FAQs - How TETRA works
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 (pages 47 – 48) contains a simple summary of how the TETRA network for the emergency services operates. To see this report, click here.
Do TETRA transmitters pulse at 17Hz?
No they emit a continuous signal – see the FAQ section on Transmitters and Base Stations for more detail of how they work
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.
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.
How do TETRA handsets compare with the old analogue radios used by the police?
The power used by a TETRA handset is typically up to 1.8 watts. 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 Terminals.
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’
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 milliseconds and off for 42 milliseconds with a repetition rate of 17.6 times a second. This is why the handsets are said to pulse.