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Electronics Production | June 21, 2011

Impact of materials on microwave cable performance

Cables are often the last component considered during system designs. In many situations, cables are really the system’s lifeline.
For example, if the cable system used for data transmission in a spacecraft fails, the communication between the craft and the ground station could be lost. Cable reliability is based on both durability and signal integrity, and the materials used to engineer the assemblies have a direct impact on their life in any environment.

Introduction

The environments in which microwave cable assemblies are being used today are becoming more challenging with exposure to such conditions as extreme temperatures, chemicals, abrasion, and flexing. Additional challenges include the need for smaller, lighter packaging for cable systems that last longer and cost less.

To ensure signal integrity and product reliability, it is essential to identify the electrical, mechanical, environmental, and application-specific constraints that can affect the cable’s overall performance. These variables have a direct impact on the materials used for cable dielectric and jacketing as well as the construction of the cable. Also, testing and data analysis are key to ensuring that the cable will, in fact, perform reliably in a specific environment.

Identifying constraints

Environmental influences are having more of an impact on microwave/RF cable assemblies. Electrical performance is probably the first and foremost consideration, and many factors can potentially compromise signal integrity, such as internal and external electromagnetic interference (EMI), voltage standing wave ratio (VSWR), and insertion loss. Electrical performance is typically very reliable when no other environmental factors are involved; however, when mechanical, environmental, or application-specific stress is added, maintaining reliable electrical performance can be more challenging.

Mechanical stress occurs when cables are exposed to various types of movement. Flexing creates kinetic energy in the cable, which can cause severe damage if not properly managed. One of the biggest causes of mechanical stress on cables is when the cable is part of equipment handled by a person. An operator can kink, pinch, or crush a cable by stepping on it or rolling over it. Therefore, crush and tensile strength is essential in overcoming mechanical stress. Also, cables used with portable equipment can come into contact with sharp surfaces that cut cables or expose them to abrasion. When the com- plexities of compensating for vibration or gravity are added, mechanical stress can significantly compromise stability and cause premature failure of a cable.

Environmental stress results from the physical area in which the cables are used. Extreme temperatures and pressures affect cable materials. Low temperatures make them brittle, and high temperatures cause them to become very soft. Vacuum leaches oils and additives out of a cable, contaminating a cleanroom manufacturing process, while hydrostatic pressure causes gas or liquids to per- meate cable jackets.

Radiation can damage both dielectric and jacket materials depending on the type and dosage level. Friction resulting from cable movement can compromise cable jackets by causing particulation, while contaminants such as mud, chemicals, or metal chips can damage the cable jacket. Environmental stress can significantly compromise dielectric and jacketing materials, so these issues must be taken into account when designing a cable assembly.

Application-specific stress results from constraints that are unique to the application in which the cable will be used. In aerospace applications, cables need to be the lightest and smallest possible size in order to minimize mass during take-off. If the cables are used by technicians or other personnel, safety issues such as flammability, voltage, and halogen use are factors.

One of the added complexities of designing cable assemblies is that electrical, mechanical, and environmental performance is interwoven. Each has a direct impact on the other; so the design must be thoroughly tested in the specific application.

Choosing the right materials

Ensuring high-quality signal stability means evaluating the dielectric and jacket materials for attributes that account for the harsh elements of the application. The dielectric materials used in signaling cables affect the signal integrity as well as robustness of the cable. The material used in an outer jacket affects maximum voltage and resistance to abrasion. Jacket materials must survive most of the external factors (e.g., temperature, friction, liquids, and gases) to protect the conductors inside the cable.

The list of possible materials used in cable dielectric and jacketing is very long, and many of these have been developed for specific applications. Because each material has unique properties, some are more appropriate than others for use in microwave cables engineered for challenging environments.

- Silicone
- Polyurethane
- Polyethylene
- Fluoropolymers
- Engineered Fluoropolymers
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Author: Paul Pino, Application Engineer, W. L. Gore & Associates, Inc. More can be found here.

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