Clearing up the feeder cable debate
The ubiquitous coaxial feeder cable provides the radio frequency link on land mobile base stations from New York to Los Angeles. Bought by the foot, its buy-and-install cost is negligible compared with the total base station price. In many quarters, the feeder system is regarded merely as a land mobile network “consumable,” with the thinking being “a cable is a cable is a cable.”
However, some experts — including network operators and planners and site installation and maintenance crews — believe such thinking is wrong. If poorly selected and managed, the potential total life-cycle costs of a transmission line system can be very high indeed.
One such expert is Aaron Wilson, technical maintenance manager with CMS Wireless, a wireless base station developer headquartered in Little Rock, Ark. Wilson’s company provides end-to-end wireless project development services, from site acquisition and base station design, to installation and maintenance.
It is at the maintenance end that Wilson sees feeder costs really blow out.
“We often get called out to repair faults on non-CMS Wireless sites,” he said. “The majority of the feeder cable problems we see here are as a result of poor ‘connectorizing,’ problems at bends, and cuts and crush faults in the cable run.” The result is poor signal strength or intermodulation problems.
According to Wilson, it’s a frustrating and costly business for the operator. “The end-user is putting out maintenance money to repair what should’ve been done right in the first place,” he said. The cost isn’t limited to that of raw maintenance; there are also costs resulting from base station downtime and resultant subscriber churn.
Wilson cited the limited nature of the feeder system commissioning/testing regime as a factor, as the common voltage standing wave ratio (VSWR) sweep test doesn’t truly measure the long-term quality of the install. “You can tighten a connector down and make the sweep pass today, but over time and temperature cycles, a poor install will deteriorate,” he said. “You’ll start seeing reflective power over time.”
The challenge facing both land mobile operators and installation crews lies in dealing with the number of variables in the transmission line equation. These include a wide selection of feeder technologies — such as corrugated cable, high-performance corrugated cable, and rigid smooth-wall cable — plus a dizzying range of accompanying connector systems and tools. There also is a wide diversity in installation crews’ skill levels and experience, plus a broad range of site layouts and weather conditions. This mix makes achieving repeatable long-term feeder performance a challenge.
“We’re well aware of this real-world, multi-variable situation for base station feeders,” said Chris Adams, transmission lines global product manager for Meriden, Conn.-based Radio Frequency Systems. “All too often, feeder cable technology groups focus too hard on achieving performance in one or two key areas — say attenuation, flexibility or connector VSWR.”
The end result, according to Adams, is something that might perform well in the laboratory, but is a disaster in the field. “It’s about striking a balance and remembering that the feeder systems are ultimately destined to be installed by real installers on real base station sites, not in labs,” he said.
Accommodating cable bends — sometimes up to six bends in a single run on a modern urban land mobile base station site — is an area where problems can occur. Most vexing is the rigid smooth-wall feeder cable. Without the corrugations of conventional cable, it exhibits minimum bend radii of up to two and a half times that of corrugated cable and bending moments as much as six times greater.
“From an installer’s point of view, this is the hardest cable to install,” Wilson said. “It’s hard to bend, and if you re-bend, it tends to kink. If you bend it once and need to bend it back, it generally snaps.”
The kinks that ultimately occur in rigid smooth-wall cable represent a great deal more than visual blights — they are weak points that crack or deteriorate over time and, ultimately, reduce signal strength.
For these reasons, the appearance of smooth-wall feeder cable on the base station scene is something of a curiosity.
“To date, rigid smooth-wall feeder cable has mostly been used in the U.S. and elsewhere in its native application — that of cable television signal routing,” Adams said. “Here, it’s buried deep beneath the ground — where the thermal conditions are comparatively stable — and routed in long straight runs with few bends, so it thrives.”
In the vastly different environment of the land mobile base station, conditions are quite dissimilar. “When we get called out on a site repair where rigid smooth-wall cable has been used, we always check for the kinks first,” Wilson said. “We also check the ground kits and connectors, as cutting into the outer conductor seems to be a common problem with smooth-wall installs. If it’s cut, then thermal contraction and expansion eventually make an opening for water to get in.”
Some high-performance (reduced attenuation) cables also can pose site problems. In the quest to minimize attenuation, some manufacturers have used dielectric foams with densities so low that crush resistance is severely compromised.
“They’re looking at gaining a quarter of a decibel, but losing crush strength in a big way,” Wilson said. “When you take the cable out and put a hoisting grip on it, it puts indentations in the cable because it’s so thin.” This is an even greater problem on co-location sites, he said, where obstructions and bends are numerous.
Adams pointed out that a great deal of thought was put into the attenuation/crush strength trade-off during the development of RFS’s reduced attenuation feeder system. “We knew crush strength was vital on site,” he said, “so we didn’t compromise here, while dropping the attenuation up to six percent.”
Dressing or “connectorizing” the cable — finishing the cable so it is fitted with a universal 7/16-inch DIN connector — is the second area where Wilson sees problems that often are caused by cable manufacturers. “There are some brands out there that have three different types of connectors and connector tools, to do three different cable models, all by the same manufacturer,” he said.
He added that such a scenario is a recipe for disaster. “We’ve actually just repaired a site where the installer made the wrong connector fit the cable. There was no signal strength at all — it was completely open.” Though the cable and connector were of the same brand, the models didn’t match.
Worse yet, the wide variety of tools required to handle a multiplicity of connectors can be expensive, bulky and difficult to use. As a result, simply ensuring one has the right tool for each job can be an uphill battle for a field technician, according to Wilson.
Adams agrees that cross-range and backward-compatibility of connectors, plus simplicity of tooling, is essential for getting the job right on site. Equally important though, is ensuring the RF and electrical robustness of the connector design itself. There are three basic designs for connectors — two of which rely on clamping down on the outer of the cable, which is ultimately problematic.
“Smooth-wall cable is an entirely glued assembly — inner conductor to dielectric to outer conductor to jacket,” Adams said. “This means you can’t flare the outer to electrically clamp it from both sides, as you do with corrugated cable connectors,”
As a result, the cable outer is clamped from the outside only, using a slip ring. “Over time, the outer starts to collapse under the slip ring and gives way. Installers tell us that you can leave an installation nice and tight, then return to find the connector can almost spin on the cable. This leads to inconsistent behavior, dropped calls, intermodulation and so on,” Adams said.
The other basic design that applies pressure to the cable outer conductor is used with some corrugated cable connectors. While the outer is flared and clamped in the conventional manner, a ring of ball bearings within the connector head is used to achieve extra electrical connection on the ridge of one corrugation. However, this design is flawed, as the bearing ring only grabs a small portion of the available conductor, according to Adams.
“Instead, we use a slotted brass finger claw on the connector to maximize the contact with the outer,” he said. “It gives around twice the electrical contact and better long-term performance.”
Water ingress in the cable is another area of debate. Some argue that the smooth-wall cable’s glued construction prevents water ingress, whereas flexible corrugated cable is susceptible. Others believe such an argument is nonsense.
“I have been using corrugated cable for many years now and have never experienced water ingress problems,” said Stefan Kraege, project leader with the German installation group Hestra-Antennenmontage. “If the connectors are correctly installed and sealed, you have no problems. We have base stations that are almost a decade old, and none have experienced water problems.”
Adams agreed: “The glued assembly of the rigid smooth-wall cable stops water moving from within its connector to the cable. But water in the connector in a cellular installation is enough to totally disrupt transmission. Our view is that the only place water belongs is outside the transmission line.”
He added that water generally gets in via a cut in the cable outer or a poor quality connector. Keeping water out of the connector comes down to two factors — simplicity of connectorizing and quality three-point sealing, rather than using simple crest-seal O-rings, Adams said.
Minimizing feeder system total life-cycle costs starts and finishes at the site. “Stick with what works,” advises CMS Wireless’s Wilson. “To me, that’s corrugated cable. My suggestion is to find a cable with just one type of connector and stick with it. Make sure you keep the crush strength up there, as well.”
Jôerg Springer is the chief marketing officer for Radio Frequency Systems, a global designer and manufacturer of cable and antenna systems plus active and passive RF conditioning modules, providing total-package solutions for wireless infrastructure. He can be e-mailed at [email protected].