Decode TETRA as easy as FM on SDR Sharp
No, really.. It’s simpler now to listen to unencrypted (clear) Tetra on SDR Sharp than it is to listen to FM
DSD isn’t needed, because it doesn’t support Tetra, and i thought one of the unpleasant bits of getting SDR Sharp set up – was having to ‘send’ the audio output from SDR Sharp – with a ‘virtual cable’ over into DSD or DSD PLus, both of which work well, but The UI of the DMR decoders is pretty messy and unintuitive, with several separate windows springing open it’s and a mess of separate windows opening up.
You don’t need anything except the file I’ve zipped up for you here, an aerial and an RTL-SDR Dongle – and of course, a Tetra signal.
I have zipped up a working SD Sharp folder ready with the Tetra Decoder already built in. Here is the link below.
it will ask you for a password, type in MBARS
To use, just unzip it to anywhere on your C: drive. It contains all you need. You may need to run Zadig if you do not have SDRsharp another SDR installed already. instructions below.
Running ZADIG (if you need to)
select Options – then List All devices and then from the drop-down, select “Bulk In Interface 0” In the drop down box, choose Bulk-In, Interface (Interface 0) though it might also show up as something like ‘RTL 28320’ or something similar, and that’s ok.
Just make sure that ‘WinUSB’ is selected as the target driver, and then click on ‘Replace Driver’.. you may have to reboot, but next time you start SDRsharp, your RTL SDR should now be in the list of available devices, select it.
Preparing SDR sharp for Tetra decoding.
Once you have selected the correct USB device and have SDR sharp running, you can test it out on normal analogue transmissions and, once you are happy, It’s time to go and search out some unencrypted Tetra.
Firstly, set SDR Sharp up, you can either be in Wide FM or Normal Narrow mode – all you need to know is that whatever FM mode you choose, set the filter width to 25khz. i use NFM and zoom in, and drag the passband out wider that way, but you can use the Wide FM or the option in the Radio menu, it’s more precise, but it’s not too critical, unless you go too narrow.
Now in the Audio tab, make sure that the’ filter audio ‘ button is NOT checked. then go to the Tetra Demodulator panel and select the first option, Demodulator and if you are looking for `DMO Tetra, you must tick the DMO checkbox to it’s right also.
Also, underneath the config button.. click & select the Voice tickbox and click AUTO.
These are your 4 Tetra timeslots, so they need to be set at AUTO unless you are interested in monitoring just one particular slot. If you are listening to a Tetra base station, or DMO just leave it to AUTO
When you have found a valid Tetra frequency, the red text “Received’ will appear – along with any errors, like in weak signal areas or mistuning.
In the UK, there is clear Tetra above 420mhz, some pirate activity around 420.125 mhz where non techs and non Hams set up, as many codeplugs on the UK secondhand market come with some DMO frequencies here. This is illegal but is often referred to as DMO 1 and so on for other channels. Up to and above 460mhz too you may find Tetra, and often hiding in between the usual DMR, FM and carriers – tune carefully,, if you find a valid Tetra signal – a TMO one, get to recognise the harsh high pitched nasty sound of a TMO transmission the Tetra panel will display ‘”Received’ in red font and for more info you can click the Net Info button and serch through there and see the cell info and all kinds of lovely tech info, and whether there is encryption.
You want to see ‘Air Encryption = 0’ towards the bottom of the info cell panel, though there are other Group call panes too, once you do hear things.
What You want to see – .
You will see what talkgroups are being used also in the Groups and more detailed info in the Calls tab too.
Of course current cell refers to the Tetra base station that you are currently tuned to, and as Tetra is Trunked, Neighbour cell is, as the name suggests, the next cell on the Trunking system. With another USB SDR receiver, and a little plugin called Tetra Trunk Tracker will enable you to follow calls from site to site. I didn’t include Tetra Trunk Tracker because it’s at an early stage of development sill, and not simple to set up and get working.
There is no DMR or other digital voice decoding in this build of SDR sharp. just the Tetra plugin. When youclick the Tetra decode button ON the volume slider on SDR Sharp should have a cross through as Tetra mutes it, to use FM again, uncheck the Tetra demodulator tick box, and you can then slide the SDR Sharp volume slider back up.
We use Tetra around Morecambe, on 430.3125 GSSI 10 (talkgroup) we think the 430.000 Tetra frequency is a bit of a poor choice as you will still be radiating outside of the Amateur band. That’s why we use 430.3125 locally and It’s only a 5 minute job getting most commonly available Motorola Tetra gear on to 70cms anyway, and most Tetra sets are the 380mhz to 430mhz band only, not the preferred 410-470 mhz but the 380-430mhz band sets are perfectly fine a few mhz up from 430.000 anyway.
This is just a basic How To on how to hear non encrypted Terra. It’s up to you to get scanning the airwaves and discover things in places you wouldn’t normally expect.