TETRA - reaching for a higher standard

A TETRA network is often chosen for applications needing high levels of security, resilience and reliability. Carl Jones, MD of Skymasts Antennas, explains why higher standards are required for TETRA antennae

TETRA - reaching for a higher standard

The TETRA standard has been developed to offer the very best quality, high capacity, multi-user infrastructure for such applications, and the world’s leading manufacturers have developed cutting edge radio and network equipment to supply these services.

The radio equipment used will have been stringently tested and type approved to ensure it is up to the job, but the antennae have had no such testing or approvals. When selecting antennas, their performance in a radio network is often evaluated using planning software in conjunction with performance data from the manufacturer, but this only reflects ideal circumstances and other problems are often discovered a long time after the network is deployed.

The TETRA standard offers a communications system intended for multi-user use and service providers are keen to maximise the capacity of the network and revenue from the infrastructure investment. To increase the capacity of a TETRA base station, multiple radio transmitters and receivers will be used and combined into a single antenna system. This presents several problems to the antenna, which can only be overcome with careful design and manufacture. If attention is not given to these issues, the deterioration of the antenna over time will impair network performance, often as the user base is increasing and compounding the problems.

Passive intermodulation
The first well-known problem to overcome is passive intermodulation (PIM), which becomes an issue when the same antenna is used for transmit and receive. PIM is an unwanted signal that is produced when two or more RF signals from two transmitters are mixed in a device which has non-linear characteristics. Semiconductors such as diodes and transistors are examples of non-linear devices, but non-linearity can also be found at a junction between two dissimilar metals, across a loose connection or where there is corrosion, for example, within an antenna. These unwanted signals will occur at sum and difference frequencies of the original carrier frequencies, which gives them the potential to fall across the receive band at comparable levels to the signal received from the mobile, interrupting communications. This is bad news for a digital communications system, where noise has an impact on data rates, and where the low transmission power available from battery powered units can be the limiting factor in base station coverage area. If it is not suitably designed, the PIM performance of an antenna will worsen over time, creating a time bomb for network operation.

The peak voltages, which can be present inside an antenna also could be a problem to the designer, an issue which is not always considered and will cause faults that are difficult to diagnose. Antennae are often designed to handle a certain power level, and it is assumed that a combination of transmitters can be used whose total power does not exceed this.

However, using multiple transmitters will generate much higher peak voltages within the antenna than a single transmitter will at the same combined power level. If the antenna is not capable of handling these voltages, several problems can occur, ranging from noise generated by the antenna as sparks jump across gaps between electrical conductors to a complete breakdown of materials within the antenna ,where the electrical potential across them is too high.

As antenna sharing increases to keep ongoing site costs down and technological developments give rise to better receiver sensitivity, the performance of the antennas becomes paramount. But just as important is the protection of sensitive areas within the antenna to ensure that this performance is retained over time. As users are added to the system and the network gets busier, these problems can exponentially increase.

Resilient antenna design
Skymasts Antennas has invested 18 months in developing a new range of TETRA antennas intended to survive in the most extreme environments and retain such high levels of performance.

To achieve a low PIM, design has required careful selection of construction materials, cables, connectors and precision engineering and manufacturing; but just as critical is the protection of the antenna from the environment to ensure its continued operation. To supply the antennas to a major UK-wide TETRA network, Skymasts’ products were designed to ensure that they would operate in the most hostile parts of the country, and even survive the fastest ever-recorded wind velocity in the British Isles. This has involved extensive testing combined with careful review of design, to be sure that all component parts are up to the job and all calculations and specifications used in the product design process are verified.

Also, the development of the production processes and testing procedures are important to ensure consistent performance. Without the correct test equipment the PIM performance of an antenna is impossible to determine, and it can be generated at high levels by imperfections almost invisible to the naked eye.

Broadband for high capacity
Skymasts’ new antenna products are also designed for the full UHF TETRA 380-430MHz band, which allows for greater capacity and site sharing. These antennae will be used for the 2012 Olympic Games site communications network, where a dedicated TETRA network will be in operation alongside the existing network.

Attention to PIM performance becomes paramount, as a wide spread of frequencies gives a greater potential for the third order harmonic – typically the highest in level – to occur in a receive band. There is also a broadband indoor antenna covering 380-470MHz, allowing TETRA operators and local PMR systems to share the same antenna network.

Written by Wireless magazine
Wireless magazine

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