What do normal people use SDRs for? I'm genuinely curious. Or is it just cool to do?
Mostly it's just fun. I like peeking at all the ambient transmissions in the area.
A while back I was more deeply involved in the SDR space and even reverse-engineered some pager signals. At least a few years ago, pagers were still widely used in hospitals (totally in plaintext, by the way).
This always strikes me in the "oh no look at all the cheating!" anti-ai takes.
Like, the cheating only blew up because there was/is the gap in teacher/professor/moderator understanding of the tools. Kids were copying wikipedia back in the day, and if the kid had a decent vocab, went completely undetected for a time. Then everyone thought they could do it and scorers got suspiscious enough to create a demand for an "is this wikipedia" software such that companies made them and scorers implemented them. We even saw reversions to written exams, and have had continuous use of written exams in 'critical' tests like SAT et al.
And of course, the only reason that the teacher's ensure that the students aren't cheating is because they know the students are heading for tests they can't cheat. So, there's new cheating, there will need to be new detection.
A motivated learner today has unprecedented capabilites compared to a motivated learner of even 10 years ago.
Assuming, of course, that the learner is practicing universally accepted study/research methods such as never implicitly trusting single sources, especially when they're third hand summaries of unverified sources.
I've had proffesors give me incorrect information with every bit of certainty an LLM offers, but it wouldn't be their fault if I just nodded along with either of them.
Everything in SDR has an analog equivalent, from local oscillators to PLLs to filtering. In fact most of it simply is a direct implementation of decades-old analog concepts.
But the underlying math is what I found most incredible, where the "true" representation of a signal is complex, and that every real (both in the ordinary and mathematical sense) signal is actually the sum of a positive and negative frequency, such that the complex component cancels out. Phase becomes much more comprehensible as simply the angular component of the signal, and frequency is the derivative of phase. All of this had the indirect effect of improving my understanding of complex numbers as well.
Claude is surprisingly good at mapping out complex multi-repeater trunking systems, and identifying DMR vs P25 vs NXDN signals, so even if you know nothing about digital radio, you can be listening in minutes.
With some help, it downloaded the fcc licensor databases and ingested them into a SQLite database, which allowed it to identify voice frequencies that correspond to an identified control channel.
You can then use tools like dsd-fme to decode and playback digital voice channels and scan entire trunk systems. I’ve been having a blast lately and learning a lot.
The RTL SDR is the gateway drug into SDR's. The bandwidth is limited, so don't expect to be capturing HDTV Streams. But it's certainly possible to listen to AM/FM, Ham Bands, VHF/UHF signals, etc.
If you don't want to buy an RTL-SDR, you can search KiwiSDR which is a streaming SDR for frequencies under 30MHz. You can also connect to KiwiSDR's worldwide.
Then if you get the itch, you can go get your a HackRF, a ham license, or whatever else scratches it.
Stick an RTL-SDR dongle on your computer and use the program rtl_433 to monitor that traffic. It comes with built-in decoders for the protocols for a very large number of those sensors.
I've got such a setup and I can see not only my one sensors (An AcuRite outdoor thermometer, and the two sensors from an AcuRite fridge/freeze thermometer) but also a few neighbor's temperature sensors, humidity sensors, soil moisture sensors, wind speed and direction sensors, rain gauges, and their tire pressure.
You can set up MQTT and have rtl_433 report the data that way, and you can have Home Assistant get that and display it. It knows how to handle a variety of sensor types.
I haven't done that yet. I just have rtl_433 log everything by appending JSON records to a file. A script reads that, finds the sensors I care about, and stores their data in an sqlite database.
All via a tiny web server in an ESP32 with a radio attached. And the parts I bought for decoding the signals were cheap and let me replicate the process easily. Which reminds me I should push the Claude skill I created for doing this up into that repo. I even had Claude build me a Streamlit app to teach me how to read the signals. https://ook-signal-explorer.streamlit.app/
Ham radio operators looking forward using them for 13cm and 5cm band stuff.