If you want to monitor the Police in your town, you only need three things:
1. To look up which radio frequencies they use: https://www.radioreference.com
2. This $40 USB dongle radio receiver: https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/
3. This free software to decode their digital transmissions: https://github.com/DSheirer/sdrtrunk