How do you receive HF signals using your RTL SDR? You’ll probably say that the easiest way is to get an upconverter. That’ll certainly get you the best quality results. You might also suggest modding the hardware itself. This also works, but can be tricky if you’re not used to soldering.
There is a third way, however, and that is to hack the software so that the RTL-SDR dongle thinks the signal is bypassing the tuner on the dongle, as is what happens with the hardware modification.
You can get your RTL-SDR to go into direct sampling mode by simply using a modified RTLSDR.dll file. Full details on where to download the file and some extra explanations are available here on the www.rtl-sdr.com website.
So how well does it work? Well, signal quality isn’t great using this method compared to hardware mods or using an upconverter. At the moment, I can really only get “strong” signals from 7000kHz to about 7500kHz. The bad quality and limited range may be in part due to the antenna I’m using: about 1m of wire wrapped around the stock antenna, hanging vertically from a curtain rail inside a bedroom. I will have access to a better outdoor longwire at the weekend and I hope to do more testing then.
But here’s a sample of what I can hear using this basic setup. I managed to get China Radio International on 7405kHz and 7415kHz, and you can listen to them here and here, respectively. Note how the quality in the first file (7405kHz) increases substantially at around the 1 minute mark.