A cable is a cable, is a cable - is not

The ubiquitous coaxial feeder cable provides the RF link on mobile base stations from Melbourne to Minneapolis. Bought by the metre, its 'buy and install' cost is negligible on the total base station price scale.

In many quarters, the feeder system is regarded merely as a network 'consumable'. Some believe that 'a cable, is a cable, is a cable'.

"Wrong," say the experts - the network operators and planners and the site installation and maintenance crews. If poorly selected and managed, the potential total lifecycle costs of a transmission line system can be very high indeed.

"We attach great importance to the reliability and quality of feeder cables, as they directly influence both the quality of the services we provide to our subscribers and the ongoing expansion costs of the system as a whole.

"It is for this reason we search out a specific grade and quality of cable," says Valery Ulianov, director of regional network development and technical director of Moscow region, with Russian mobile operator, Vimpelcom.

Ulianov is involved in the operator's network expansion program that will see Vimpelcom's 'BeeLine GSM' service further expanded in the so-called macro-regions of Russia.

Mounting costs

Fast forward from base station planning and roll-out, to where the feeder system costs really mount up: during long-term maintenance and repair. Aaron Wilson is technical maintenance manager with CMS Wireless, an Arkansas US-based wireless base station developer.

His company provides end-to-end wireless project development services, from site acquisition and base station design, to installation, commissioning and maintenance.

And 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. The majority of the feeder cable problems we see here are as a result of poor connecting, problems at bends, and cuts and crush faults in the cable run," says Wilson.

These manifest themselves in 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 says.

The cost isn't limited to that of raw maintenance; there are also the costs of base station downtime and resultant subscriber churn.

Wilson cites the limited nature of the feeder system commissioning/testing regime as something of a problem. The common voltage standing wave ratio (VSWR) 'sweep' test simply 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 says. "You'll start seeing reflective power over time."

The challenge facing both operators and installation crews is dealing with the number of variables in the transmission line 'equation'. These include a wide selection of feeder technologies - such as corrugated cable, low-attenuation 'high-performance' corrugated cable and rigid smooth-wall cable - plus a dizzying range of accompanying connector systems and tools.

There is also wide diversity in installation crews' skill levels and experience, plus a broad range of site layouts, weather conditions and so on.

This mix makes achieving repeatable long-term feeder performance a challenge.

"We were well aware of this real-world, multi-variable situation when we developed our Cellflex 'A' corrugated feeder cable," says Chris Adams, global product manager of Transmission Lines, with RF technology group 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 is they have 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!"

Bend and crush

Accommodating cable bends - sometimes up to six bends in a single run on a modern urban site - is an area where problems can occur.

Most problematic 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," says Wilson. "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 hasn't been seen much in European base station applications, but it's common all over the world in its 'native' application - that of cable television (CATV) signal routing," says RFS' Adams.

"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 environs of the mobile base station, things are quite different.

"When we get called out on a site repair where rigid smooth-wall cable has been used, we always check for the kinks first," says Wilson.

"We also check the ground kits and connectors - 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 can also pose site problems. In the quest to minimise 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 dB, but losing crush strength in a big way," Wilson says.

"When you take the cable out and put a hoisting grip on it, it puts indentations in the cable 'cause it's so thin." This, he says, becomes an even greater problem on collocation sites, where obstructions and bends are many.

"We put a lot of thought into this with the development of Cellflex 'A', explains Adams.

"We knew crush strength was vital in the field, so we didn't compromise the crush resistance, while dropping attenuation up to 6%."

Make the connection

Dressing or 'connectorising' the cable - finishing the cable so it is fitted with a universal 7/16" DIN connector - is the second area where Wilson sees problems.

These, he believes, are often 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!" Wilson says.

He firmly believes this is a recipe for disaster - and he's witnessed the results.

"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!" The well-known cable brand name matched that of the connector, but the models didn't match up.

Worse yet, the wide variety of tools can be expensive, bulky and difficult to use. As Wilson explains, it can be an uphill battle ensuring you have just the right tool for each job.

RFS' Adams agrees that cross-range and backward compatibility of connectors, plus simplicity of tooling is essential in getting it right at 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. This means you can't flare the outer to electrically clamp it from both sides, as you do with corrugated cable connectors," Adams says.

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 behaviour, dropped calls, intermodulation and so on."

The other basic design that applies pressure to the cable outer conductor is one 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.

"This design is flawed, as the bearing ring only grabs a small portion of the available conductor. Instead, we used a slotted brass finger claw on the Rapidfit connector to maximise the contact with the outer," Adams says. "It gives us around twice the electrical contact and better long-term performance."

The water myth

Water ingress into the cable is another area of debate. The rigid smooth-wall camp claims its cable's glued construction prevents water ingress, whereas flexible corrugated cable is susceptible.

Nonsense, say the users.

"I have been using corrugated cable here in North Germany for many years now and have never experienced water ingress problems," says 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 concurs with this view.

"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 mobile installation is enough to totally disrupt transmission!" he says.

"Our view is that the only place water belongs is outside the transmission line. Where it gets in is either via a cut in the cable outer, or via a poor quality connector. Keeping water out of the connector comes down to two factors - simplicity of connectorising and quality three-point sealing, rather than simple 'crest-seal' O-rings."

Minimising feeder system total lifecycle costs, it would seem, starts and finishes with just what is installed at site and how.

"Stick with what works," advises 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."

While feeder cable might be purchased as a site consumable, its potential long-term cost implications are in the 'major capital item' league.

Ulianov's experience with Vimpelcom suggests this is very much the case and is the core reason Vimpelcom opts for Cellflex 'A'.

"The vastness of Russia, coupled with the fast-paced and all-embracing nature of our services expansion program, forces us to work under diverse economic and climatic conditions," Ulianov concludes.

"At the same time, we insist on providing unique quality standards in all the services we offer. For this reason, we take the choice of the right cable systems very seriously."

The bottom line, it would seem, is that subscribers the world over justifiably expect operators to get it right the first time. There are no 'second chances' - feeder technology needs to be chosen accordingly.