Maria Calia, Director – Space and Missiles
Times Microwave Systems
Originally published with Connector Supplier
The commercial space industry is going through exponential growth, fueled in part by low-earth orbit (LEO) satellite applications. LEO satellites operate at lower altitudes and travel at high speeds around the Earth, in contrast to the traditional GEO (Geostationary Orbit) satellites used by government agencies such as NASA.
Each LEO satellite completes an orbit in less than two hours and covers only a small area of the Earth’s surface at a given period of time, due to its proximity—only 500-2,000 km—to the planet. This concentrated field of view results in a shorter communication latency, making LEOs ideal for applications that require real-time data transmission and global coverage. For example, companies like Amazon (Project Kuiper) and SpaceX (Starlink) are launching thousands of LEO satellites to provide global broadband internet coverage. Thousands must be launched into orbit, one following the other, to create “constellations” that ensure consistent and comprehensive coverage.
When it comes to achieving fast, reliable communication and high-performance operations, the RF cable assemblies used in LEO satellites play a crucial role. RF interconnects act as the vital bridge between many critical systems, including payload, communications, signal transport, and processing.
However, the extreme environmental conditions of space present unique challenges. That’s why material selection is essential when choosing a coaxial cable assembly for LEO satellites. Factors including radiation resistance, shielding effectiveness, outgassing, and whiskering must be evaluated, as well as cable performance parameters such as phase stability over temperature, stability over flexure, and attenuation.
The materials used within the cable in an RF assembly significantly impact its performance. For example, cable assemblies for commercial space applications can be subjected to 30 Mrads of radiation and temperatures anywhere between -90 °C to 150 °C. Therefore, the materials used within the cable must be able to withstand extreme conditions. To ensure optimal performance, the following factors must be considered.
Radiation exposure can change the cable dielectric and degrade electrical performance. The cable location within the system determines how much radiation it will be exposed to.
Depending on construction, different cables will be rated to withstand varying amounts of radiation. System designers carefully mitigate radiation exposure within the satellite through the use of metal shields to keep components, including RF cables, below the total exposure limits. Sometimes, however, critical cables have to be routed without shielding 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. They must be protected or constructed using materials that will be affected less than plastic polymers.
Cables must have effective shielding to prevent electromagnetic interference (EMI). EMI is an undesirable phenomenon when an outside signal or source causes a disturbance. Without effective shielding, cable assemblies would have to be kept 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.
When exposed to a vacuum environment, many non-metallic materials outgas. This includes plastics commonly used in coaxial cables, such as PTFE, FEP, and PE. Emitted gases can recondense on critical components and degrade performance. Materials used for cables and assemblies going into space must meet industry standards for low outgassing.
Tin is commonly used in solders for coaxial connector terminations. Metals such as pure tin can grow whiskers in vacuum and high-temperature environments and are generally prohibited from spaceflight use. Coaxial cable assemblies must be built with tin/lead solder alloys to avoid whiskering. For LEO applications, smaller diameter RF cable assemblies minimize the number of solder joints and are designed to fit in tight spaces.
Maintaining cable performance once an assembly is installed is necessary for the function of a LEO satellite. Three major parameters that influence cable performance include phase stability, stability over flexure, and attenuation.
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 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 phase change is problematic for applications where phase stability is crucial.
Proprietary fluorocarbon dielectrics such as TF4 can mitigate the issues PTFE presents regarding phase stability over temperature. TF4 eliminates the non-linear phase performance that occurs between 15 °C and 25 °C.
In tight or in-the-box spaces, cable flexibility is crucial for routing. However, a flexible cable needs to maintain stable performance over flexure.
The conductivity of the conductors, the diameter of the cable, and the dielectric constant are three properties that define the attenuation, or signal loss, of a coaxial cable.
High-conductivity materials, such as silver and copper, provide low attenuation per unit length but are often heavy or expensive. Lighter materials such as aluminum and stainless steel reduce overall mass but are poor conductors. Cable manufacturers frequently optimize conductor designs by cladding or plating a lightweight, low-cost base metal with higher-conductivity copper or silver for the RF path.
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; a tight bend will cause the cable to kink or become oblong, causing an impedance mismatch and excessive return loss.
A lower-loss dielectric generally will be lighter because it incorporates more air. 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.
LEO satellite applications can be very different from a design perspective than their GEO counterparts. One key difference is faster development times, which means readily available cable assemblies are a must-have for rapid product development and testing. Once testing is complete, these solutions should be easily customizable to meet the unique needs of different applications.
Traditional semi-rigid RF cable technology may not be suitable for smaller LEO satellites—where space constraints are significant—due to routing limitations and the need for flexibility during installation. The InstaBend® Space cable assembles are a new class of compact and flexible RF cable and connector technologies specifically designed for small satellites and similar applications. These innovative solutions can be easily routed through tight spaces without compromising performance or reliability.
These highly flexible cables can bend closely behind the connector, allowing maximum routing flexibility in small spaces like interconnects between RF circuit cards, modules, and enclosure panels. The essential advantage lies in the unique clamp technology, which eliminates the need for solder connections traditionally found in larger-diameter cable assemblies. They also offer a comprehensive solution with a triple-shield construction, broad frequency range, and exceptional durability.
A phase stable option of this type of cable assembly is also available. InstaBend PhaseStable assemblies are low-loss, ultra-flexible foam-core micro coaxial cables that eliminate the PTFE phase change around 19°C, 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. Both types of InstaBend space assemblies are designed to withstand extreme conditions in space and function continuously over a long period.
Furthermore, low-loss RF cables are crucial for LEO applications, as they can reliably maintain signal integrity over long distances. This is particularly important when transmitting high-frequency signals, as losses can degrade the signal quality, leading to errors or data loss. Low-loss RF cables also minimize power losses during signal transmission, ultimately ensuring that transmitted signals reach their destination with minimal loss in power or quality.
An example of an ultra-low-loss assembly that can meet the needs of commercial space applications is MaxGain Space assemblies, a range of high-performance coaxial cable assemblies built with a unique spiral outer conductor technology. This lightweight cable construction produces a reliable, high-frequency interconnect solution suited for applications like LEO satellites where low loss and high performance are required.
However, it is important to note that even though the design approach differs, RF assemblies used in commercial space applications are still required to meet the rigors of the environment, including the considerations outlined above. Working with a supplier with a large manufacturing footprint, extensive heritage, and access to the right materials and technologies is recommended to provide the best solution.
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