Radiating cable improves in-building wireless
Coupled with a DECT-standard system, radiating cables reduce the required number of base stations and extend coverage into interior spaces.
Wireless communications systems are becoming increasingly important, both to business and personal life. It is equally important that they work everywhere that people go. For system coverage in open areas, antennas are almost always sufficient. In buildings and other enclosed areas, there are frequently difficult propagation issues, making coverage of the interior spaces by exterior-mounted antennas inadequate. This not only limits communication, it can limit revenues for systems providers.
There are several ways to achieve better RF coverage within a building or other interior space. One method is to point an outside antenna at the structure. Depending on the structure’s composition (wood, brick, stone, concrete, steel, glass, etc.), directed RF will vary in effectiveness. Another method is to mount an antenna inside the building. Again, effective coverage depends on the RF-shielding properties of the interior walls. If long corridors or tunnels are involved, the RF energy will usually not propagate far along their length. Many antennas and many radio base stations, at great cost, may be required to achieve good coverage.
The preferred solution to achieving RF coverage in interior spaces is the use of a radiating cable. This cable is designed to couple energy along its length. In effect, it acts as a long, low-gain antenna. The net effect is that the cable loss is increased only slightly, compared to a conventional coaxial cable, while enough energy is coupled to the environment to achieve good RF coverage in virtually any configuration of interior space. With a radiating cable, a single base station may be able to provide coverage over a large area, reducing the cost of system implementation.
In the trial installation described here, a new-design radiating cable was evaluated. The balance between downline attenuation and coupling to the environment is controlled by the design of the outer conductor. For applications in subways and tunnels, where the lengths of cable used may be several kilometers, the downline attenuation must be minimized to produce sufficient signal at the end of the run.
For a building application, where the runs are rarely more than a few hundred meters, a different balance between downline attenuation and coupling to the environment is required. There, the attenuation of the cable can be somewhat higher, while a greater degree of coupling to the environment is desirable, to help the signal to penetrate walls, floors and partitions.
Digital enhanced cordless telephone (DECT) is a reliable standard for cordless telephony that is commonly used in many parts of the world. It operates on a frequency range of 1,880MHz-1,900MHz, and the radiated power is 250mW. The mean coverage area typically extends about 50m from the antenna in an indoor application and about 300m in an outdoor application.
To cover a large indoor area, several base stations or repeaters are required. A central unit controls the base stations, centralizes the communication control and achieves a real network with roaming and seamless handov er between cells. In this way, the DECT system is similar to other cellular and PCS systems in use today.
Because the DECT system allows the use of multiple cells coordinated together to allow hand-offs, it can be used to cover virtually any area. When a customer orders a system, the requirement is usually to cover the entire facility. This will frequently include areas such as storerooms, basements and garages, where there are few users and there is little demand for capacity. With a standard antenna-based system, a separate base station usually will be needed to overcome the RF shielding of these areas.
A large paper factory with a total area of 5 million square feet required 63 base stations to achieve coverage using a conventional antenna-based system. Because base stations are expensive (and because in many cases they still do not give good RF coverage in an enclosed area), a trial was undertaken on the use of radiating cable in a DECT system at the Kabelwerk Eupen factory, Eupen, Belgium.
Each base station has two antennas to provide space diversity, increasing the quality of the communications. The signals from the two antennas are compared, and the better signal is used by the system. The antennas were replaced by radiating cables in several configurations, with good results. In order to match the coverage achieved to the space, the following configurations were tried: * Radiating cables were attached to each of the two antenna ports, run in opposite directions. This eliminates the diversity and allows the largest area to be covered by the base station. It results in some degradation in signal quality because of the lack of diversity, but may be suitable for some rarely used large areas.
* Radiating cables were attached to both antenna ports and run in parallel. This provides the best performance, because of the cable diversity. It is the best solution for heavy traffic areas where there is frequent use and quality of coverage is important, such as heavily used corridors or tunnels.
* An antenna was attached to one port, and a radiating cable was attached to the other. This allows the use of a single base station to cover a large open area and an adjacent hallway or corridor.
By carefully combining these configurations, the number of base stations required to implement a system can be reduced by more than half, depending on the building layout. The quality of the coverage achieved will usually be much better in a system with radiating cables. There is also a reduction in system complexity, increasing its reliability and decreasing maintenance requirements.
An omnidirectional antenna radiates equally in all directions and would be the best solution to cover a single, uniform room. Radiating cables provide a good solution to cover a long corridor, a great hall with several obstacles, or several rooms with the same communication cell.
Radiating cables transmit the signal in their longitudinal direction and simultaneously radiate a part of the energy along their total length to the surrounding area. Also, the reception of a signal from the outside is possible. Radiating cables can be considered as elongated antennas that are specially appropriated for long, extended areas. Currently, most radiating cables are used for mobile communications in tunnels, subways and mines, but the development of new in-house radiating cables also permits their use in buildings.
By using radiating cables, wireless coverage within buildings can be accomplished in a way that is more economical and more complete. The amount and complexity of the base station equipment required can also be reduced, resulting in a more reliable system requiring less maintenance over time. When combined with conventional antennas, radiating cables provide an excellent tool for the designer of indoor wireless communications systems.
Haas is RF sales manager and Veithen is RF engineer for Kabelwerk Eupen A.G., Eupen, Belgium.
