Tetra has been known among radio amateurs for some time, but until recently its use was almost entirely limited to DMO (Direct Mode Operation). With recent developments in software-defined radio and amateur radio projects, this is now changing. It is possible to put a fully functional Tetra TMO base station on the air using relatively simple hardware, including network functionality.
This opens up a world of possibilities that goes far beyond a conventional point-to-point QSO.
From DMO to TMO
With DMO, Tetra radios communicate directly with each other. This can be point-to-point, but recent equipment can also operate as a DMO repeater. Tetra makes clever use of its different timeslots for this. What is received on one timeslot is retransmitted on another timeslot on the same frequency. A DMO repeater therefore requires only one frequency. A duplexer is not required either. The disadvantage is that DMO repeaters cannot be directly interconnected. Connecting different sites, for example, is possible via RoIP (Radio over IP), but this is a workaround rather than a native Tetra network solution.
With TMO (Trunked Mode Operation), things are different. TMO uses a base station that not only provides the radio link, but can also form part of a network. This makes functions available that we know from professional Tetra networks.
The TMO hotspot
TMO requires a base station. Professional Tetra base stations are, however, difficult to find on the second-hand market. Even when one becomes available, it may not be suitable for amateur use, and the price can be considerable. Recent developments by and for radio amateurs have changed this situation. Using software-defined transceivers, it is now possible to build a TMO base station with relatively simple hardware. One example is the HAT from Z32IT. Combined with a Raspberry Pi 4 or 5, the HAT can put a full-duplex Tetra TMO network on the air. Hardware is only part of the story, however: suitable software is also required.
At present, three solutions are circulating amongst Tetra enthusiasts:
- BlueStation – the original solution, providing the basic functions.
- FlowStation – a branch of BlueStation with many additional features and an attractive dashboard.
- Nexus-BS – a more recent development which, for the time being, provides mainly the basic functionality.
For this article, we will concentrate mainly on FlowStation.
Installing FlowStation
The safest approach is to start with a completely fresh installation of Raspbian Trixie 64-bit, without a desktop environment. The required dependencies must then be installed. The BlueStation documentation describes this process BlueStation documentation and dependencies. FlowStation can then be installed. A detailed guide is available here FlowStation installation guide
It is advisable to follow the documentation step by step. Anyone familiar with Linux will recognise much of the process, but for those more accustomed to Windows, the installation may initially seem rather challenging.
Frequencies and duplex shift
One of the most important parts of the configuration is setting the transmit and receive frequencies. Obviously, the frequencies used must always comply with the licensing conditions and the applicable band plan. Choose an appropriate shift of, for example, 5, 7 or 7.6 MHz, depending on the frequencies being used and the capabilities of the equipment. The lower frequency is the receive frequency of the base station. At the same time, this is the transmit frequency of the handheld or mobile radio.
The selected shift must also be entered in the [cell_info] section, using the duplex_spacing parameter. There is an important peculiarity when programming Tetra radios: you do not simply enter both frequencies. In a Tetra radio, you programme only the receive frequency, i.e. the transmit frequency of the base station. In addition, you programme an index number referring to the duplex shift to be used. The handheld or mobile radio contains a table in which each entry specifies a duplex shift. The default FlowStation setting is: duplex_shift = 4
This means that the base station tells the radio to use duplex shift number 4 from its table. According to the ETSI standard, this corresponds to a shift of 5 MHz. There is an important pitfall here: the difference between tx_freq and rx_freq must correspond to the value configured as duplex_spacing. If it does not, the software will not start.
Network codes: MCC and MNC
The settings under [net_info] also require attention. MCC stands for Mobile Country Code and MNC for Mobile Network Code. These terms originate from the mobile telecommunications world and identify the country and network respectively. For the Netherlands, the MCC is 204, while Belgium uses 206. The MNC often used in the Netherlands is 7373. These values must not only be configured in FlowStation. The Tetra radios must also be programmed with the correct MCC and MNC. Otherwise, they will not register with the base station.
Identifying the base station
Another important configuration step concerns identification of the base station. In the Home Mode Display section, the lines from [cell_info.home_mode_display] through to text = "FlowStation" must be enabled by removing the # characters. The source_issi parameter can, for example, be set to the DMR ID with SSID. The text FlowStation can then be replaced with your own amateur radio callsign. The base station will then transmit an identification. For an amateur station, this is of course an important feature to ensure that the station identification complies with the applicable regulations.
Once these basic settings have been entered correctly, the base station can be started and you can check whether the radios are registering successfully.
More than just a hotspot
Once the basic connection is working, the really interesting possibilities begin. FlowStation can, for example, be extended with TetraPack and DAPNET. TetraPack can be used to establish a connection to a DMR network. This allows a local Tetra network to become part of a much larger network. The familiar DMR talkgroups can also be used. A good example is talkgroup 204: when the Tetra network is connected to the DMR network via TetraPack, a Tetra radio can use talkgroup 204 to participate in a QSO that is also taking place on DMR talkgroup 204.
Programming Motorola radios
Tetra radios were originally designed as professional equipment. This means that they generally require a little persuasion before they can be used in the amateur bands. Some tweaks and modifications are therefore required. These should always be carried out at your own risk. With professional Motorola equipment in particular, it is important to proceed carefully: an incorrect programming operation can leave a handheld or mobile radio unusable. A good description of the required modifications and programming procedures can be found at STTA: STTA – Tetra Ham Documents
The Z32IT base station has been successfully tested with, amongst others, the Motorola MTP700, MTH800, MTP850S, MTM800E and MTM5400.
What can a TMO base station do?
The combination of the HAT and FlowStation offers a surprisingly extensive range of functionality. Anyone accustomed only to DMO suddenly gains access to capabilities that are very similar to those found in professional Tetra networks. A TMO base station has four timeslots. TS1 is used as the control channel, while TS2, TS3 and TS4 are available for calls. This means that up to three different QSOs can take place simultaneously.
Group calls
As with DMR, group calls can be made. Everyone who is registered on the same talkgroup can participate in the QSO.
Private calls
Tetra also supports private calls, allowing one participant to ‘call’ another directly. Interestingly, this direct communication is possible even when the two participants are in different talkgroups. So in this situation Tetra differs from DMR where a participant can’t be found as long as the talkgroup is unknown.
Full duplex
Tetra goes one step further. A full-duplex connection can be established with another participant. In effect, this creates something resembling a telephone call over the Tetra network. This function does, however, require two timeslots.
Dynamically assigning talkgroups
Another interesting feature is the ability to assign talkgroups dynamically to users via the dashboard. A radio can therefore be placed into a talkgroup that does not even have to be pre-programmed into the radio. This makes management of a TMO network particularly flexible. The talkgroups familiar from the DMR world can also be used directly within Tetra. Once the base station is connected to the TetraPack network, for example, a Tetra radio can use talkgroup 204 to participate in a QSO on DMR talkgroup 204.
This creates an interesting combination of two digital radio worlds.
Not a turnkey project
A Tetra TMO base station offers radio amateurs an enormous range of possibilities, but it is definitely not a turnkey solution. Anyone wanting to get started will need at least some basic knowledge of Linux. Configuring FlowStation also requires accuracy and some understanding of how Tetra networks operate. Then there is the programming of the radios. Tetra CPS software is generally considerably more complex and less user-friendly than the programming software familiar to many amateurs from DMR equipment. Careless actions can have unpleasant consequences. A handheld radio can become a paperweight and a mobile radio a doorstop.
But that is also part of the appeal of the project.
For those who like to experiment
Tetra TMO brings a wide range of functions to the amateur bands that, until recently, were difficult to access without professional infrastructure. With a relatively compact combination of a HAT, Raspberry Pi, software and a suitable Tetra radio, it is possible to build a genuine network station. The project does require some perseverance. Linux configuration, network parameters, frequency planning, Tetra-specific settings and the sometimes rather quirky CPS software together make for quite a puzzle. But for anyone who is not afraid to experiment, search, troubleshoot and occasionally venture down a path that is barely marked at all, this is a fantastic amateur radio project.
Tetra TMO is therefore not simply a way of connecting a handheld radio to a hotspot. It is, above all, an exciting introduction to the technology behind professional digital radio networks.