small in diameter by about .3mm and does not make good contact to any mating BNC-F connector. So, the connectors themselves are really bad. Nonetheless, I created an open and short by filing down the center conductor and then soldering a small tinned copper disk for the short, just for the fun of it. The result was smith chart that looks like Indiana Jones whip after calibrating. For open, short and load (even below 1GHz) the smith chart does not even get close to where it is supposed to be and jumps all over the place when just looking at the connector. I really wonder how the guy from the website linked above managed to get anything out of these connectors, but it may also well be my soldering skills.