Although today's cable assemblies are smaller and lighter, the market demands that they last longer and cost less. Yet they are being used in more challenging RF and microwave environments, where they are exposed to extreme temperatures, chemicals, abrasion, and flexing. Noting that these requirements are basically at odds with each other, W.L. Gore's Lead Design Engineer, Paul Pino, has created a white paper titled, "Impact of Materials on Microwave Cable Performance." To guarantee signal integrity and product reliability, he emphasizes the need to identify the constraints that can affect a cable's performance.

The eight-page white paper notes that cables are usually the last component to be considered in the system-design process. Yet they are often a system's lifeline in terms of the critical capabilities that they enable. Because a cable's reliability is based on durability and signal integrity, the materials used to create the assemblies are crucial.

Pino notes that environmental influences are becoming more of an issue for microwave/RF cable assemblies. Electrical performance is probably the primary consideration for cable performance, for example. When no other environmental factors are involved, however, electrical performance is typically very reliable. It becomes a challenge when mechanical, environmental, or application-specific stress is added. Environmental stress also can compromise dielectric and jacketing materials.

The paper tackles materials as well. Silicon, which is primarily used as a cable jacket, is flexible even at low temperatures. Yet it cuts easily and has a high coefficient of friction, thanks to its sticky surface. Silicon also has low tensile strength and tear resistance, which require it to be thicker than other jacket materials. It has very good radiation resistance, but is not recommended if weight or flexibility are important factors in an application. The paper also evaluates polyurethane, polyethylene, and fluoropolymers.

Although the latter are not very resistant to abrasion and cut-through, certain fluoropolymers can be engineered to enhance their physical, chemical, and electromagnetic attributes. Ethylene tetrafluoroethylene (ETFE), for example, can be irradiated to improve its mechanical properties and chemical resistance. Yet the irradiation process also increases stiffness. The paper ends with a section on design verification, which underscores how cable reliability can be verified through electrical, mechanical, and environmental testing.

W.L. Gore & Associates, Inc., 555 Paper Mill Rd., Newark, DE 19711; (410) 506-7787, www.gore.com.