You should test your cables regularly. Some may go bad, especially cheap or older ones, with poor connectors or breaks.
I use a lot of short connectors to hook up SDR receivers and antennas. Many of these are short lengths, often purchased online for cheap. As shown above left, these may have BNC, SMA or F connectors. I have RG-58, RG-59, RG-316, even RG-6. Many are salvaged from old CATV installations over the years.
I find that the impedance mismatch between 50Ω and 75Ω cables does not really matter much for HF receiving. Of course, for transmitting, I use proper RG-8 or similar cables.
But what does matter is the quality of the connection. Over time, breaks can occur in the shield. And, with twisting and bending, the connectors may work their way loose. So, once a year, I test all my cables for continuity and intermittent performance. I just did my annual test.
Guess what? Roughly half of my cables produced intermittent results. With a slight twist or bend, many would stop working properly. This is especially so for my older salvaged CATV cables, which I use with F to BNC adapters.
I find that a good way to test your cables is to hook them up to a vector network analyzer, in my case the wonderful little NanoVNA. One end hooks into the VNA, the other into a 50Ω terminator, as shown upper right above.
Test Your Cables and Avoid Surprises
When you run your NanoVNA, you should see a smooth impedance trace at approximately 50Ω , as shown above, from 1 to 30 MHz. Your trace should not change as you wiggle the cable or its connectors. If you see some weird results or discontinuities, then you know that you have a bad cable, or at least an intermittent one.
VSWR should read close to 1:1 and the time domain test should indicate the length of the cable under test with some accuracy.
Sadly, now it’s time to replace about half my cables in use.