Cables, connectors, assemblies, and other RF solutions are integral components of spacecraft systems. Inside these crafts, room for equipment is extremely limited. Minimizing the size and weight of cables and assemblies are key, as they need to take up the smallest footprint possible. The high-frequency cables required for space applications have a shorter range, so to operate spacecraft require a dense network of antennas. Lightweight cables and assemblies are ideal for high-density connections, especially for intricate installations in tight spaces. Minimizing space between cables and connectors is necessary for interconnect systems to survive high vibration and other harsh environmental conditions.
High shock and vibration, extreme temperatures, and intense radiation are all common environmental and technical conditions of space applications; space rated RF cables and assemblies must withstand these extraordinary conditions reliably and consistently over long periods. Tight phase control is crucial for optimized system performance. The ultimate challenge for interconnect design engineers is finding flexible dielectric materials that meet the physical requirements and can also be phase stable across a broad temperature range.
Selecting RF Cable Assemblies for Space Applications
The selection of appropriate coaxial cables for space applications depends on phase stability, return loss, materials, cable performance, and many other factors. Maintaining cable performance once an assembly is installed is crucial for the function of a satellite. Some important factors to consider when selecting coaxial cables for space related applications include:
- Phase Stability: In phase-sensitive systems, compensating for the “knee” demonstrated by PTFE multiple times per orbit as the spacecraft moves through its operating temperature range is a challenge that ultimately limits overall system performance. In addition to controlling overall phase change vs. temperature, designers may need to characterize the hysteresis of the phase change across the operating temperature range.
- Electrical Length: Factors like frequency, time delay, physical length, dielectric constant, and propagation velocity impact electrical length. A cable assembly will get electrically longer as it gets colder and shorter as it gets warmer.
- Return Loss: Consider the electrical performance of a cable over radiation exposure. Plastics such as PTFE and TF4 will degrade over time and increase return loss. For long duration or missions with higher radiation exposure, materials such as SiO2 have better shielding than a plastic.
- High Density: Choose high-performance cables that can accommodate the extremely restricted space restraints of space applications. Cables built with inside-the-box applications in mind can minimize footprint, save room, and simplify cable routing.
- Materials: Be careful of what materials a coaxial cable is built with, as not all materials are suitable for the environment of space. Material selection for space qualified cables has a significant impact on performance.
- Shielding Effectiveness: Cables require effective shielding to prevent electromagnetic interference (EMI). Without effective shielding, cable assemblies must be kept farther apart, which isn’t possible in densely packed systems like those for space. Consider cables with multiple internal shields, as they are more effective at mitigating EMI.
- Environmental Considerations: Radiation resistance is key for preventing changes in the cable dielectric that degrade electrical performance. Also consider Outgassing when looking for space qualified RF cables; when exposed to a vacuum environment, many non-metallic materials outgas, including plastics commonly used in coaxial cables. Cables and assemblies going into space must meet the standards for outgassing rates defined by NASA and the European Space Agency (ESA). Additionally, avoid whiskering of metals such as pure tin by building coaxial cable assemblies with tin/led solder alloys. Other environmental considerations for space applications include sandblast storms, extreme temperatures, and pressure variations.
- Stability over Flexure: In tight, dense, or in-the-box spaces, cable flexibility is crucial for routing. Flexible cables used in these spaces need to maintain stable performance over flexure.
- Attenuation: Three properties define the attenuation of a coaxial cable: the conductivity of the conductors, the dielectric constant, and the diameter of the cable. Taking all three of these properties into account when selecting a cable for a space application is crucial.
Factors to Consider for a Space Qualified RF Cable Supplier
For space qualified microwave cables and connectors, choosing a supplier for space cables and space grade connectors is a critically important decision. Some suppliers offer standard qualifications and documentation for basic products in hopes of simplifying this process. Designing a crucial interconnect system that will perform well and withstand the extraordinary environmental and technical conditions of space, reliably and consistently over long periods of time, is not like designing just any RF interconnect system. The conditions encountered in space are unique and require special, highly customized solutions to prevent failure.
- Qualifications and Heritage: Look for partners that have experience with designing for space applications, as well as other areas such as military and defense, who can help meet your unique requirements.
- Dedicated Technical Experts: Choose a supplier who works collaboratively as an extension of your design team and involve their technical experts. Don’t work with a supplier that’s committed to selling you the same product they’re selling everyone else. Your supplier should help you understand the electrical and mechanical trade-offs particular to your application.
- Breadth of Products: A partner with a broad range of products can better handle providing the right system and products for your unique application. Having the opportunity to select the right material, choose from multiple cable constructions, various connector designs, and provide input on assembly techniques is a key benefit to selecting a qualified supplier. In all applications, but particularly for products to be deployed in space, technical standards like using only acceptable materials or mil-spec cable construction must be met. However, since there currently aren’t any standards for applying these materials to construct an RF solution that is reliable time after time. This is where a supplier’s expertise and access to a full range of product options are crucial.
- Manufacturing Execution: Ideally a supplier has both the technology and products needed and understands how to put them together for a final product. The next qualification to consider is manufacturing operations. Does the company have robust manufacturing facilities and processes to support execution? Cleanroom manufacturing capabilities and traceability are key for managing all the pieces to create complicated assemblies. Be sure you know what quality standards your supplier follows and what extended services they offer.
