Note: more measurements added after graphs below
I recently acquired a RigExpert antenna analyzer and I am trying to learn how to use it to test cables. I took two cables to start with to try and understand what it shows and what the graphs mean.
- LMR-240, 7.9m long (2 months old)
- RG-8/U, 18.3m long (>25 years old)
I thought I would use these two cables to learn how this works. I ran the TDR (Time Domain Reflectometer) sweep on both cables, with an open circuit at the far end. The LMR-240 matches one of the book's SR (Step Response) graph samples, the RG-8/U does not match any of the samples. The book doesn't really explain much about what they mean and I haven't been able to find anything online that explains how to interpret the SR graph.
I get that reflections show up as peaks on the IR graph, is that it? Since there is an open circuit at the end of these cables, I can see the electrical length from the IR graph, about 6.5m (21 ft). The physical length is 7.9m (26 ft), so the velocity factor is 6.5/7.9 or 83% for the LMR-240.
What does the SR (Step Response) graph actually display and what does it mean?
I don't understand what it means when the SR goes negative on the RG-8 cable. The cable is inductive? I tried testing from the other end of the RG-8 cable instead and I got the exact same SR graph, so it's not that something is wrong with that end of the cable. I'm thinking the capacitance of the dielectric is low, making the cable more inductive, messing up the cable's impedance, but that's just a guess based on sample graph for an Inductive Terminated cable.
The detail data for the RG-8/U shows that the impedance is only 20.8 at 16.96m down the cable. Obviously it should be 50 ohms, but perhaps the dielectric is dried out or something like that is bad.
* New Information *
I measured the characteristic impedance for the RG-8 cable and it comes out to 32 ohms, off from the expected 50 ohm impedance.
I also reran the TDR scan using the AntScope2 software (PC software provided by RigExpert) and the graph is quite different. First the IR graph is the same, but the SR graph is different, instead of going negative it stays flat. A more normal graph similar to the LMR-240 cable's graph. The odd part though is that it shows the cable end at 4.2 meters instead of at 18.4 meters. For some reason AntScope2 thinks the cable is much shorter.
SR graph samples: