RTL-SDR Tutorial: Following Trunked Radio with Unitrunker

RTL-SDR Tutorial: Following Trunked Radio with Unitrunker

The popular trunking decoding software Unitrunker now supports the RTL2832U R820T RTL-SDR directly in its new version. This means that extra SDR receiver software like SDR# is no longer required to use Unitrunker.
You can download the latest version of Unitrunker here.
In a normal radio system, one company (or talkgroup) might use a single frequency for radio communications. However, this is very inefficient as the frequency may not be in use for the majority of the time. In a trunked radio system, a small set number of frequencies are shared between a large number of talkgroups. Each radio receives a special computer controlled control channel. The control channel determines a vacant frequency that a particular talkgroup should use. This helps to make radio frequency allocations more efficient.
Because a talkgroup might switch between various frequencies often, it can make listening to a conversation difficult for radio scanners. Unitrunker can be used to decode the control channel and follow a voice conversation as it hops across various frequencies. With two RTL-SDR dongles you can set up a trunking receiver station with just Unitrunker. What follows below is a tutorial on how to set this up.

Unitrunker Voice Trunking Following TutorialTo set up a trunking station we will assume that you have been through the Quickstart guide and thus have a working RTL-SDR setup. In addition you will need
  1. 2x R820T RTL2832U RTL-SDR dongles. See the Buy RTL-SDR dongles page if you don’t already have two. You will need one dongle to receive the control channel and one dongle to receive the voice channel. Only the R820T appears to be supported at the moment.
  2. The Unitrunker software (Choose the latest version from http://www.unitrunker.com/download/)
  3. One antenna that splits into two connections for the two dongles, or two antennas that can receive the trunking frequencies you are interested in.
Now follow these steps to set up Unitrunker with the RTL2832U.

Setting up the Signal (Control Channel) Receiver

  1. First determine a trunking control channel that you are interested in monitoring. You can use SDR# or a similar program to search for these first. They will be signals that are continuously transmitting with no breaks. Here are some examples of some common trunking modes. The are commonly found in the 400 and 800 MHz bands.

Motorola 2F1D
MPT 1327

  1. Plug in both dongles and then open Unitrunker.
  2. Press the ‘Add new receiver’ button add_new_rcvr_button, and then on the bar that pops up click on the RTL2832 button.
    Add new receiver
  3. In the new window that pops up choose your first RTL-SDR under “RTL Device”.
  4. Enter that units PPM correction value under “Correction”. Note: Make sure that you are entering the correct value for the correct dongle. If you are unsure which dongle is which then you may like to try labeling the dongles with rtl_eeprom first.
  5. We also recommend enabling “Baseband AGC”, turning on “Auto Gain”, enabling “Drift Correct”, setting the sample rate to 2.048 msps and settings “VCOs” to 1 VCO. If you have an older and slower PC you may wish to set the sample rate lower.
    Set sample rate, gains and PPM offset
  6. Click on the first VCO tab at the bottom of the window.
  7. Change the “Role” to “Signal”.
  8. Change the “Park” value to the frequency in MHz of the trunking control channel that you want to monitor. For example if you wish to monitor a control frequency at 862.839700 MHz, then enter 862.839700 into the box.
  9. Under the “Signal” heading ensure that all the control channel protocols that you want to possibly decode are checked and that “Discover” is also checked.
    Set the role to Signal, set the park frequency and choose the protocols
  10. Now at the top of this window press the Play button  .
  11. If everything is tuned correctly a new Site window should soon pop up and you should audibly hear the control channel sound coming through your speakers. In the VCO tab you should also see under the “Signal” heading the protocol dectected and it’s “Health”, which measures the quality of reception.
    1. If every thing worked correctly, then go back to the first VCO tab, and ensure that “Mute” is checked. This will mute the control channel audio.
    2. If no new site window popped up and/or you don’t hear the control channel then go back to the first VCO tab and ensure the “Audio Outout” is set to your speakers, and that the park frequency is set correctly. Also go back to the Info tab and check that the PPM Correction is set correctly. You may also need to adjust the gain settings – try manual gain by unchecking both “Baseband AGC” and “Auto Gain” and using trial and error to choose a gain value between (0 – 500). Monitor the “Health” reading of the signal under the “Signal” heading in the VCO tab. If it is below 100 it may indicate reception problems which will need to be solved by tuning the signal correctly, adjusting the gain, or using a better antenna.
  12. The Site window shows the frequencies in use, and logs each call. If the frequencies are not showing, click the ‘Wizard’ button unitrunker_wizard_button whose icon looks like a calculator. Then choose the system used in your country. If unsure, try ‘Standard’ first as this is the most commonly used.
    The new site window
  13. (Optional) If you have a RadioReference.com account, you can click on the gear icon and enter your RadioReference account credentials. This will download information provided by RadioReference.com, including site information and some talk group labels.
  14. Close the Receiver R820T options window for the signal channel, move the new site window out of the way (don’t close it) and go back to the main Unitrunker Window.

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