Arlyn S. Powell, Jr.
Coaxial-cable connectors may seem like dinosaurs--large, heavy and bulky--but they are in no danger of becoming extinct. In addition to serving the large installed base of Ethernet and other local area networks that depend on coaxial cable, these connectors are seeing something of a resurgence in use in cable television`s hybrid fiber/coaxial-cable installations and the telephone company`s broadband services. Coaxial cable and components are also used in emerging microwave and wireless applications.
This means that manufacturers continue to look for ways to improve their products--and distinguish themselves from the competition. From the standpoint of cabling installers, there certainly seems to be room for some improvement.
Harry Connell, manager of new business development for cabling contractor Data Cable Services (North Lake, IL), sees a number of issues that could be addressed. "One problem we have with coaxial-cable connectors," he says, "is that they are more labor-intensive to put on and they have to be installed correctly. For example, the braid has to be attached just right. It`s complicated, like a Rubik`s cube."
Attaching a coaxial-cable connector is a multistep operation that requires a special crimping tool and takes two or three minutes (see "Checklist for Stripping and Terminating Coaxial Cable," page 14). The core conductor and outer braid have to be terminated to the connector to complete the circuit. "Many of the problems we get called in on come from people who try to do it themselves," Connell observes. "It`s an acquired skill. We have to bail them out, because they don`t do it all the time. They lose the touch."
Quick-termination twist-on connectors are available, but their reliability has been questioned. "We don`t recommend twist-ons," Connell says. "They tend to be unstable. When we send someone out on a job, it`s done the old-fashioned way--with crimp-ons."
All the news about connector termination is not bad, however. Although you need some training to put on coaxial-cable connectors, and steady practice to maintain the skill, the task itself is not too difficult. Because coaxial cable tends to be larger and stiffer than unshielded twisted-pair cable, it is easier to handle. "I suppose you have the same problems with RJ-45s," Connell concedes. "You still have to apply the correct techniques."
Another issue that affects the use of coaxial cable and connectors is cost. The cable is thicker and more complex than unshielded twisted-pair cable, resulting in a higher cost per foot. Coaxial-cable connectors are also expensive because they are made from such metals as copper and aluminum. The plugs and jacks used in Category 5 installations, on the other hand, comprise mostly plastic. In addition, specialized installation tools such as a coaxial-cable crimper could cost $30 or $40; a different crimper may be required for each manufacturer`s product, because there is little standardization in this area.
This lack of standardization can be a problem in other ways. Cable nomenclature follows no pattern, such as the higher the number, the larger the cable. RG-8 Thicknet cable, for instance, is larger than RG-58 Thinnet. Because each cable type has a different impedance and varying electrical characteristics, each is suited for only a single application.
Connell adds, "One problem we have with 50-ohm cable is that occasionally people mix it up. People will switch an RG-62 cable for an RG-58 and, of course, they`ll have problems with the ohm ratings. It won`t work correctly."
This situation has arisen because coaxial cable and components for use in premises applications grew up in computer networks in the 1960s, when all systems were proprietary and specific to individual vendors. Coaxial-cable technology still reflects this piecemeal history, whereas premises applications of UTP cabling technology are recent enough to be comfortably rationalized into generic structured cabling systems.
Because the structured system is much more appealing to the installer and end user, the contractors steer their customers toward twisted-pair systems whenever possible. "If a person has a coaxial-cable network installed," Connell says, "and the network does not have to be moved--and it works--there`s no incentive to make any changes. But more and more, if people have to move, then they can justify the capital expenditure to switch to structured cabling. The people we talk to who want to move into the future are all installing Category 5 cable in the horizontal and fiber in the backbone."
Even if the installation is not being moved, there can be reasons to switch to a structured cabling system. Maintenance can be a problem with coaxial-cable networks, and troubleshooting can be a nightmare. "There can be real problems when the end user moves a terminal," Connell says. "The `T` connectors in a coaxial-cable network can pop out or bend, which leads to service calls. With one customer, we get a call approximately every three days, believe it or not. Maybe the cleaning people come in and bump something. And with some networks, you have the old Christmas-tree light problem: If one terminal goes down, they all go down. It can also happen that two users are sitting side by side, with one up and one down. That can be a bit confusing."
Growing up piecemeal
Troubleshooting is also much more time-consuming and expensive with coaxial-cable networks than it is with UTP cabling. One reason for this is that there is rarely a cable map, and the coaxial-cable system has grown up piecemeal rather than systematically. "It`s very difficult sometimes, if you have 150 nodes, to locate a suspect cable," Connell adds. "You have to first find out where the run goes, and there`s no cable plan because the network has evolved over time: The user started with 100 nodes, and then added five more, and then three more. The company has no idea how many cable segments they have, or what the linear footage is for each segment. It looks like an octopus when you open the ceiling or ductwork."
This cable-locating issue is not always apparent to the customer. "We have to put a tone on a line," Connell says, "and then we go up into the ceiling, and over here and over there. It eats up a lot of the customer`s time. We tell the customer that sometimes these things are not easy to track down. We had a problem the other day that took two or three hours just to locate--because there were add-ons to the network, and the run went through manufacturing, and up into the ceiling. We had to go through tons of coaxial cable that dated back to prehistoric times.
Not surprisingly, manufacturers accentuate the positive when it comes to coaxial-cable connector technology. According to Al Melite, director of sales and marketing for radio-frequency products at ITT Cannon (Santa Ana, CA), "It is a truism that connectors cost less than the labor it takes to install them, especially for coaxial and fiber-optic cables. But that doesn`t have to be true. By redesigning connectors, manufacturers can help their customers cut labor cost."
Melite also points out that conventional BNC connectors cost approximately $1 but take 2.5 to 3.5 minutes to install. At today`s labor rates, that works out to $3.50 to $4.50 per termination. "And a better connector design could not only save time, but it could also mean more consistent, reliable connections," he says.
A major problem with BNC and similar coaxial-cable connectors is that termination with them is skill- and labor-intensive. "There are a lot of opportunities for mistakes that will affect the performance of the cable," Melite says. A standard BNC or similar connector comes in four to seven pieces. The installer must separately crimp a center contact to the center conductor and then crimp a ferrule over the outer cable. This may require two crimping tools. So, using special tools and handling small parts, the installer must perform several steps per termination
"The work may be done under less-than-ideal conditions," Melite adds. "Installers may be high on a ladder or packed into a tight equipment bay. What happens if they drop a part? They can either climb down and fish out the part, or use a new connector, both of which waste time and money."
ITT Cannon, as well as other companies, has addressed these problems by trying to redesign coaxial-cable connectors to meet market demand for quick installation, reliability and lower price. "They should also fit within the outline profile of the standard connector and use the same stripping dimensions," Melite says. The ITT Cannon solution, which it calls the QT connector (for "quick termination"), can be installed in less than 30 seconds. It is also pre-assembled, so that it has only two parts to install. A push-on technique is used to attach the center conductor to the connector, while the outer ferrule is crimped to the cable using a standard hexagonal crimper. The item has been successful in Europe and will be introduced in the United States later this year.
Bob Hoff, general manager at Tru-connector Corp. (Peabody, MA), a manufacturer of specialty coaxial-cable connectors, agrees there has been a resurgence of interest in the quick-connect connector, especially where mating and unmating can be reliably performed. Asked about trends, he said miniaturization is becoming increasingly important.
Shrinking component size
Kris McGuire, an applications engineer at Hirose Electric (Simi Valley, CA), also singled out shrinking component size as a trend. "We`re seeing more wireless and cellular-phone applications," she says, "so everything, including connectors and cables, is getting smaller.
"We also try to get the expense down as much as possible," McGuire adds. Small, low-profile connectors are being stamped now rather than cast, but alternative connector materials such as composites are still on the horizon. To keep costs down and to avoid field termination problems, Hirose depends on factory termination of cable assemblies.
The BNC, a bayonet connector, has been the industry standard for decades. The TNC is threaded to increase reliability and performance. However, according to McGuire, "The latest trend is toward a simpler push-pull design."
The 7/16 DIN coaxial-cable connector recently introduced by Tru-connector Corp. (Peabody, MA) comes with either a standard hex nut or a new low-profile design that features a knurled nut.
Checklist for Stripping and Terminating Coaxial Cable
1. Get ready.
Here`s a short procedure to use as a guideline for stripping and terminating coaxial cable. For best results, the following universal tools and connector types are recommended:
Two-step rotating stripper
Ratcheted crimping tool
Captive pin connectors
Although other connector systems may be used, captive pin connectors ensure positive retention of the center conductor.
2. Cut the cable.
Make a straight cut in the termination end of the cable.
3. Insert the cable into the stripper.
Adjust the stripper to meet the desired cable diameter and stripping requirements. For best results, the stripper should be adjusted to expose 0.25-inch of the conductor and 0.25-inch of the insulation. Insert the cable into the stripper.
Rather than adjusting the stripper each time, set and use different strippers for varying cable diameters.
4. Rotate the stripper.
Rotate the stripper three to five full turns.
5. Pull the cable out of the stripper.
Pull the cable out of the stripper and inspect the stripped section. Make sure that the center conductor and the insulation are not nicked or scored and that stray strands from the braid are pushed away from the conductor. You are now ready to terminate the cable.
6. Seat and crimp the pin on the conductor
Seat the connector system pin on the exposed conductor. Crimp the pin to the conductor, using the small-diameter die in the crimping tool.
7. Insert the sleeve and connector onto the cable.
Slide the connector system sleeve over the pin and exposed insulation. Place the connector body on the cable end. Align the connector body so that its shaft fits over the pin and between the braid and the insulation. This should spread out the braid. Check again for stray strands and push them out of the way. Make sure the pin flange rests on the exposed insulation. To do this, hold the cable in one hand and use your other hand to push down on the pin.
8. Crimp the connector onto the cable.
Slide up the connector system sleeve to cover the exposed braid. Make sure the crimping tool is fitted with the proper hex die. Place the crimping tool over the connector sleeve and squeeze the tool until the die is completely closed. Inspect the neatness and tightness of the termination.
This sidebar was prepared from the Dupont Blue Book, Revised Edition (1992), and is made available thanks to Anne McIntosh and Bill Digel, Dupont Corp., Wilmington, DE.