RF Cable Assemblies to Meet the Demands of Commercial Space Applications

Originally published with SatNow

The commercial space industry is experiencing a period of significant growth and development. Within this industry, the most common commercial space application is low-earth orbit satellites, also known as LEO satellites. These satellites orbit the Earth at an altitude of 2,000 km or less. LEO satellites are commonly used for Earth observation, surveillance, telecom, space telescopes, Earth sensing, and space cubes.

LEO satellites and other commercial space applications rely on durable, dependable RF cable assemblies that consistently offer high performance. However, space presents many extreme environmental conditions which pose distinct challenges for RF cable assemblies.

As the exploration of space continues, the availability of cable assemblies that can withstand these extreme conditions and function continuously over a long period of time is an increasing concern.

Selecting the Right Cable Assembly

When choosing a coaxial cable assembly for a commercial space application, there are several important factors and rules to consider regarding materials and performance.

Materials

The materials a coaxial cable is built with have a significant impact on its performance in space. It is crucial to consider if the materials used are suitable for this environment.

Radiation Resistance

When choosing a cable assembly for space, consider how much radiation it must withstand. Radiation exposure can cause a change in the cable dielectric and degrade electrical performance. The cable location within the system determines how much radiation it will be exposed to. Different cables will be rated to withstand varying amount of radiation, depending on their construction materials. System designers carefully mitigate radiation exposure within the satellite through the use of metal shields to protect and keep components, including RF cables, below the full exposure limits; however, critical cables sometimes may have to be routed to remote locations. For example, RF cables exiting the safety of the bus to reach a remote antenna will be exposed to increasing amounts of radiation and must be protected or constructed using materials that will be affected to a lesser extent than plastic polymers would. In these scenarios, radiation resistance is critical.

Shielding Effectiveness

Cables need to have effective shielding to prevent electromagnetic interference (EMI). EMI is an undesirable phenomenon when an outside signal or source causes a disturbance in the signal of interest. Without effective shielding, cable assemblies would have to be kept at a greater distance apart to avoid EMI, which isn’t possible in densely packed systems like LEO satellites.

Cables constructed using internal multiple shields are better at mitigating EMI. The RF interference also has to be carefully considered and evaluated to achieve the least possible interference.

Outgassing

When exposed to a vacuum environment, many non-metallic materials outgas. This includes plastics commonly used in coaxial cables, such as PTFE, PVC, and PE. Emitted gases can recondense on critical components and degrade performance. Materials used for cables and assemblies going into space must meet the standards for outgassing rates defined by NASA and the European Space Agency (ESA).

Whiskering

Tin is commonly used in solder for coaxial connectors terminations. Metals such as pure tin can grow whiskers in vacuum and high temperature environments and are generally prohibited from spaceflight use. To avoid whiskering, coaxial cable assemblies must be built with tin/lead solder alloys.

Cable Performance

Maintaining cable performance once an assembly is installed is crucial for the function of a satellite. Three crucial parameters that influence cable performance include stability over flexure, phase stability, and attenuation.

Stability Over Flexure

In tight or in-the-box spaces, cable flexibility is crucial for routing. A flexible cable needs to have stable performance over flexure.

Phase Stability Over Temperature

Both Polytetrafluorethylene (PTFE) and TF4® dielectric cables are fit for commercial space applications, but each dielectric has its place depending on the requirements of the specific use case. PTFE has a high melting point along with its excellent dielectric properties, making it popular in many microwave applications. However, around 19°C, PTFE exhibits a non-linear change in phase known as the “knee.” This change in phase is problematic for applications where phase stability is crucial. Times Microwave Systems’ proprietary fluorocarbon dielectric, TF4®, addresses the issues presented by PTFE in terms of phase stability over temperature. TF4 eliminates the non-linear phase performance that occurs between 15 and 25°C.

Attenuation

Three properties define the attenuation of a coaxial cable: the conductivity of the conductors, the dielectric constant, and the diameter of the cable.

In general, larger diameter cables provide lower attenuation per unit length than comparable smaller diameter cables, but this comes at the cost of increased weight and a wider minimum bend radius. Larger cables cannot be bent as tightly as smaller cables, and an overly tight bend will cause the cable to become oblong or kink, causing an impedance mismatch and excessive return loss.

High-conductivity materials such as copper and silver provide low attenuation per unit length but are often heavy or expensive. Lighter-weight materials such as stainless steel and aluminum reduce overall mass but are poor conductors. Cable manufacturers frequently optimize their conductor designs by cladding or plating a lightweight, low-cost base metal with higher-conductivity copper or silver for the RF path.

A lower-loss dielectric generally will be lighter because it incorporates more air into the media. More air in the dielectric materials lowers the effective dielectric constant, meaning the transmitted signal encounters less resistance or loss, making performance closer to the ideal of a wave traveling in a vacuum.

Finding the Right Supplier

To alleviate the challenges of selecting cable assemblies for space applications, it is recommended to work with a supplier with a heritage of building such assemblies and has access to a variety of materials and technologies to provide the best solution. An example of this type of partner is Times Microwave Systems, the preeminent brand in innovative RF and microwave interconnect assemblies, cables, and connectors. Our commercial space cable assemblies are designed with low outgassing materials (per ASTM E595) and vented connectors, if applicable, to deliver reliable performance and adhere to stringent safety requirements. With Class 100,000 cleanroom manufacturing, our cable assemblies are optimized for the lowest attenuation, radiation resistance, ultra-stable performance with flexure, and phase stable performance over temperature.

InstaBend® Space Cable Assemblies

InstaBend Space assemblies are flexible coaxial microwave assemblies designed for interconnects between RF circuit cards, modules, and enclosure panels. These cables can bend not only in the middle of the cable, but also closely behind the connector and allow for flexibility of routing in tight spaces. Click here to learn more.

MaxGain® Space Assemblies

MaxGain Space assemblies are high-performance, ultra-low loss microwave coaxial cables. Built with our unique spiral outer conductor technology, this lightweight cable is a reliable, high frequency interconnect solution. These cable assemblies are suited for applications like satellites where low loss and high performance are required. Click here to learn more. 

InstaBend® Phase Stable Space Assemblies

InstaBend PhaseStable assemblies are low-loss, ultra-flexible foam-core micro coaxial cables, eliminating the PTFE phase change occurring around 19°C and making it ideal for applications demanding stable phase performance over temperature. This high-performance cable has the same triple-shield construction as several of our popular cables, along with a broad frequency range and strong durability. Click here to learn more.