@p @jeremiah

> How does someone stop you from doing it?

The same way they stop you from doing anything else, if they care (and if you, in turn, care that they care). The ham radio community is kind of like gun boomers, though. Lots of people who think it's their patriotic duty to snitch the moment someone hits the gray zone.

> SDR

The little RTL-SDR dongles are pretty fun, but the real fun comes from something like the HackRF or USRP, which also let you transmit (low amounts, you'd need a linear amp to boost it). Combine that with GNU Radio and really learn the modules and how to chain signals together, and you'll be listening to all kinds of weird signals. There are open-source modules for making your own cell tower. Michael Ossman has a great tutorial series on how to think about phase shift keying as a 3D spiral like a slinky.