FAQs - How TETRA works

1 How does the system operate?

2 Do TETRA transmitters pulse at 17Hz?

3 What is transmit inhibit?

4 What is a leaky feeder or cable?

5 How do TETRA handsets compare with the old analogue radios used by the police?

6 Do handsets emit RF when they are not being used to transmit?

7 What is time-slot sharing?

1 How does the system operate?

A TETRA base station transmits and receives information from around 16 terminals. Base stations are connected together via cables to create a complete network. A base station generally comprises four transmitters, amplifiers and antennae. The radio frequency emissions are directed into a beam.

Appendix C of the 2007 report published by the Mobile Telecommunications and Health Research programme (page 47 – 48) contains a simple summary of how the TETRA network for the emergency services operates. To go straight to this report, click here ().

2 Do TETRA transmitters pulse at 17Hz?

No they emit a continuous signal – see the FAQ section on Base Stations for more detail of how they work.

3 What is transmit inhibit?

Transmit inhibit is a feature that TETRA offers so that the user can prevent the radio transmitting but can still receive incoming communications. This facility can be useful in places like hospitals.

4 What is a leaky feeder or cable?

A leaky feeder or cable is an alternative to a mast-mounted base station. A special cable can be laid to provide a means of signalling between handsets and a central control. This is used, for example, in railway and underground situations where there is a need for coverage in tunnels and along tracks between stations.

5 How do TETRA handsets compare with the old analogue radios used by the police?

The power used by a TETRA handset is typically around 1 watt, about the same as that of a torch bulb. This is lower than the power used by the analogue radios that preceded the TETRA-based system.

In practice the power used by a TETRA radio can be much lower; if the handset is being used close to a base station adaptive power control can reduce the power to around a thirtieth of a watt. The power consumption is highest if the handset is being used inside a vehicle or a building.

See also the section of FAQs on Handsets and Devices.

6 Do handsets emit RF when they are not being used to transmit?

Yes they do, but only for very short times, perhaps a few seconds during an entire 8 hour shift for occupational users. This is because the handsets maintain occasional brief contact with the nearest base station – these contacts are called ‘handshakes’.

7 What is time-slot sharing?

Four handsets can share a frequency when communicating with a base station by compressing speech into one of four time slots, each operating sequentially. A base station can handle four different frequencies so can communicate with up to 16 handsets at any one time. This means that a handset is effectively ‘on’ for 14 millseconds and off for 42 milliseconds with a repetition rate of 17.6 times a second. This is why the handsets are said to pulse.