The search for extraterrestrial intelligence SETI
What does radio have to do with SETI ? Here's the link: GNU (GNU = Open source software) https://www.seti.org/gnu-radio-and-seti
Just in case someone wants to look up.
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Based on the link you provided, it seems your RTL-SDR dongle provides built-in support for the "direct sampling mode". My RTL-SDR dongle did not have built-in support for direct sampling, so to experiment with "direct sampling", I had to open up the dongle and stick in a wire to connect an antenna directly to one of the pins on the internal chip.
Unfortunately, I found that direct sampling was really insensitive, such that holding a VFO directly against the antenna wire (a clip lead of maybe 30 cm length) gave only a tiny, barely-detectable blip on the SDR display, whereas we should normally expect a huge and overpowering peak of energy for a VFO held directly against the antenna wire.
That's odd that I had such extremely low sensitivity, because I recall you mentioned that your RTL-SDR (which, based on your above link, is using direct sampling) could detect the oscillation from your regenerative receiver: https://www.theradioboard.org/forum/main/comment/627dd9976143d50017711d7c .
Due to the horribly low sensitivity I experienced, I gave up on direct sampling and built an upconverter instead (a crystal oscillator plus a homebrew diode-ring mixer). This shifts all signals upwards by 49 MHz, so that for instance 7 MHz signals get upconverted to 56 MHz. This 56 MHz signal can then be received by the RTL-SDR as a VHF signal, with good sensitivity.
This upconverter works much better and is much more sensitive than the direct sampling, in my experience. However, there is still at least 6 dB of loss through the passive mixer. A better, though more complicated, circuit would use an active mixer chip with conversion gain, like the SA602 (which is probably several generations behind state of the art, but should work fine in this application).
I'd be interested in hearing about your experiences with an HF preamp, should you choose to get one.
Basically, GNU Radio software will allow you to write your own SDR application instead of using the Airspy software. You will have access to the raw IQ data stream from the RTL-SDR hardware, and then in GNU Radio you will need to drag and drop the various signal processing components to change the raw IQ data into audio, while applying any filtering or mixing operations that you want.