The importance of coaxial cable to base station performance Low-loss flexible cable provides an alternative to semi-rigid corrugated cable with advantages in loss, handling, ease of installation and ruggedness compensating for tradeoffs in shielding and i
Coaxial cables are vital to the performance of mobile radio systems. Selecting the best coaxial cable for an application within a mobile radio base station has become more complicated over the past few years as more suppliers have offered a broader range of products. The cable selected will affect the system’s cost, coverage and reliability. The new choices that are available can frequently allow for better overall system performance at a lower price than the older alternatives.
Until recently, the choices were generally limited to corrugated copper cables, conventional braided cables and air dielectric braided cables. Generally, corrugated copper cables were chosen for applications requiring the lowest loss, such as antenna feeders. Braided coax or RG-type cables were used for applications requiring the greatest physical flexibility. Air dielectric cables seemed to offer a compromise, with relatively low loss and good flexibility, but their construction leads to performance problems. These include moisture accumulation in the air space, radial movement of the center conductor at bends, resulting in VSWR degradation, and axial movement of the center conductor relative to the outer conductor, resulting in connector failures.
In recent years, several new types of cables have been introduced for communications applications. These include low-loss flexible cables, which offer loss comparable to corrugated copper cables, but with much better flexibility. In addition, several manufacturers have introduced new types of semi-rigid cables with smooth outer conductors. New suppliers of corrugated copper cable have also entered the market. To select from this greatly expanded universe of choices, it is helpful to consider the most important characteristics of coaxial cable more carefully.
Signal Loss Because the function of a coaxial cable is to transmit RF energy from one point in a system to another, efficiency is the most important factor in selecting a cable. The loss of a cable is measured in dB/100ft, which is a logarithmic expression of the ratio of the output power from the cable to the input power to the cable. The loss of a cable is determined by the conductor loss and the dielectric loss. Dielectric loss remains essentially constant as the size of the cable changes, whereas conductor loss decreases as cable size increases, much as the resistance of a wire decreases as the size of the wire increases. The need for low loss, rather than requirements for high power handling, dictates the size of large cables in mobile communications systems.
Because all of the cable types being compared use low-loss dielectrics and high-conductivity conductors, the losses of similar size cables are close, as can be seen in Table 1 on page 20. This is not the entire story with regard to total signal loss, because the semi-rigid cables generally require the use of jumper cables at each end in order to be routed to the radio equipment and the antenna. These jumper cables add loss to the feeder run. Flexible cables generally can be run without jumper cables, lowering total signal loss, or alternatively, allowing the use of a smaller cable to achieve the same loss.
Table 2 at the left shows a performance comparison of 150-foot feeder runs of flexible and semi-rigid cables.
By eliminating jumper cables with the 5/8″ and 7/8″ flexible low-loss cables, performance similar to the next-larger-size corrugated copper cables can be achieved with substantial cost savings. The elimination of four connector junctions–two on each of the jumper cables–greatly increases the reliability of the system while simplifying and speeding installation. These savings are being realized by system operators who have chosen the 5/8″ flexible cable to replace 7/8″ corrugated copper cables. The difference in total signal loss is only a few tenths of a decibel for lengths as long as 200 feet, an insignificant difference in system performance. The cost savings are about 50% in materials and a substantial savings in labor. An additional savings may result from the reduction in tower loading with the use of smaller cable.
Shielding Another important characteristic of a coaxial cable is shielding effectiveness. This is a measure of the ability of the cable to keep signals in the cable from leaking out and signals outside the cable from leaking into the cable. A transmit cable with poor shielding may allow RF energy to leak out and interfere with nearby receive cables. Conversely, a poorly shielded receive cable will allow RF energy from the environment into the system and may cause interference. Typically for antenna feeders, shielding is not an important issue because the antennas allow a large range of RF signals into the system, which must be filtered out before the receiver.
Semi-rigid cables with solid outer conductors provide the best shielding, typically better than 120dB. Flexible, low-loss cables have bonded metal tape outer conductors with braided overshields. This construction provides better than 90dB shielding. With either semi-rigid or flexible cables, the weak point in shielding is the interface between the cable and the connector, which will typically limit shielding to about 90dB.
Because the concern is typically the leakage of signal from a transmit cable into a receive cable, the effective shielding provided is the sum of the shielding of the two cables–better than 240dB in the case of semi-rigid cables or 180dB in the case of flexible cables. These levels result in signals that are far below the receiver sensitivities of any practical mobile radio system. In any case, these cables are being used to feed antennas. Typically, the isolation between receive and transmit antennas will be far less than the isolation between the cables; therefore, cable shielding is not the limiting factor in these applications.
Intermodulation distortion Intermodulation distortion or passive intermodulation distortion (PIM) has been a topic of much discussion in recent years as a contributor to performance degradation in mobile radio systems. When two high-power signals are present in a device with a non-linear junction (such as a semiconductor or ferro-magnetic material), a third signal is generated at a frequency equal to two times one of the frequencies minus the other frequency (sometimes referred to as 2A-B). Actually, a whole series of additional frequencies will be created, with the 2A-B frequency being referred to as the third-order intermod product. In high-power systems, such as broadcast, the power levels of the intermod signals can be quite high relative to receive signals and can cause major problems with other co-located systems. Within mobile communications systems, the power levels are relatively low, and the frequencies that are used are usually selected to minimize the probability of within-system interference from intermodulation. The primary concern is active devices that usually produce intermodulation products at levels much higher than passive devices, such as cables.
PIM levels are typically expressed in dBc (decibels below the carrier level). The following estimated values are based on two carrier tests with carriers in the 900MHz range at +43dBm (20W)
Semi-rigid cables with properly designed and attached connectors can provide PIM levels of -160dBc and better. Flexible low-loss cables, with properly designed and attached connectors, provide PIM levels of -130dBc or better. The limitation in both cases is the connector-to-cable interface and the design of the connector.
These levels are far better than the levels provided by typical active components, such as power amplifiers, and are more-than-adequate for most system applications. In the most-common system configuration, additional protection for the receiver is provided by the use of separate receive and transmit antennas. The separation of the antennas results in at least 60dB of path loss, which reduces the level of the intermod products in the receive path. In systems using the same antenna and transmission line for transmitting and receiving, the additional path loss is not available, and the immunity of the system to intermod is decreased.
VSWR VSWR is one way to express the amount of the power directed toward a device that is reflected back to the input. Other ways of expressing this phenomenon are reflection coefficient and return loss. In any case, achieving low values of reflected power depends on maintaining constant impedance along the length of the device. In a cable, low reflected power is important for proper system performance and is also used as a “figure of merit,” with lower values of reflection corresponding with “better” cables.
Achieving low values of VSWR in a cable depends on proper design of the cable to achieve the desired impedance (in the case of wireless communications, typically 50 ohms). This characteristic impedance is determined by the relative dimensions of the inner and outer conductors and the dielectric constant or velocity of propagation of the dielectric. In addition, impedance variations along the length of the cable, especially those that occur at evenly spaced distances along the length of the cable can result in high reflections at specific frequencies, because of an additive effect. These periodic variations can be introduced in the manufacturing process by gears, pulleys, or any other moving part.
Typically, all of the cable types described can be provided with VSWRs of 1.3 or better over specific communications bands–more than adequate for most applications.
Weather sealing For cables installed in outdoor environments, the ability of the cable to withstand environmental extremes over a long time is critical. This includes temperature extremes, humidity and water, UV radiation, vibration from the wind and the loading of ice and snow. Semi-rigid corrugated copper cables have a long history of successful service in many thousands of tower installations. The flexible low-loss cables have been used in tower applications for more than three years in a broad range of climatic conditions with success. Their construction is based on materials used in cable TV applications for more than 20 years. The other smooth-wall, semi-rigid cables mentioned are also based on cable TV technology.
For installations requiring direct burial, or in especially severe environments, flexible low-loss cables are available in versions with a flooding material included in the braid. This prevents any moisture that enters the cable because of damage to the jacket or poorly sealed connectors from traveling along the length of the cable.
Installation time and cost Semi-rigid corrugated cables can be difficult to install, especially where they must be routed in tight spaces, including building-top sites and monopole towers. Flexible low-loss cables can be routed much more easily because of their substantially lower bending moments. Flexible low-loss cables are also virtually immune to kinking, even if they are bent on a tighter radius than their minimum bend radius. This is because they get their strength from thick polyethylene jackets rather than from a thick metal outer conductor. Because polyethylene is a resilient material, flexible low-loss cables are much less subject to damage than the solid-metal outer conductor semi-rigid cables.
For 7/8″ and 11/4″ diameters, flexible low-loss cables actually have higher crush strength than a corrugated copper cable. At these sizes, with the low-density foam dielectric used and thin copper-tube center conductors, corrugated copper cables are quite susceptible to kinking and crushing, whereas the low-loss flexible cables are much more rugged.
The installation of connectors on the low-loss flexible cables can be accomplished in a fraction of the time that it takes to install a connector on a corrugated copper cable. Trim tools are available for the low-loss flexible cables to trim them back to the proper dimensions for connector attachment. The attachment of the connectors for the 5/8″ and larger flexible low-loss cables can then be completed in less than 2 minutes per connector. This process can be time-consuming for a semi-rigid cable and may require expensive special tooling, especially in the case of the smooth wall copper cable.
Conclusion Low-loss flexible cable provides an alternative to semi-rigid corrugated cable with advantages in loss, handling, ease of installation and ruggedness. Although there are tradeoffs in shielding and intermod performance compared to semi-rigid cables, their performance is more than adequate for most mobile radio applications. These cables are being used in antenna feeder and system interconnect applications by more and more original equipment manufacturers and system operators worldwide.