Unitrunker SDR Setup – Low-cost Trunked Radio System "Scanner"

Trunked radio systems, like those used by many police and fire services, can be received using a Windows PC, low-cost hardware, and some free software. For less than $30 a receiver can be created. This is a real radio receiver  – i.e. it does not rely on having Internet access. This is benefical in an emergency but it also works great for everyday use as well. 

Follow these easy steps to create your own trunked radio receiver.

Materials needed

  1.  A windows desktop or laptop with at least one available USB2.0 or USB3.0 port. Systems before Windows 10 may need additional drivers (not discussed here)
  2. Unitrunker software – Free - http://www.unitrunker.com/download/
  3. VB-Cable Virtual Audio Device – Free/donationware - https://www.vb-audio.com/Cable/
  4. Zadig USB Driver software – Free - https://zadig.akeo.ie/
  5. DSDPlus – Free - https://www.dsdplus.com/download-2/
  6. DSDPlus DLLs -Free - https://www.dsdplus.com/download-2/
  7. RTL-SDR Device - ~$22 - https://www.amazon.com/NooElec-RTL-SDR-RTL2832U-Software-Packages/dp/B008S7AVTC
  8. Trunked Radio system config file or radioreference.com membership – Free / $15 (180 days package) - https://www.radioreference.com/premium/
  9. SDR# Software – Free - https://airspy.com/download/

Overview: What is all this doing?

Each part has its role:

  1. Zadig provides the right driver so Windows will ‘see’ the RTL-SDR device
  2. The RTL-SDR receives the radio signals and presents them to the computer
  3. Unitrunker both decodes the “control” channel for the trunked radio system and tunes into whichever frequency is actively transmitting. The frequencies and their names are provided by a config file or via Radioreference.com
  4. Because communications are mostly digitally encoded, VB-Cable is used to send encode audio to DSDPlus
  5. DSDPlus decodes the audio for listening
  6. SDR# software will be used to calibrate Unitrunker though it is a great SDR application on its own!

Note: Some communications, particularly police communications, may be encrypted. DSDPlus is not capable of decrypting these communications (nor would it be legal to do so without proper authority). Decoding does not equal decryption.

RTL-SDR Setup

For windows to use the RTL-SDR it will need a driver (code that tells the OS how to use a device) provided by the Zadig application.

1)      Connect your RTL-SDR device to the computer

2)      Run Zadig as administrator by right clicking the zadig exe file and selecting “administrator”

3)      In Zadig, select the “Options” menu and select “List all devices”. Your listed devices should contain a device named “RTL2832U”

4)      The RTL device should be displayed next to the word “Driver”. Ensure that “WinUSB” is selected in the dropdown window to the right of the arrow. Click the “Replace Driver” button

 

5)      You should see a message that states “The driver was installed successfully”

 

6)      The RTL-SDR will now show up as a USB device. Seen here on the Windows Taskbar

 

Calibrate RTL-SDR

Using the SDR# application we will identify the skew between the frequency the tuner says it is tuning and what it is tuning. The skew or “correction” value identified here will be supplied to the Unitrunker software later.

1)      Download and unzip the SDR Sharp (SDR#)

2)      Open a command prompt and browse to you SDRSharp folder

3)      run “install-rtlsdr.bat” to ensure that SDRSharp can see the RTL-SDR

4)      run “SDRSharp.exe” and click on the gear icon

5)      In the device dropdown, select your RTL-SDR device, select “Close”

6)      In the main window press the play icon

7)      Tune to the control frequency for the trunked radio system (found via radioreference.com) in this case 853.8625