Impact of materials on microwave cable performance Part 2

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.

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, see Table 4.

Table 4: Properties of fluoropolymers.

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.

FEP can handle temperatures ranging from -250 to 150°C, while PFA ranges from -250 to 200°C. PTFE is suitable for temperatures from cryogenic to 260°C without losing flexibility.

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, like temperature resistance, flexibility varies depending on the specific material. PFA is the stiffest followed by FEP and PTFE and engineered PTFE is the most flexible.

Anything that is added to a cable’s dielectric, jacket, conductors or shield wires will outgas in a vacuum.

When materials outgas, particulate matter condenses on cooler surfaces, which are typically the work surfaces in the application area. In a satellite, optics can become fogged by silicone oil or other processing lubricants that outgas from a cable.

PTFE is chemically inert and does not contain any process additives, oils, lubricants, or plasticisers, 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.

Specialised technologies have been developed to engineer PTFE so that it can withstand a wide variety of environmental and mechanical challenges, see Table 5.

Table 5: Enhanced properties of engineered fluoropolymers.

The dielectric materials used to insulate conductors can significantly affect insertion loss, cable size and flexibility. The lower the dielectric loss, the less insertion loss the cable exhibits.

Typical fluoropolymers have a dielectric loss of 2.1. To reduce cable size PTFE can be engineered to have a dielectric constant of 1.3. At the same time, its dielectric withstanding voltage can be increased by a factor of 2.5 while achieving a very low loss tangent of 0.00015 at 10 GHz compared to PTFE’s standard construction.

With these attributes, a conductor insulated with a 50-micron layer of engineered PTFE can be rated for use at 1000 V.

Another version of engineered PTFE can be made semiconductive and used to increase the effectiveness of a cable’s shield.

For issues of abrasion or cut-through resistance, PTFE has been engineered to attain a tensile strength that is 50 times greater than standard PTFE and to withstand temperatures from cryogenic to 300°C.

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 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 minimise 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.

So phase tracking and stability should be thoroughly tested in the environment in which the cables will be used.

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.

Random flexing is a frequent issue with a handheld test instrument because the cable assembly is often wrapped around the instrument to carry it. The impact of these movements on system performance must be evaluated during system design.

In the lab, a technician could roll over the cable with a chair, which means that crush strength is also an issue. Random flexing motion is very difficult to model in a test lab, but the worst-case scenario can be modelled using a tic-toc test with repeated bending of 180° or more.

Then a pull test can simulate a cable being used as a tether. During these tests insertion loss and VSWR should be evaluated.

The cable’s electrical performance should also be measured while simulating the environmental conditions in which it will operate - conditions such as temperature, altitude and pressure extremes; vibration and acceleration; exposure to liquids or gas; or humidity.

It is important to monitor impedance during altitude change, mechanical shock and vibration tests. Vibration and shock can cause mechanical and electrical failure due to metal fatigue or cracking of solder joints.

Temperature changes have a direct impact on phase length. As the temperature approaches an extreme, the electrical length will change; if it does not change at the same rate as the temperature when returning to normal (a state known as hysteresis), it is very difficult to apply error-correction techniques to the signal.

Adding a clamp force during a temperature cycling test allows monitoring of the cable’s dielectric withstanding voltage to see how the jacket and conductor change.

After the cable is put through substantive mechanical and environmental tests, the manufacturer should again verify that the electrical performance, dielectric and jacket materials remain stable within the requirements of the application.

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.

To do this means understanding the factors that can compromise cable performance; selecting the right materials to address these factors; and verifying the cable’s reliability through electrical, mechanical and environmental testing.