Many thanks to Ed B. for bringing up this interesting topic, and to Ed L. and Bruce for their excellent thoughts. My own approach is a little different, though. I use amateur-grade solid-state receivers exclusively (non-SDR) for my long wave listening above around 30 kHz, mainly with a 40-foot vertical mast, with great success. The radios have include unmodified Kenwood R-5000s, an ICOM R75, and more recently the receiver section of a Kenwood TS-590sg. The latter employ an OCXO and TCXO, respectively, to ensure good stability for working with slow digital modes.
I seldom use an upconverter, except when listening for actual VLF signals (those under 30 kHz, where none of those radios tune) or sometimes low LF on certain radios that exhibit greater local oscillator phase noise or internal spurious signals below 60 kHz or thereabouts (such as my R75). For aerobeacons, LowFERs, hams, time signal stations, etc., I have no need for an upconverter at all. When I do use one for the "basement frequencies," it's a custom built unit that has an excellent LPF to keep out broadcast signals that can otherwise produce intermodulation spurs, and it features a low noise, ovenized crystal oscillator to allow tuning very near to 0 Hz and to minimize drift and frequency display errors. Those errors will be far greater at 4 MHz than, say, 200 kHz.
So, having said all that, how do I reconcile my results with my endorsement of what Ed L. wrote? Simple!
The key is one seemingly mundane, too self evident to be important, plain looking, but actually extremely crucial sentence in the middle of Ed's post. Namely: Longwave antennas need impedance matching to the 50 Ohm input impedance of your receiver.
If the board software would let me, I'd put that in large gold letters on a midnight blue background. That one fact alone accounts for nearly all of the "poor LF performance" complaints one hears about otherwise very capable modern desktop radios. For solid state, 50 ohms means 50 ohms, and no practical wire antenna is going to come anywhere near that at longwave! By actual measurement, my old faithful R-5000 suffered 30 dB sensitivity loss due to antenna impedance mismatch alone at 185 kHz, and worse than 60 dB at 60 kHz. I improvised an antenna tuner, but continual re-tuning was a pain.
How come most upconverters and so-called active whips don't seem to suffer this dreadful deafening fate? Same reason a lot of the classic tube LW radios didn't: they make no attempt to present a true 50 ohm load to the coax!
There are, however, plenty of good technical reasons to both source and terminate a transmission line with the correct impedance value, most of which become a bigger deal where solid state devices are concerned. For that reason, I built a two-transistor buffer amplifier which I use as a wideband impedance matcher which is high-Z at the antenna terminals yet drives the coax at a very comfortable 50 ohms. No voltage gain is needed, merely current gain. This was a design I published in The LOWDOWN back in 2012, intending it to be a three-part article starting with the simplest and cheapest Radio-Shack-parts-only design and evolving toward an only slightly more complex but more universally applicable version. I completed the second version, which proved so satisfactory in my low-RF-pollution environment that I allowed myself to be sidetracked and never proceeded to the third part, which currently consists only of hypothesis and blather in an unfinished "Part 2.9"...but I'll be glad to separate the parts, trim the fat, and re-post them in the LWCA Library this week if there's any interest.
John