Yesterday evening I left the Juma TRX2 connected up to my MFJ magnetic loop logging PSK31 spots to Propagation Reporter. This morning I switched to 20m and re-tuned the loop to 14.070MHz and saw some clear vertical lines at 100Hz intervals on the waterfall.
My first thought was "mains hum". I am not using transformer-isolated audio connections from the radio to the computer, though the truth is, I never have done so in 10 years of data modes operation, and the only time I did use an isolated interface it actually caused hum due to the laptop computer "floating", having no ground connection through its power supply.
I de-tuned the loop so I could see the hum lines more clearly and was surprised to find that they reduced in intensity along with the signal. They were definitely audio-derived, though. When I moved the Juma VFO the PSK signal traces snaked about but the thin hum lines stayed put.
I hadn't noticed these hum lines on 30m. It appears that they peak up when the magnetic loop is tuned to 20m. But only on the Juma. When I connect the loop to my Elecraft K3, these lines are not present. And when I connect the multiband dipole to the Juma, they are not present either. So the MFJ loop is somehow picking up strong mains hum when tuned to 20m.
I tried grounding the Juma (which doesn't have a ground terminal) but it didn't make any difference. So I'm somewhat stumped at the moment. I know the Juma TRX2 is a direct conversion receiver. And I vaguely recall reading that direct conversion receivers can suffer from hum problems. But I don't know any more than that. Finding out why is one more item to go on my frighteningly long ham radio "to do" list.