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By Gene Coll and Gary Knaus, Thomas & Betts Cable Communications Division

Don't let your drop hold you back. The reliable performance of the drop clamp method of cable attachment helps you maintain system reliability and customer satisfaction.

Many multiple system operators (MSOs) are upgrading their drops to meet new demands such as the addition of telephony and security system signals. Studies have shown that subscriber drops are the weak link in most systems, and are responsible for up to 70 percent of service calls.

Without secure and dependable drops, the functionality and performance of a system may severely deteriorate, leaving subscribers with poor signal quality. While several methods of drop cable attachment exist, it is becoming clear that not all "industry practices" offer the consistent performance required to support the latest technology.

In the early days of cable TV, when demands placed on systems were minimal, almost any inexpensive means of maintaining the cable plant was acceptable. Even using electrician's tape in place of straps or lashed cable supports was considered a suitable solution. Although the industry has come a long way since then and technologies have emerged to make the worst of these "remedies" a thing of the past, this mentality is not entirely obsolete.

Of the various drop cable attachment methods that have been used throughout the years, many have failed to keep up with the changing technological environment.

Spiral grips, along with non-messengered coaxial cable, were among the first methods used to attach a drop. They were an economical option that seemed suitable for simpler systems, but it was later discovered that they created problems with cable integrity, making them unacceptable for future use.

Although spiral grips provided a secure grip of the coaxial drop cable, they often left severe indentations (or "tiger stripes") in the cable's outer jacket and sometimes fractured the foil wrap. This damage left the cable susceptible to external elements, such as extreme weather, and compromised the overall signal quality, causing signal egress and ingress.

Despite their obvious weaknesses, the spiral grips worked adequately for most 12-channel systems. However, they could not provide the consistency needed for systems with higher channel capacity that used a larger portion of the cable's bandwidth. The spiral grip no longer is used in new construction, though many older systems still have subscriber drops with them in use. As with the straps and electrician's tape of the early years, the spiral grip proved to be a short-term solution to drop attachment.

Today, the "2-4-2," "3-6-3" and other types of "wrap" methods are more widely used options in drop cable attachments, but they result in problems similar to those of the spiral grip method. The 2-4-2 wrap's name reflects its approach to drop cable installation: Approximately 12 inches of messenger wire is stripped from the coaxial cable, wrapped twice around the "P" hook, four times around the messenger itself, and twice around the unstripped coaxial cable and messenger.

The 2-4-2 method forces the messenger wire into a bend radius smaller than its recommended minimum. A typical "P" hook is 0.200-inch in diameter, meaning a tight wrap of the 0.051-inch to 0.072-inch diameter messenger wire violates the recommended minimum bend radius at five times the wire diameter--or 0.255-inch to 0.36-inch.

Also, repeated flexing of the wire caused by windy conditions or other external hazards "work-hardens" the wire, decreasing its flexibility. Similar to repeatedly bending a paper clip back and forth, the cable becomes fatigued, making it hard, brittle and more susceptible to fractures. As with the spiral grips, the 2-4-2 and 3-6-3 wraps also may cause breaks in the foil sheathing that may lead to exposed lines and a significant decrease in signal quality.

Because the wrapping procedure is inconsistent for the 2-4-2 and 3-6-3 wraps, these methods also leave much room for human error. Some installers may wrap the messenger wire more tightly around the coaxial cable than others, so much so that the coaxial cable is overtightened, again causing fractures in the sheathing and foil that may lead to signal problems. As today's coaxial cables become responsible for the delivery of more channels, these "wrap" methods lack the consistent performance needed to maintain a secure drop attachment.

With today's system upgrades requiring coaxial cables to carry an increased number of channels, along with two-way communications, computer modem hookups, telephony, security systems and smart home applications, making a secure drop attachment is more crucial than ever. In response to this growing industry need, a more consistent and professional solution for drop attachment is required.

Although it borders on heresy in some industry circles to compare cable TV to the telephone company, telcos have the longest track record of consistent universal drop wire attachment. Starting in 1951, the phone company installed the first residential communication drops using a universal three-piece drop-wire clamp that has been installed throughout the country and integrated into virtually every phone system. The consistent drop installation method and performance assures trouble-free service and contributes to the well-acknowledged success of the telephone system.

Eager to make such assurances in their systems, cable TV engineers designed the coaxial drop wire clamp to offer the same consistency and reliability as the telephone cable clamp.

The coaxial drop wire clamp provides a method for drop attachment that enables the messenger wire to be secured to the clamp body at multiple points, rather than around the cable itself, thus enabling the connection to withstand greater stress without distorting the cable. The messenger wire is pulled through the bail wire, wrapped around the large radius of the clamp "nose" and then threaded through the "L slots" in the body of the clamp. The remaining messenger wire is wrapped around the body of the clamp, providing the support needed to hold the cable in place without causing damage to the cable. The clamp has a 200-pound tensile strength and may be used to support the coaxial drop at both the strand and building. With an aluminum body and a stainless steel bailwire, the drop wire clamp is not susceptible to corrosion problems and holds the coaxial cable securely.

Other types of coaxial drop clamps are used today, including one designed with a spiral-shaped body. The drop wire is inserted within the spiral body and the messenger wire is wrapped around the abutment, then around the washer and secured in the "V" slot. Both varieties of clamps protect the coaxial cable, preventing it from being compressed and damaged.

A 1997 winter storm in upstate New York provided evidence of the reliability of the drop clamp method. After enduring the ice-loading associated with multiple storms, one multiple system operator (MSO) lost more than 5,000 drops that used the 2-4-2 wrap method, while a neighboring system using coaxial drop clamps lost none. The type of reliability displayed by the drop clamps in these extreme situations is required to maintain the cable TV systems of the future.

Despite the many advantages of the drop wire clamp, many cable systems still use 3-6-3 and 2-4-2 wraps to support their systems, and a few holdouts may still have spiral grips in some older installations. In an effort to determine the most reliable form of drop installation, Thomas & Betts conducted a series of tests designed to reveal how each of the three types of attachments performed when confronted with simulated everyday hazards over an extended period of time.

Using the three different attachment methods, the test site was arranged as follows:

• Lines were set up indoors to negate the effects of variable weather conditions.
• Trishield and Quadshield coaxial cables from the industry's three major manufacturers were tested. (Cable manufacturers included CommScope, Times Fiber and Belden.)
• 100-foot drops were used with 10 feet added to attach to test equipment for monitoring insertion and return loss.
• Drops were oscillated (attached to a motor-driven rotating shaft at one end) 100,000 cycles to an amplitude of 4 inches to 6 inches at the 50-foot midpoint to simulate, in an accelerated way, the effects of wind over the drop's service life.
• The fixed end of the drop was secured to a "P" hook, mounted in a 2-inch-by-8-inch plank. Nine drops were mounted to the rotating shaft (spaced about one foot apart) for each test run.
• 60 pounds of tension were applied to each drop, resulting in a 15-inch sag at the drop's midpoint. Test equipment was set at 750 MHz.
• All cables were swept (tested for insertion and return loss) prior to the tests, after installation and tensioning and upon completion of the tests.

The test results revealed that the spiral grip showed return loss readings averaging 24 percent worse than at the beginning, and an insertion loss reading rated 1 percent worse than at the start.

The 2-4-2 wrap series all failed prior to the completion of the 100,000 cycles. Some failed at 6,800 cycles. The average time of failure occurred at 12,000 cycles. Return loss readings for these incomplete tests were 2 percent worse than at the start. (Prorated to predict the results of a complete 100,000 test cycle, the readings would have been 16 percent worse.) Insertion loss readings averaged 1 percent worse with prorated estimates at 8 percent worse.

The coaxial drop clamp series delivered return loss readings averaging 3.5 percent, or slightly worse than at the beginning of the test. Insertion loss readings averaged 0.6 percent worse.

Although it is difficult to duplicate the effects of field conditions, the Thomas & Betts' testing demonstrated that neither the spiral grip nor the 2-4-2 wrap could maintain optimum cable quality after enduring the repeated flexing and oscillations depicted in their test environment. The coaxial drop clamp delivered the most reliable performance when confronted with the external stress associated with daily weather conditions.

Although its lower initial cost may tempt some operators to believe that the 2-4-2 or 3-6-3 wrap methods are suitable for today's cable systems, the weaknesses of these methods will be more pronounced and bear a considerable expense in future systems because of their inability to provide consistent secure connections in systems with a higher channel density. Fitting a high traffic system with an inferior attachment may be a risk too great for many MSOs, especially those offering telephony service (consider the liability implications if 911 service is disrupted because of a faulty drop attachment).

Considering the rising cost of truck rolls (estimated at $60 to $75 each, and in some cases more than $100 each), the expense of preventable repairs becomes increasingly important. As with most situations, investing in quality and performance from the start may result in significant long-term cost savings.

A cable TV system is only as good as its coaxial drop cable installations. Installing systems using less-than-consistent methods may provide less-than-consistent results. Sacrificing quality in drop cable attachment by using less effective drop hardware places communications systems at a higher risk of failure. By investing in more reliable technology at the system's foundation, an operator may build a more efficient and dependable network with the flexibility to handle both today's technology and tomorrow's expansion.

Gene Coll is senior applications engineer for Thomas & Betts Cable Communications Division, and Gary Knaus is RF test lab supervisor for Thomas & Betts. They may be reached at , and , respectively.

 Back to June 2001 Issue

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