Nice! I built the EMF detector on a raspberry pi zero but never thought about using an RTLSDR dongle for RF. Can you go into any specifics about what modes and capabilities it has?
Sure. I'm using a Pi 3B and a NooELEC Nano. I'm still running Raspbian on it, so I have some upgrades to do. So far:
- Can specify 0 for squelch, which I suppose is kind of the barest minimum
- Can specify hop mode: forward sequentially, reverse or random hops
- Can configure delay between hop (including setting it to random)
- Can specify one or multiple frequency ranges to hop, and specify the gap between frequencies when sweeping (ie, even 12k)
- Can pipe output through sox for real time filters. Dynamic noise reduction can create some damn weird effects
- You can save the feed to disk at the same time you're listening
- You can pipe output to an icecast server, so multiple people could be listening at the same time streaming to their own phone or whatnot
- Added a RTC module so the damn thing could keep date/time correctly
Random hop + random delay returns some really interesting results. You wouldn't think you'd hear anything but gibberish, but weirdly enough you can hear complete phrases. It is 100% command line right now, but I am working on using a small screen for changing modes, etc.
Currently using MJAWS bluetooth earbuds. They're decent at noise cancelation.
I have it connected to a 10000mah battery, so it's completely mobile and fits in the palm of your hand. There's an external antenna, and the way I have it right now when you're holding the entire thing, your hand wraps around the antenna which I kind of like (using your you body as an antenna). I'm using a Pi 3B instead of a newer Pi 4 because the 3B still has a normal headphone jack. So if the earbuds go dead or whatever, you still have the option to plug in.
There are a few drawbacks I've run into so far:
1) If you want handheld and mobile, you're probably only going to be able to do FM. The dongle I have has direct sampling, so theoretically I can do AM without an upconverter, but I've never been able to receive AM without using a Ham it up converter. I suppose using one of their nano upconverter in a proper case could work. I mean, you CAN sweep AM, it's just unlikely you'll get much of anything unless you live in a city.
2) It probably can't sweep as fast as a an actual radio like a SB7, but it's pretty fast so I'm not sure if it matters. Sometime soon I'll record a minute or two at the fastest hop and post it.
3) SDRs get hot. The newer/better ones are specifically designed to distribute heat to their metal cases. This creates some design issues, especially when designing a case. It also makes adding other sensors like temp and humidity kind of pointless, because those dongles put out a lot of heat after they've been running a few minutes. The one I use now just has the Pi in a standard black case, with the battery underneath and a piece of velcro holding everything together. Not very elegant, but I don't have to worry about the sdr dongle melting my case or something.