If you want to receive digital trunked radio signals (like many police and fire departments use) for well under $50, this video is for you. I'll walk you through setting up an RTLbased SDR and then installing the open source sdrtrunk software.

PLEASE NOTE: The inexpensive RTL SDR dongles have limited bandwidth. As a result, you may find that you are not able to follow all trunked channels on your system of interest since the software has to keep the control channel within the bandwidth of the dongle. If you see debug messages about "No tuner available" for some of the trunked channels while listening, this is likely your issue. There are a couple of ways to fix this. One is to add additional RTL SDR dongles to increase the spectrum the software can consume. Another is to use a more powerful SDR receiver, like an SDRplay device or similar, BUT to make an SDRplay receiver work with the sdrtrunk software requires extra (pretty technical) steps not covered in this video.

Links referenced in the video:
https://github.com/DSheirer/sdrtrunk
https://jdk.java.net/archive/
https://zadig.akeo.ie/
https://www.rtlsdr.com/