Coaxing the Most Out of Coax Cable

As the saying around the oases in Arabia goes, once one lets the camel's nose into the tent, the rest of the camel is sure to follow. Similarly, some people believe the emergence of the hybrid fiber-coax (HFC) network means that coaxial cable is in danger of being nosed out of the market. While fiber will continue to make inroads in the personal communications systems (PCS) and CATV markets, there is no cause to expect coaxial cable to be kicked out of the tent anytime soon. "Fiber is wonderful, but co-ax is a viable solution," says Kimberly Driggs, president of Covid Cactus Cable in Tempe, Ariz. "Coax will not go away any time in the near future." Interestingly, Covid got into the fiber business years ago because the company's leaders anticipated a move away from coaxial cable to fiber. However, especially in the U.S. market, it has not turned out that way. "Domestically, people like to use what they know and what they know works. That is coax," Driggs continues. She predicts network demands-the sources and displays-are what will dictate the types of cable used in the networks of tomorrow. "Fiber is good for really big pipes or a small pipe with big bandwidth," says Galen Gareis, senior advanced product engineer for Belden, Richmond, Ind. "But most jobs with typical bandwidth to 100-foot lengths you can do with coax. Why change from coax if what you are doing is working?" he asks. Today's coaxial cable has tremendous bandwidth capability and electronics. Belden has coaxial cable with 30+ dB return loss. "You don't put rocket fuel in your car just because it's available," Gareis says. "Of course, you could run a turbine car, but it's awfully expensive." He says the same holds for the fiber vs. co-ax argument. With coaxial cable, the pipe is still big enough and installation is much cheaper. Coaxial cable is called "coaxial" because it includes one physical channel that carries the signal surrounded (after a layer of insulation) by another concentric physical channel, both running along the same axis. The outer channel serves as a ground. Many of these cables or pairs of coaxial tubes can be placed in a single outer sheathing and, with repeaters, can carry information for a great distance. A protective, cylindrical sheath encased in a dielectric surrounds the inner conductor in the typical coaxial cable. An outer conductor acts as a return path for low-frequency currents or as a waveguide for higher frequencies. The shield prevents the cable from either picking up or emitting electrical noise. These features have made coax a mainstay in the CATV market. The CATV market According to David Cyganski, an instructor at Worcester Polytechnic Institute, Worcester, Mass., everyone knows coaxial cable is used to transmit 100 TV channels into the home via CATV coaxial cable. These cables provide nearly 1 GHz of bandwidth to the home. These same cables are capable of transmitting many GBps of information. In fact, Cyganski maintains that the research and test deployment of CATV-based Internet delivery systems is currently a growth industry. According to Cyganski, in many businesses today, coaxial cable is the major delivery system for 10 and 100 Mbps Ethernet computer network data signals, for hop distances of 500 meters and 185 meters respectively for the larger and smaller diameter cables (RG-8 and RG-58) that are in common use. Admittedly, fiber deployment in this traditional coaxial stronghold is expected to experience solid growth. Today, cable companies are using fiber to upgrade their coaxial cable in order to provide two-way transmission and new applications such as video-on-demand. The MultiMedia Telecommunications Association, Arlington, Va., predicts fiber deployment will peak in 2001 at 4.5 million kilometers, then fall off a bit in 2002. The cable industry represents over 30 percent of annual fiber installations. Bruce Carlson, vice president of network division engineering for Trilogy in Jackson, Miss., says coaxial cable was once used for the entire plant for CATV, from back-end to the home. "The game now is HFC, with fiber to a neighborhood and co-ax to the home," he said. "I feel pretty strongly that this will be the way to deliver high bandwidth to the home and still have the backfeed that the systems require." Telephone companies also resort to coaxial cable to bridge larger distances with higher rate digital connections. One example Cyganski gives is the use of coaxial cable to transmit 140 Mbps data signals between telephone switch buildings with a hop distance of up to 2 km. Gareis says telephone companies can afford fiber optics because they spread the cost over so many households. "You're paying for those 1550 nM laser transponders over many homes. But look at what it costs to use fiber closer to the building," he says. "It's just a romantic invention of the fiber market to say their product solves all problems." Gareis says single-mode fiber connectors are expensive and tricky to use in the field, despite recent innovations. He points to both the previous Olympic Games and the upcoming Games in Sidney, Australia to make his point: "They used relatively little fiber for setting up the broadcasts," he says. "It's good, old co-ax they used to wire it up. The preponderance of cable in the Olympics is copper-they need to make sure the network will work." Carlson, too, is bullish on the future of coaxial cable, especially since the entire networking business segment is growing rapidly. "The market is growing so fast that there is not a big concern," he says. The downside Coaxial cable has a long history. It was invented in 1929 and first used commercially in 1941. But 60 years of success do not ensure eternal life. Indeed, some observers see limits to coaxial cable's use. According to Mitchell Snier, author of the Dictionary of CP Hardware & Data, published by O'Reilly & Associates, coaxial cables are generally falling out of favor for the following reasons: - They are too single-purpose; you need a different type for each application, and you can't use any of them for token ring, FDDI, RS-232, telephone, or ISDN. - For the same length of cable, coaxial cable can have five to 500 times more attenuation (depending on many factors, such as the type of coax and the frequencies used) than fiber-optic cable. - Running coaxial cable between buildings creates problems of ground-potential difference (the building grounds will be at different voltages), so the coax shield must be insulated from building ground in at least one of the buildings. - Outdoor runs need lightning protection. For the last two points, fiber-optic cable is a better choice, as it is an insulator (sometimes called a dielectric), so it does not require lightning protection. Fiber scores some good points. Preston Buck, manager of premises applications for Corning, Inc., Corning, N.Y., concedes that copper (including co-ax) will continue to be used for a large percentage of desktop links because it provides a less-expensive, short-term solution than fiber. "But with Gigabit Ethernet, I hope the industry is beginning to realize that upgrading their copper cable isn't really buying them that much," he says. "The upgrade still is basically the same material in a slightly different configuration. Perhaps it's time to truly upgrade the network cabling to optical fiber and reap the benefits for years to come." Just as one can argue that a station wagon was a poor compromise between a passenger car and a pickup truck, so too can one point out that a single-purpose coaxial cable is best used for specific applications, rather than as an "all-purpose" solution. Look at specs Coaxial cable usually has low transfer impedance, obtained by two or more shields. Coaxial cable usually has impedance of between 50 and 100 ohms. For example, an RG-58 coaxial cable might be rated at 52 ohms. RG-6 is typically around 75 ohms, and RG-62 is 93 ohms. However, this can not be measured with an ohmmeter. By contrast, a typical Category-5 twisted pair cable might have 100-ohm impedance. Therefore, when a cable is connected to a device with 100-ohm impedance, the cable, regardless of length, appears infinitely long to the circuit. So, the voltage-to-current ratio is the same all along the cable. "Our goal is to improve the performance of our coaxial cables," Driggs said. "That's possible with better shielding, going to air foaming or a cellular structure, and better dielectrics. You have to keep up with the resolution and frequencies of computers and match the demands of the source devices." Driggs expressed concern over the discrepancy between the size of co-ax and that of fiber. "A 20-gauge cable is so large and a fiber is so much smaller for what it can carry." While Carlson also concedes that fiber optics has its advantages, he sees coaxial cable retaining a grip on the PCS market, which is a huge growth area today. Co-ax is used on the short run between the radio in the hut wall. A 1.625-inch co-ax runs from the shelter up the 200-foot tower. A short half-inch jumper is found at the top. "There is between $10,000 and $20,000 in co-ax at any given site," Carlson says. Due to loss, and the fact that the U.S. PCS market is based in the 2 GHz area, the 1.625-inch is used. In Asia and South America, where similar systems are in the 800-1 GHz band, they use 7/8 coax. Coax has a major advantage in the tower business. Remember your physics: the antenna on any tower is electrical and has to be activated by electrons. The photons in fiber can not activate unless an optical-to-electrical converter is installed on the tower. "The tower is essentially a big lightening rod," Carlson said. He also expressed reservations about putting expensive conversion equipment on a tower. In addition, when was the last time anyone saw optical computers deployed in an office? Or, the first time for that matter? Gareis predicts that co-ax will have the inside track as long as computers are electronic, not photonic. Coax growth Another growth area for coaxial cable is in subways and tunnels, where coax can function like a long antenna to deliver radio signals to police and transportation officials. Called radiating cable, it has slots cut along its length and is strung along the wall of the subway. Carlson explains that his company's product, marketed as AirCell, works like a water hose with numerous cuts along its length. Just as the hose leaks a controlled amount of water, so the radiating cable "leaks" a controlled amount of radio signal along the tunnel. Carlson sees similar use for coax in buildings, especially in Asia where people in places like Singapore and Hong Kong are telephone crazy and want to be in touch via cell phone everywhere. Citing coaxial cable as the tried-and-true solution, Gareis warns against using fiber just to appear advanced. "The input/output transceivers in fiber are very expensive. Fiber is not as permanent and long lasting as people would have you believe. The battle is not as clear-cut as the fiber forces would maintain." Since optical fiber is so new, users are encountering some problems as new laser technology comes into the market and as different types of fiber expand into the marketplace. Buck disagreed. "Fiber-optic cable is relatively straightforward to install. There aren't any twists to count or 3/8-inch insulation to remove. The jacketing materials are there to support and protect the fiber. They play no part in the transmission of the light and can not affect the performance of the network." He also notes that the connectors are not part of a fiber transmission path. Still, Gareis advised, "Stay with tried-and-true. The coax pipe is still big enough and it's cheaper to use." With strong applications in use, and with market projections showing rapid growth in almost all areas, the communications tent could expand rapidly enough to create room inside for both coaxial cable and the fiber-optic "camels" in the networking tent of the future. Harler, a contributing editor to Electrical Contractor, is based in Strongsville, Ohio. He can be reached at (440) 238-4556 or via e-mail at

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