Connector selection design criteria
Transmitting signals to and from tower-mounted antennas is critical to wireless system performance. Usually, designers use foam-dielectric corrugated-copper outer conductor cables to transmit the signal because they have 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.
A variety of manufacturers offer connector designs to address some of these concerns, but few address all of them. The selection process requires an understanding of the connector options available and how they affect issues such as electrical performance, mechanical performance, weatherproofing, ease and repeatability of installation, and long-term reliability.
Connector Sealing One critical decision is the method used to form a waterproof seal between the connector, the corrugated outer conductor of the cable and the cable jacket. The intrusion 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 jacket of the cable.
Unfortunately, the corrugated copper outer conductor is an imperfect surface. Because of the manufacturing method, 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. If this seal fails, it creates two problems. First, 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 and increases vulnerability to damage from lightning strikes.
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 to produce 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 intrusion 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 some appropriate uses for O-rings exist. For example, O-ring sealed connectors are ideal for temporary installations because they are easy to reuse, and they provide adequate protection from moisture intrusion for a few months. With the RTV silicone method, the connector can be cleaned and reused once the silicone hardens, but it is time-consuming.
Outer conductor prep and clamping There is a choice of trim dimensions for the different cable components and methods to achieve the dimensions. The concept of cutting the cable flush for connector attachment 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 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 conductors. 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, the outer conductor is pulled away from the dielectric with pliers and stripped even with the back portion of the connector. The result is a flared conductor, with no copper-chip contamination.
The method used to achieve high-contact pressures also is critical to connector performance. 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 is in the normal installation environment, and “metal creep,” which decreases contact pressure over time, degrades connector performance and loosens the cable attachment. Spring contacts for both center and outer conductors can be used to eliminate the potential of reducing the outer conductor contact pressure.
How many pieces? From a production perspective, it is economical to produce the connector in many parts and assemble them. But it is easy to lose the small parts in the field, especially when installing them on a tower. A chance of incorrect assembly also exists. In either case, the connector will not perform to its design specifications or may not be installable at all.
Another option is to assemble most of the parts in the factory. This assures that the pin depth is set correctly in the finished product, which is essential for proper return loss performance. It greatly 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 for the system to perform as required. In addition, the connectors must be designed to be installed 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. Allowing the intrusion of water into the interface will corrode the contact parts, which will degrade electrical performance. In particular, contact surface corrosion will produce PIM, which can degrade system performance and result in dropped calls. In addition, 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 to identify because it typically occurs as a gradual degradation rather than as 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 points of contact exist 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 PIM 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.
Andreescu is connector design manager for Eupen, Belgium and Perelman is vice president of sales and marketing for Eupen Cable, USA.