I really enjoyed making antennas and capturing data from satellites.
Getting trunking receivers (eg unitrunker) to listen to local first responder radio.
Decoding pagers is fun
As others have said, decoding various devices that talk on the various ISM bands is interesting.
Just band surfing at night is a lot of fun imho. Finding random signals and trying to figure out what they are is cool.
Unlike a traditional scanner which only monitors one frequency at a time, if two things are happening simultaneously then you hear them both. This would be annoying or unlistenable in a busier area, but in my area it works fine.
If I were to do it over, I'd likely want to optimize things quite a bit to get the sensitivity up.
The amazing thing is, though the RTL-SDR dongles are only 8 bit devices, when you're down-sampling 2Mhz of them to audio frequencies, you gain quite a few bits of resolution. I routinely pick up signals below the quantization threshold.
I wonder what the lower bandwidth limit truly is, assuming a stable reference clock.
I bought both a HackRF and an Ettus USRP B210 years ago for playing with.
I am yet to actually do anything of interest! Anyone want to share anything cool they’ve done in the SDR world?