Having used a rtl-sdr V3 usb dongle for a number of years now, I was curious to try out their new sdr V4 model. The V4 has redesigned circuitry from improved HF performance. The device specs show that the unit works down to 500 khz. Of course, I couldn't resist trying it out below that lower listed frequency.I was pleasantly surprised to see that it would indeed give results below its rated specs. My first stop was the 630m band. I set it up to decode wspr as it's handy to have a ready reference of signals that print out how well you're "hearing". I ran two instances of wsjtx... one fed by the rtl-sdr V4, the other fed by my Airspy HF+. Both sdr's were tied to the same antenna using a splitter. I found that the rtl-sdr was very close in rx performance.. just a few db down from the more expensive airspy.
Encouraged, I tuned down lower and familiar beacons were visible and easy to copy. Tuning much further down in freq, WWVB looked good. Tuning even lower brought the "big 3" military stations in quite well at roughly 24 khz. So, the V4 did surprisingly well below its rated specs. I would have liked to try decoding some Navtex at around 518 khz, but at the time of test it wasn't available. Since 630m was a success, I'm sure a Navtex session would work out well. Bottom line is that I'm really impressed with this very inexpensive sdr.
Caveats ... a simple device like this is prone to overload. Care must be used to use the right amount of gain from antenna to PC. You can't just throw any antenna into it and the gain controls in software need to be carefully adjusted to obtain the best results. For me, the V4 is something to test and compare its results to the more expensive sdr's. It's well worth the money and is a nice tool to have in your toolbox. It can do much more than just lf-hf monitoring.
Btw, the antenna used for this test was a L400b e-probe... a very good antenna for this V4 sdr device.