Impact of materials on microwave cable performance Part 1

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.

Testing and data analysis are also key to ensuring that the cable will perform reliably in a specific environment.

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 are 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 complexities 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 permeate 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 to minimise 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.

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 signalling cables affect the signal integrity as well as the 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 (eg, 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 is primarily used as a cable jacket and is very flexible even at low temperatures, see Table 1. However, it cuts easily and its sticky surface results in a high coefficient of friction, so it is not good for cleanroom environments.

Table 1: Properties of silicone.

Silicone’s tensile strength and tear resistance are low, therefore requiring it to be thicker when compared with other jacket materials. Some surface treatments are available to reduce the coefficient of friction but these tend to wear off over time.

Silicone has very good radiation resistance but the grades of silicone used for cable jackets are known to outgas silicone oil in vacuum applications such as a thermal vacuum chamber. If weight is an issue, silicone is not the optimal choice.

If flexibility is important and weight is not a factor, silicone is a good choice. However, it is more labour intensive to gain access to the conductors, which results in higher costs for termination.

Polyurethane is a good jacket material but it is not used as a dielectric material because its dielectric withstanding voltage is low when compared with other materials, see Table 2.

Table 2: Properties of polyurethane.

Halogen-free grades are available. Mechanically, polyurethane is flexible and it is very resistant to cut-through and abrasion. Treatment for flame resistance does not reduce its flexibility. However, the more flexible grades tend to be sticky or tacky, which results in a higher coefficient of friction.

Environmentally, polyurethane is resistant to solvents, UV rays, radiation and fungus. Polyurethane does not have a very broad temperature range; it becomes brittle around -40°C, and its upper temperature limit is around 100°C. It cannot survive the chemicals used for cleaning.

Polyethylene is most appropriate as a dielectric for conductors because polyethylene jackets tend to be stiff, which affects the flexibility of the cable, see Table 3.

Table 3: Properties of polyethylene.

Polyethylene has good dielectric constant properties when used in conjunction with foam. Mechanically, high molecular-weight polyethylene is abrasion-resistant and low-friction, but it is also stiff when compared with other materials. Like polyurethane, polyethylene’s temperature range is rather limited and it is difficult to bond chemical boots to polyethylene cable jackets.

Overall the mechanical properties of polyethylene are reduced by flame-retardant treatments.

Part 2 will conclude this article soon.