//What I learned about Software Defined Radios in a few days…

As usual, it has been quite a while since I’ve posted anything to the site, but I figured since I spent some time learning about Software Defined Radios (now that I finally got a bit of down time from work) that I’d share what I learned.  One caveat, I did have a purpose in learning more about SDRs and that was to be able to listen to and record all the Chicago Police and Fire dispatch radio traffic on the old analog radio system.  After a bit of trial and error and testing a few of the hardware and software options I settled on a working configuration that I’ll share in the video series below.

Background

SDRs are hardware devices, controlled by software and the configuration of that software, that allow us to use the radio hardware to tune to and receive radio transmissions, and in some case transmit radio as well, within the spectrum covered by the hardware device.  These devices range from $25 on the low end to $1,000’s on the upper end, and in this post I’ll mainly focus on the lower end devices (RTL-SDR and Nooelec SMART NESDRs).  The links to the hardware, software, and other sites mentioned in the video series are posted below for reference.

Objectives

The objective, as mentioned above, was to be able to receive and record radio transmissions.  These transmissions happen to be in the 460 MHz range (from 460.025 to 460.525 MHz) as Chicago is still using an old analog radio system for dispatch (yes, SWAT and other have moved to a digital encrypted system already).  The issue I ran into was there is a lot of information on how to setup and configure a digital, or digitally trunked system, but not a lot on how to setup the hardware and software for older analog radio systems.  Turns out, it is not that difficult…

The Videos

I figured this post was better suited for a series of videos that cover the basics of the radio spectrum through the installation and configuration of hardware and software to meet my objectives.  Starting with the basics, Video 1 below covers:

• The basics of the radio spectrum in the US
• An overview of my use case and objectives
• Sites that can be used to lookup radio frequencies
• Hardware options (handheld scanners) and types of radio systems (analog, digital, and trunked)

In Video 2, I’ll cover:

• SDR Hardware options (limiting to $350 and under)
• Software options
• The hardware and software I chose for my use case
• Where can I stream these transmissions? OpenMhz!

Video 3 gets us started on the build, and covers:

• Connecting our SDRs to Windows and Linux
• Using SDR#
• Installing rtl_eeprom, docker, and getting trunk-recorder to a working state

Finally, in Video 4, we work on:

• Determining your error correction or PPM using GQRX
• Configuring trunk-recorder (config.json and talkgroup files)
• Determining a center frequency for our use case (analog 460 Mhz transmissions)
• Pushing our recordings to OpenMHz and listening locally

And one final bonus video that shows the quick and dirty way to get trunk-recorder working using a pull from Docker Hub of the latest robotastic/trunk-recorder docker.  In this we will use docker -run and map some volumes and devices between our OS and the Docker container and get up and running quickly.

Finally, here are the links:

• https://www.rtl-sdr.com/
• https://www.nooelec.com/store/
• https://airspy.com/
• https://shop.hak5.org/collections/featured-makers/products/hackrf
• https://www.andrewmohawk.com/2020/06/12/trunked-radio-a-guide/
• https://github.com/robotastic/trunk-recorder
• https://github.com/AndrewMohawk/trunkrecorder-helpers
• https://www.radioreference.com/
• https://openmhz.com/