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.
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.
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. 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. However, it cuts easily, and its sticky surface results in a high coefficient of friction, so it is not good for cleanroom environments. Silicone’s tensile strength and tear resistance are low, therefore requiring it to be thicker as compared to 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 labor-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 to other materials. 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. Poly urethane does not have a very broad temperature range; it becomes brittle around -40 °C, and its upper temperature limit is around 100 °C.
Polyethylene is most appropriate as a dielectric for conductors because polyethylene jackets tend to be stiff, which affects the flexibility of the cable. 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 to 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.
Fluoropolymers such as fluorinated ethylene propylene (FEP), perfluoroalkoxy (PFA), and polytetrafluoroethylene (PTFE) are excellent jacket materials, particularly in applications when the cost of system failure is high. The dielectric withstanding voltage of fluoropolymers is among the highest of any dielectric material. Fluoropolymers can withstand extreme temperatures, but each material has its own range. Fluoropolymers can also withstand exposure to chemicals, acids, and aggressive solvents, and they are naturally non-flammable. PTFE and its co-polymers also have the benefit of low outgassing, which is critical for ultra-high vacuum (UHV) environments. Most fluoropolymers are flexible, but flexibility varies depending on the specific material. PTFE is chemically inert and does not contain any process additives, oils, lubricants, or plasticizers, which makes it the best material for vacuum environments.
One of the few negatives of fluoropolymers is that they are not very resistant to abrasion and cut-through. Certain fluoropolymers can be engineered to enhance their physical, chemical, and electromagnetic attributes, which improves a cable’s ability to withstand the specific challenges of a microwave application. Ethylene tetrafluoroethylene (ETFE) can be irradiated to improve its mechanical properties and chemical resistance; however, irradiation increases stiffness, so there is a significant decrease in flexibility. PTFE is naturally thermal-resistant and chemically inert, so its temperature and chemical properties are not altered when engineered to enhance electrical or mechanical attributes.
Standards and Testing
Some industries have defined safety, environmental, and performance-related standards for cables, but many rugged applications that use microwave cables require going beyond the standards. In these kinds of situations, the manufacturer may need to develop additional tests that evaluate the cable’s electrical performance while simulating mechanical and environmental stress similar to that in the application. It is essential to monitor electrical performance and signal integrity throughout all of the testing, and the specific type of testing that is needed depends on the environmental constraints of the application.
Phased-array applications require close phase tracking of multiple assemblies of the same type and length to minimize residual systemic error. These errors eventually affect system range, clutter and jamming resistance, and overall accuracy. Problems with phase tracking most often occur either because of poor materials and process control during cable assembly manufacturing or because assemblies from different manufacturers’ components were combined.
Mechanical testing verifies electrical performance while the cable is operating in environmental conditions such as crushing, abrasion, potential cut-through, tight bending, continuous flexing, shock, and vibration. Using microwave/RF cables generally means that the application requires excellent phase stability, which can be affected with bending and flexure, whether during installation, routine maintenance, or actual use.
For products that will be used in demanding environments, the consequences of cable failure are usually high. Therefore, it is essential to ensure the electrical and mechanical integrity of the cable for the life of the application.
This article was written by Paul Pino, Application Engineer, at W.L. Gore & Associates, Landenberg, PA. For more information, visit http://info.hotims.com/34459-542.