Triaxial radiating cables improve subway communications:
Achieving proper radio frequency coverage for emergency police communications has become an increasing concern for subway transit authorities. Electrical, mechanical and combustion characteristics are important.
Obtaining adequate radio coverage in enclosed areas such as subway tunnels and underground transit stations poses a challenge. The use of point-source antennas results in dead spots, and RF energy will not propagate along the length of a tunnel.
Many terms such as radiating cables, leaky feeders and antenna cables have been used to describe a cable designed to provide RF coverage in enclosed or confined areas where point-source antennas are not practical. These cables have been used as a way to provide RF coverage for two-way radio, paging, cellular and, most importantly, radio communications for police and emergency workers in the event of an accident or fire within a tunnel or enclosed area.
In 1985, the New York City Transit Authority (NYCTA) began a system-wide upgrade of its transit police radio system. This upgrade involved the removal of an existing twin-lead antenna cable and replacing it with radiating cable. The NYCTA was looking for improvements in coverage of communication dead-spots within the tunnel system, improved reliability and the capability to add communications for other purposes in other frequency bands.
The NYCTA approved two radiating cable constructions to replace the existing twin-lead design. One construction was the traditional slotted corrugated copper design. The second construction was a split-shield triaxial design. After testing and qualification, the low-smoke-producing, flame-retardant triaxial design was chosen over the conventional slotted corrugated design for the IRT, the largest portion of the New York Subway System, extending into all five boroughs.
Fischbach and Moore Transit Group was chosen to install the more than 300,000 feet of TRC-1250-size cable (1.760″ outside diameter) that was used. Because of the height restrictions within the tunnel, a maximum reel size of 72 inches was allowed, which provides as much as 2,000 feet of continuous length to be installed before any connector attachment is needed. These cables are directly mounted to tunnel walls, providing easier installation.
Cable design As with all RF cables, the most important function of a radiating cable is to transmit RF energy from one point in a system to another with minimal degradation or loss of signal power. The difference between radiating transmission lines and conventional shielded coaxial designs is that this cable must also allow a controlled amount of RF energy to couple out of the cable and into the surrounding environment without sacrificing too much in downline attenuation.
Figure 1 on page 10 shows the construction of the triaxial cable design. Depending on the cable size, either a copper-clad aluminum conductor or a hollow copper tube is used for the center conductor. The dielectric material is a high-velocity, closed cell structure of foamed polyethylene, which offers a tough, low-loss medium for the transfer of RF energy between the center and outer conductors. The outer shields consist of a semi-circular bonded smooth aluminum tape, which is separated by a layer of solid polyethylene. Braided drain wires are used to provide contact with the two outer shields when connectors are attached.
The outer jacket consists of a dual extrusion of low-smoke, low-toxicity polyolefin material. This proprietary flame-retardant material offers a tough weather- and ozone-resistant jacket, while maintaining the flexibility that is required when installing these cables within enclosed areas.
Theory of operation All radiating cables are affected by their environment. The environmental effect on the cable is a degradation of downline attenuation and coupling loss of varying degrees. The extent of the degradation depends on both the cable design and the environment in which it is installed. Cable performance will exhibit different results under different conditions, such as free space, inside a building or next to a conductive or high-dielectric-constant material (such as lying on the ground or attached directly to a tunnel wall.) The triaxial cable designs exhibit a more stable and predictable performance over these environmental conditions than the conventional slotted corrugated designs because of the different operating mechanisms. With the corrugated constructions, rows of milled openings are created, which provide a means for coupling energy to the surrounding environment. These rows of oval apertures occur periodically along the length of cable and, therefore, contain electromagnetic fields that have a component in the direction of propagation of the cable, which results in the total energy not being transmitted in the transverse electromagnetic mode (TEM). With the triaxial design, the aperture that is created by the separated shields does not store energy but transfers a small amount of energy to the outer surface. These cables are continuous and, therefore, have no fields in the direction of propagation. Figure 2 on page 11 demonstrates the effects of a section of steel piping on the attenuation performance of the two radiating cable designs. Both constructions exhibit similar attenuation characteristics from 100MHz through 1,000MHz. With the introduction of this piping at the input end of the cable, the triaxial design was virtually unaffected, whereas the slotted design showed a significant effect. This is important to system installers because cable standoffs are no longer required, decreasing cable installation costs.
An added benefit of direct mounting is the elimination of hanging cables, which offers a more appealing cable installation. Another benefit is the additional protection from the effects of vibration and damage from contact with moving vehicles.
Electrical performance The attenuation and coupling loss of the triaxial cable is controlled by the separation of the split shields. This approach optimizes the tradeoff between downline attenuation and coupling loss, and they are tightly controlled by the shield width tolerances, as well as dimensional tolerances on both the dielectric core and interlayer. Table 1 above gives values of three of the standard size triaxial designs. The attenuation and coupling loss values are typical of directly mounted conditions. These cables were all mounted directly to the tunnel walls within the New York City transit system, which resulted in easier cable installation and reduced overall cost.
Mechanical considerations In addition to the electrical design performance of these radiating cables, other considerations such as mechanical, physical and environmental issues had to be addressed. The NYCTA required a series of qualification tests to be performed before any cable would be considered for installation within the transit system. Table 2 on page 17 is a partial listing of the environmental, physical and mechanical specifications that the cables were required to meet.
Because of the harsh physical environment that these cables would encounter, additional testing was performed to determine the crush strength of the triaxial designs. Both the corrugated construction and the triaxial design were subjected to a flat plate crush strength test, using the reverse mode on a tensile testing machine. The triaxial cable exhibited increased crush strength to more than twice that of the corrugated copper design. This is primarily due to the tough foamed polyethylene dielectric, along with the protection of the layers of polymeric jacketing material. In addition to the overall higher crush strength, the elastic memory of the materials allowed the triaxial design to return to within one-half ohm of its original impedance, whereas the corrugated construction remained in a crushed state. This increased crush strength and memory may provide the difference in replacing a cable that has been damaged by some outside physical force. The triaxial design is also much more flexible and easier to route than the corrugated copper cable.
Fire safety There have been well-documented mass transit system fires over the years. Death and injury due to toxic products of combustion and smoke have accompanied several of them. As a result, in the last decade, many mass transit systems worldwide have required low-smoke-producing, flame-retardant materials for all products installed in subway tunnels. Events such as the fire at the London Kings Cross Road subway and the tragic loss of 300 lives in the Baku (former Soviet Republic of Azerbaijan) subway fire have ledto an increased awareness of the safety of the materials that are specified and used within enclosed are as.Table 3 on page 17 lists some of the flame performance and products of combustion testing that the triaxial cables meet. With the recent advances in flame-retardant technology and available materials, the triaxial cables are able to meet the required level of flame performance without the need for expensive barrier tapes. In addition to the flame retardance, all materials used in the construction of this cable are non-halogen. When subjected to a flame source these cables exhibit extremely low levels of smoke generation, toxic emissions and corrosive offgassing.
System interconnects For those sections within the transit system that did not require RF communications, or where system interconnects were needed, shielded low-loss coaxial designs were used. These flame-retardant constructions provide lower downline attenuation because of the non-radiating nature of the designs. Both the low-loss coaxial cables and the triaxial radiating designs can be routed much more easily than corrugated copper cables. The flexibility and non-kinking nature of these cables allows them to be routed and bent into tighter areas, reducing installation time. The interconnect cables are also required to meet all the physical, mechanical, environmental and combustion properties of the radiating cable designs.
Because of the varying nature of environments in which radiating cables will be installed, exact electrical performance can be difficult to predict. The triaxial cable designs have exhibited that environment affects performance minimally. These triaxial constructions were chosen by the NYCTA for the various benefits and predictability of performance. The installed cables were tested after installation and exhibited high-quality signal levels when subjected to survey test procedures that were required prior to operation of the system-wide police radio system. The ease of connectorizaton, elimination of cable standoffs and the flexibility of these constructions were also benefits provided to the installation contractors.
Over the past three years, the transit police radio communications system has been operating with no defects or downtime.
Fedor is product manager, cable, for Times Microwave Systems, Wallingford, CT.
