Saving time and money at the site
Site installers and technicians often work in harsh environments and under severe time constraints. Extreme temperatures, foul weather, poor lighting and difficult-to-reach areas are just a few of the obstacles with which they must contend.
Furthermore, equipment that has been onsite for many years can present its own challenges. Installers use various methods to weatherproof new cable connections when tower sites are installed. Schemes to keep moisture from penetrating coaxial cable through the connections typically include a combination of butyl or vinyl tapes and cold or hot shrink-wrap applied in layers.
With time and exposure, weatherproofing material can become difficult to remove. Butyl can become tar-like, and tape adhesives can become tacky. Disconnecting cables to perform radio frequency (RF) tests at a tower site requires a good measure of physical strength and dexterity even under the best conditions. Knives and cutting pliers are used to slice through and peel away layers of old weatherproofing. Often the materials were hastily applied, and the extricated connectors remain smeared with the tar-like substance.
Even when weatherproofing is properly applied, the removal process makes it easy to spread the adhesives around the connector coupling nuts or threads. Extremely hot or cold weather makes matters much worse, and the adhesives are not easily cleaned off.
The photo below on the left shows the end of an actual jumper cable that has been disconnected for testing. The coupling nut on this cable no longer rotates freely and requires a significant amount of applied torque. In low temperatures, rotating the coupling nut most likely would be possible only with a wrench. At room temperature, the latent adhesives cause the cable to stick to one’s hand.
Time also works against the technician, as an idle site causes problems for the system operator. Whether troubleshooting, repairing or testing a system after changing out failed equipment, the goal is always the same: minimize down time. Site technicians report they need to mate and de-mate test and system cables a dozen times or more within a span of 10 to 15 minutes — almost one per minute.
In addition, properly calibrated torque wrenches are not always available on site (while perhaps not visible in the left photo, examination of the coupling nut shows teeth marks from a pipe wrench or similar tool). Even when they are available, achieving the proper torque specification can be time-consuming and often is overlooked. This can yield connections that may loosen with time and allow moisture ingress that can degrade system performance.
Over-tightening is of equal concern. This can yield a ripped cable outer conductor, stripped threads or broken system components. The failure can be immediate or, worse, latent. Because of their constant and repeated use, RF test cables often are the first to require replacement. The photo at right shows the size of a typical site analyzer test cable with a 7-16 DIN female connection, compared with the site jumper cable. This analyzer cable has an outside diameter of about 0.3 inches. Small diameter cables are preferred by site analyzer test equipment manufacturers because they coil more tightly and fit easily into the analyzer carrying case, but their small size makes them more difficult to handle onsite and more easily broken.
Dents, loose outer braids and failed attachments result in cables that exhibit unacceptable return loss or are unstable with flexure, a common occurrence. A site’s often harsh operating environment, the glue-like weatherproofing materials used on connectors and the hurried nature of the work add up to a need for a significant upgrade in RF test cable ruggedness.
One solution to these myriad challenges can be found by applying a rugged handgrip around the connector at the system interface end of the cable assembly. Such a device allows the user to easily hand apply as much resistance to a torque wrench — or even a pipe wrench — as is necessary to mate to, or de-mate from, connectors on the system.
Inside the handgrip, connector attachment is achieved via a solder-clamp approach. Solder-clamp has proved to be the single most reliable method for connector attachment on high-performance RF cables in constant-flexing applications. The connector clamp area is machined into a long, solid metal sheath. The sheath and high-performance RF cable are then encased in an oversized crimp ferrule that is added over both to effectively increase connector retention to more than 200 pounds of pull force.
The entire system is located inside the grip. The inside of the grip is asymmetrical and matches an asymmetrical connector. While retaining screws are visible, they are not required to prevent connector rotation relative to the grip. The strain relief on the handgrip is similarly heavy duty and uses a material and tapered design that matches the flexibility of the armor. This results in a perfectly smooth transition to a highly flexible armored RF cable.
The application of the handgrip in this fashion addresses the need to make and break RF connections both quickly and reliably under harsh conditions, and eliminates the most common problem plaguing test-cable makers: premature test cable assembly failure at the connector-to-cable attachment area. The ability to securely hold both the test cable and a device under test and add sufficient resistance while applying sufficient torque during the coupling process also minimizes the risk of damage to system jumpers and often difficult-to-replace system components.
The photo above on the left shows a rugged handgrip-equipped cable ready to be mated to the site jumper shown in the left-hand photo on page 30. The above photo on the left shows the mated pair. Unlike a standard test cable, the functional diameters of both the test cable and the cable under test are similar, so hand torque is similar. Thus a cable so equipped makes it possible to make or break connections while wearing heavy workman’s gloves. Also, armoring the coaxial cable makes it highly resistant to damage; however, the flexibility of the armor can be matched to the cable, so it does not inhibit working in tight quarters or coiling for storage.
Paul Tusini is a 28-year RF industry veteran, with 19 years experience in coaxial cables. Currently he is product manager for test cables at Times Microwave Systems, which manufactures SilverLine test cables equipped with TuffGrip connectors. Prior to joining Times Microwave, Tusini was a founder of Volex RF Technologies, a Wilmington, Mass.-based manufacturer of cable assemblies. He also worked for Amp/Tyco — where he was involved in all aspects of marketing, sales and applications engineering for the company’s high-performance coaxial cable division — and was director of sales and marketing for Qualify Microwave Interconnects.