Caring for Connectors

Transmitting signals to and from tower-mounted antennas is critical to wireless system performance. Usually, designers use foam-corrugated-copper outer conductor cables to transmit the signal because they are low loss and have proved reliable in outdoor tower-mounted applications. For the past five years, carriers have used the 7/16 DIN European interface to the antenna, which is designed for reliable sealed performance in an outdoor environment. The engineering challenge is to design a transition from the corrugated-copper transmission line to the 7/16 DIN interface that does not degrade the RF system performance, has good mechanical and electrical stability, is completely weather sealed, provides long-term field reliability and is easy for field technicians to install.

Many manufacturers offer connector designs to address some of these concerns, but few address all of them. As you begin the selection process, you need to better understand the connector options available and how they affect performance issues such as electrical, mechanical, weatherproofing, ease and repeatability of installation, and long-term reliability.

Connector Sealing One critical decision you have to make is what method you will use to form a waterproof seal between the connector, the corrugated outer conductor of the cable and the cable jacket. The ingress of any amount of water vapor over a long period of time can corrode contact surfaces. The result is an increase in passive intermodulation (PIM) distortion, which degrades system signal-to-noise ratio and eventually leads to dropped or non-completed calls, revenue loss and customer dissatisfaction.

The most common method of achieving this seal, the use of one or more O-rings, is not necessarily the best option. O-rings are designed for making a reliable seal between two machined, nearly perfect metal surfaces. They are used with a high degree of reliability to provide a weather seal in the interface between the male and female 7/16 DIN connectors. This is a different application from using an O-ring to form a seal between the connector and the corrugated-copper outer conductor or between the connector and the cable jacket.

Unfortunately, the corrugated-copper outer conductor is an imperfect surface. It is not perfectly round, often has longitudinal scratches from the drawing dies used in the manufacturing process and has a weld seam, which is not perfectly flush with the surface. As a result, an O-ring does not provide a reliable seal to the corrugated-copper outer conductor.

The seal to the polyethylene jacket can create other problems. The jacket's surface is even more imperfect than the copper outer conductor because the manufacturing tolerances are greater, and it is a softer material subject to more damage. When this seal fails, water that seeps under the jacket of the cable at the connector interface can migrate along the cable under the jacket and corrode the ground straps. This leads to poor grounding, which means lightning strikes will damage the system.

A hole in the jacket along the cable also can lead to problems. The water can migrate along the cable to the connector. If there is not a good seal between the connector and the jacket, moisture can get into the connector interface and cause corrosion. Although these problems are more serious in cables with helical or screw-thread corrugations, even annular- or ring-corrugated cables are not sealed between the jacket and the outer conductor, which allows water to migrate along the length of the cable in this interface.

A properly selected polymeric sealant, such as room-temperature-vulcanizing (RTV) silicone, is one reliable method that produces a long-term seal to the corrugated copper. This material is injected into the back end of the properly designed connector to completely encapsulate the end of the jacket, the corrugated copper and the connector body. It hardens within a few hours of application and fills in all of the irregular areas on the outer conductor. Appropriate RTV sealants provide a high degree of adhesion to all of the materials, including the copper of the cable outer conductor, the connector body and the cable jacket.

These sealants actually absorb water as they harden, removing any residual water vapor that may have been in the connector interface. Essentially, they act as a desiccant to protect the contact parts from corrosion.

Properly installed connectors sealed with this method resist moisture ingression for many years.

On the other hand, field reports show that connectors with O-ring seals are a constant source of maintenance and reliability problems. Even the use of additional wraps of butyl rubber and electrical tape cannot keep the moisture out of the connector interface over a long period of time. But there are some appropriate uses for O-rings. For example, O-ring-sealed connectors are ideal for temporary installations because it is easy to reuse them, and they provide adequate protection from moisture ingress for a few months.

Outer Conductor Preparation & Clamping The concept of cutting the cable flush for connector attachment is attractive because it is simple, but it leads to one major performance problem: the copper chips from the saw cut get embedded into the foam dielectric of the cable and are almost impossible to remove. In fact, any method of cutting the outer conductor that requires a cut in toward the foam dielectric can introduce copper particles into the dielectric. Generally, these particles will not cause a problem until they are adjacent to the center or outer conductor. The poor contact that exists between the conductor and the copper particles will create a source of PIM.

The can-opener method, which eliminates the risk of contaminating the dielectric with copper particles, is one way to trim the outer conductor. In this method, you pull the outer conductor away from the dielectric with pliers and strip it even with the back portion of the connector. The result is a flared conductor, with no copper-chip contamination.

High-contact pressures are essential to the PIM performance of the connector and to minimize RF loss through the connector interface. In many connector designs, tightening the two halves of the connector directly applies force to the outer conductor. But with this approach, the temperature cycling in the normal installation environment can cause metal creep, which decreases contact pressure over time, degrades connector performance and loosens the cable attachment. You can use spring contacts for both center and outer conductors to eliminate the potential of reducing the outer conductor contact pressure.

How Many Pieces? >From a production perspective, it is most economical to produce the connector in many parts and assemble them in the field. But it is easy to lose the small parts in the field, especially when you must install them on a tower. You also take the chance of assembling them incorrectly. In either case, the connector will not perform to its design specifications, or you may not be able to install it at all.

Another option is to assemble most of the parts in the factory. This assures that pin depth is set correctly in the finished product, which is essential for proper return-loss performance. It reduces the potential for lost parts and increases the likelihood that the connector will be installed correctly.

System Reliability Even though a reliable cable type and a reliable interface are used for antenna feeder applications, this is one of the most troubled portions of a wireless communications system. Usually, designers do not pay enough attention to the design of the cable connector that serves as a transition from the corrugated-copper cable to the 7/16 DIN interface. Although it looks like a simple device, it must perform several functions simultaneously in a variety of harsh environments. And it must do so with a high degree of reliability in order for the system to perform with the required degree of reliability. In addition, the connectors must be designed so that they can be installed to achieve the desired performance by semi-skilled technicians in a field environment. Achieving all of these objectives is a major design challenge that requires careful attention to many details.

In addition to providing a reliable electrical and mechanical interface to the cable, the connector also must provide a reliable weatherproof seal to the cable. If it allows the ingress of water into the interface, this will corrode the contact parts, which will degrade electrical performance. In particular, contact-surface corrosion will produce high levels of PIM, which can degrade system performance and result in dropped calls. Furthermore, moisture in the cable can increase the attenuation of the signal. This can reduce the radiated signal and reduce the coverage area, leading directly to customer dissatisfaction and revenue loss. This problem is difficult to troubleshoot and identify because it occurs as a gradual degradation rather than a catastrophic change in system performance.

Electrically, the connector serves as a short section of transmission line and provides a transition from the transmission line size of the cable to the transmission line size of the 7/16 DIN or type N interface. Therefore, its performance can be measured in terms of the typical transmission line parameters -- insertion loss and return loss. In addition, because there are points of contact on both the center and outer conductors, the maintenance of high-contact pressure is essential to the long-term performance of the connector. Over time, poor contacts will result in high levels of PID and an increase in insertion loss. Applying a spring-loaded contact for use of high-contact pressure is essential to maintaining the contact pressure and preventing the connector from coming loose, which can increase PIM.

Choosing a well-designed connector will contribute greatly to system reliability. Incorrectly designed connectors will require constant maintenance and will reduce customer satisfaction and revenues. Too often designers do not give sufficient attention to this area of system design. As a result, system reliability is unnecessarily compromised.