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

8)      Notice the peak to the left of the red line in the last image, this is the actual control frequency. The difference between the two values must be compensated for. SDRSharp shows the center of the control frequency (incorrectly) as 853.818452

9)      Open the configuration menu again and adjust the “Frequency correction (ppm)” by increments of ten, then by ones until the red line of targeted control frequency (853.8625) falls in the middle of the targeted signal

10)   In this case the correction needed is ~51 ppm. Document this value as you will use it in setting up the Unitrunker application

11)   Close the SDRSharp application

VB-Cabe Setup

VB Cable creates a virtual patch cable sending the encoded digital audio from Unitrunker to DSD+.

1)      Extract the VBCABLE file from the zip file and right-click VBCABLE_Setup_x64.exe and run as administrator.

2)      Click “Install Driver”, select any defaults presented on the next window, and you should see the “Installation Complete and Successful!” message, click “OK”

Unitrunker Setup

Unitrunker can pull down the configuration of hundreds of trunked radio systems from RadioReference.com. It can also use a pre-saved files with the specifications of the trunked radio system. RadioRefrence.com is a fantastic resource and well worth purchasing if you want to explore trunked or other types of radio around us.

1)      Open Unitrunker, select “First Time Installation” and click continue

 

2)      On the Receivers tab (on the bottom of the window), the “Create a new receiver” dialog box should be open. If not open it by clicking on the “Plus” icon

 

3)      Select RTL2832 in the list of receiver types, the receiver configuration will open, along the bottom you will see a “Info” tab and three “VCO” tabs. In the following steps we will configure these

 

4)      On the “Info” tab (listed on the bottom) Change “RTL Device” to your device. It will appear as a long string starting with “RTL2832”.

5)      On the same “Info” tab change the “Correction” value to 51 identified earlier and ensure that “Baseband AGC” and “Auto Gain” are both checked. The “Correction” value is the trickiest part of the whole setup, it can differ device to device and can even change with temperature. 46.500 seems to work well across different RTL-SDR devices.

6)      In the leftmost VCO tab leave the default Role as “Signal”, check “Mute”, and uncheck “Deemphasis”

7)      In the middle VCO tab change the Role to “Voice” but leave the rest of the settings as default. This will receive any analog (non-digital) transmissions. Very few trunked transmissions are analog today

 

8)      The rightmost VCO will be used to receive digitally encoding voice transmission which will be the bulk of transmissions on most modern trunked systems. Make the following changes

a.       Change the “Role” to Voice

b.       Uncheck “Deemphasis”

c.       Leave ”Audio Output” as its default

d.       Change “Digital Output” to your VB-Cable

e.       Uncheck “Analog”

f.        Ensure “P25” is checked

g.       Check “Logging”

h.       Leave the rest of the defaults


 

9)      To create a system, fist bows to the “system” tab (bottom of the window) and click on the “plus” icon on the top bar.

10)   Double click on the new system. At this point you can either add a system using Radio Reference or by importing a configuration file.

a.        To add the system directly from Radioreference.com, add the Trunk radio system’s SID number to the “SID” line of the new system page. Next click on the “Ships Wheel” icon and add your radio reference account information. Finish by clicking “Download”

b.       To add a system using an imported configuration file, click on the black disk icon. In the dropdown change “Retro” to “XML” or one of the other supported formats. Next browse to your configuration file. In this example it is Motorola743E.xml. Finish by clicking “OK”

11)   The blank system will not change but the system will now be visible in the main Unitrunker interface. You can delete the unused blank system by selecting it and clicking on the “minus” icon

12)   Next, we need to assign the system to our receiver. To do this we click on the “Receivers” tab on the bottom of the screen and select the “Pick a site” icon on the top bar. In the “Pick site” windows:

i.         Check “Discover”

j.         Change “System” to your imported system

k.       Change “Site” to the first valid value, in the example it is “[1] Simulcast”

l.         This is again a tricky part. You must pick an active control channel. In this example (for Montgomery County, MD) the control channel (as noted earlier) is 853.86250

m.     Select the system’s digital mode, if you are unsure select all of them will work in most cases

n.       Verify your settings and select “OK”

 

13)   If everything is connected and correctly configured, a new window with the system should open, note the “health” meter at the top left of the window.

 

14)   While you will “see” the system being received you will most likely not hear anything yet, this is to be expected because DSD+ is not yet running. Leave Unitrunker running and continue

 

DSDPlus Setup

DSDPlus is a sell contained executable and does not need to be installed, this said it is helpful to copy the DSDPlus files to your “Program Files” folder so that it can be easily found in the future.

  1. Unzip the DSDPlus zip file
  2. Create a DSDPlus folder under your “Program Files” folder
  3. Move the DSDPlus files into the new folder,
  4. Open a Windows command prompt window by going to Windows icon > run > type “cmd”
  5. Browse to your DSDPlus folder
  6. Run DSDPlus.exe and Identify the numbers DSDPlus assigned to the input and output sound devices
  7. Once you identify the settings for your input and output press escape and re-run DSDPlus with the following commands, replace the input and output with your values:  dspplus.exe -i<input device> -o<output device>
  8. If working correctly DSDPlus will decode the audio, you should now hear the transmissions being received. You should see a window tracking the decoded audio
  9. You made it!

 

Reference

http://www.unitrunker.com/realtek2.html