For some time I’ve wanted a radio scanner so I could listen in on Police/Fire/EMS radio in my area but I’m not serious enough to pay for a dedicated-to-the-task scanner required to listen to today’s radio protocols. Today’s protocols are digitally based with trunking and patching systems that can both isolate calls to a local area while also allowing for nearby stations to be patched in. This is done with a constant control signal and a number of nearby frequencies that radios can hop to when they make a call. Decoding all of this requires specialized equipment or software that understands the P25 Phase 1 or 2 protocol. Radios that can do this start at around $250, go up from there and that’s before you get an antenna or anything associated with it. Additionally, I really like toying with Software Defined Radio (SDR) equipment and the idea of turning a computer into a scanner capable of tracking this radio system seemed fun.
In this post I am going to go through some of what I did to get setup to listen in on what I was interested in. While I knew that an SDR could be used for this task, I didn’t know how to put it together, what software was required and so on.
Get to know the radio systems used near you
The first thing I had to do was confirm what type of radio system was used near me. For that I turned to https://www.radioreference.com. Here, I learned that in the state of Minnesota, all Public Safety Agencies use ARMER which is a P25 Phase 1 system. Based on this information I knew better what to expect when it came to the hardware needed as well as what software I needed to research. Later, as I was setting up the software, I registered for a paid account with Radio Reference so that I could automatically pull down information about frequencies used and more.
Using the site, try to locate a tower site that is as close to you as possible and make note of the frequencies used. The important part to know
Get the right hardware
For the SDR itself, I went with this RTL-SDR Blog V3 brand unit (Amazon Affiliate Link) based on information I found that suggesting it was better supported. I also selected this unit because it has an SMA style connector for a screwed together, stronger connection to the antenna. Additionally I grabbed this antenna set (Amazon Affiliate Link) because it offered a range of antennas that would be a good fit for what I was doing.
Note that, depending on the frequencies used in your area, you may need more than one SDR in order to tune them all in. If your local system is P25 based it will use a main control channel and then a set of additional frequencies for callers to use. This is commonly referred to as a trunk based system. The frequencies in use near you need to fit in the range your SDR can tune in and listen to at the same time. The dongle you select should advertise the bandwidth it can tune to. For example, the SDR I selected advertises “up to 3.2 MHz of instantaneous bandwidth (2.4 MHz stable)” which means it can reliably listen to anything within a 2.4mhz range of frequencies. On a P25 system, the control frequency must always be tracked and all additional frequencies must be within 2.4mhz of the control frequency. If the frequencies used fall outside of this range then you may need multiple SDR adapters to hear everything.
The system near me uses two different control frequencies:
- 857.2625
- 860.2625
Callers then are on:
- 856.2625
- 857.0125
- 858.2625
- 859.2625
As long as I do not select the 860.2625 control frequency, the SDR can tune and hear any of the other frequencies in at the same time as they are all within 2.4mhz from the control frequency.
You may elect to get more than one SDR if you wish to listen to additional trunks or other frequencies at the same time. Later you will see that you can set priorities on what frequencies or trunks you want to listen to first in the event two frequencies become active.
Software
After a short bit of research I found there is a handful of software options available but I quickly settled on SDRTrunk. SDRTrunk is freely available, Java based software that will run on Windows, Mac and Linux alike. It seemed to be among the most recommended pieces of software for this task and readily available information on how to set it up. I used this YouTube video to get things setup – https://www.youtube.com/watch?v=b9Gk865-sVU. The author of the video does a great job explaining how trunking works, talk groups, how to pull in data from Radio Reference and how to configure the software for your needs.
Putting it all together
For my setup I used an older iMac running Ubuntu 22.04. I installed the SDRTrunk software per their directions and used the above mentioned video to learn how to configure the software. The system was almost ready to go out of the box but I had to install rtl-sdr from apt for the device to be recognized by the software. I used the adjustable, silver dipole antenna from the antenna kit with it fully collapsed. This was the closest I could get to the appropriately sized antenna for the frequencies used. I used this site to determine antenna length for a given frequency – https://www.omnicalculator.com/physics/dipole. I am located quite close to the broadcast tower so even a poorly sized antenna still worked. Sizing the antenna properly will assist greatly in improving your ability to tune something in. Even fully collapsing the antenna vs fully extending the antenna resulted in nearly a 10dB improvement in signal strength.
The last thing I did was setup an Icecast 2.4 based broadcast so I can tune in away from home. SDRTrunk has support for a few different pieces of streaming software and Iceast seemed to be the easiest to setup.
Finishing up
While not a full how-to, I hope my post gives you just enough information to get started. I am amazed at how well this solution, the SDR, software and more all work together so well. Better than I expected. I also like that I can repurpose the SDR for other tasks if I want, like pulling in data from remote weather stations and more. If there is something you have a question about leave a comment or find me on Mastodon.
Disclaimer: This post contains Amazon Affiliate links. If you purchase something from Amazon using a link I provided I will likely receive a commission for that sale. This helps support the site!