Phase-Stable Cables and the Challenges of Space

By David Kiesling

Originally published in Microwave Product Digest

Satellite communications are an indispensable part of global infrastructure, enabling real-time data transmission anywhere on earth and into space. As commercial industries increasingly work to advance and expand connectivity with powerful new 5G technology, space-based platforms and satellites will become even more critical.

The cables, connectors, and RF solutions deployed in space are integral components enabling industry to successfully move satellites, enable earthbound communications, and transmit information contributing to climate science, global security, communications relays between continents, high-speed Internet, and more. The enormity of global connection and data sharing needs is growing by the day—and the communication infrastructure’s performance is vital.

Therefore, 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 is not like designing any interconnect. Cable assemblies used in spacecraft need to brave high shock and vibration, extreme temperatures, and intense radiation. In addition, satellites have limited space for equipment, so minimizing size and weight are also key goals. Cable assemblies must be designed to perform reliably while taking up the smallest footprint possible.

Two primary considerations for optimizing RF interconnect systems, selecting the right technologies, and accommodating the unique requirements of deployment in space include smaller footprints—lightweight, high-density connections, and intricate installations in very tight spaces—and phase stability in terms of temperature and bending/motion. The critical requirements are detailed further below.

Smaller Space Requirements

Equipment used to power space technology must be lightweight, and RF coaxial cable assemblies must be designed to perform reliably in the smallest possible footprint. The high-frequency cables required for space applications also have a shorter range, requiring a dense network of antennas. Additionally, minimizing space between the cables and connectors is necessary for the interconnect system to survive the high vibration and other harsh environmental conditions.

Technology providers are working on advanced designs that accommodate these extremely restricted space constraints and rising operating frequencies to produce spaceflight connectors that successfully operate up to the 70 GHz range. This requires new coaxial cable and connector designs to deliver high signal integrity and reliability in increasingly dense environments.

One example of innovation in this area is Times Microwave’s new PhaseTrack? InstaBend? 047 (PTIB-047) microwave assemblies, which provide a flexible preassembled design for interconnects between RF circuit cards, modules, and enclosure panels, enabling space-efficient implementation for higher frequency systems that also need phase stability

The new PTIB-047 is a phase-stable microwave assembly designed for a range of frequencies from DC to 70 GHz. The cable utilizes Times Microwave Systems’? proprietary TF4? dielectric material for superior phase stability over temperature, eliminating the problematic non-linear phase performance of PTFE through 15 to 25oC. PTIB-047 is ideal for in-the-box applications with space constraints, including space flight, thermal vacuum, microwave test, and many other commercial and military applications. The cable can be bent very closely behind the connector, minimizing footprint, saving space and simplifying cable routing. This also eliminates the need to protect the back of the connector.

Phase Stability

Tight phase control is crucial for optimizing system performance in technologies including 5G smart antennas. Two primary elements can affect a coaxial cable assembly’s phase tracking characteristic: electrical length and temperature. For example, phased array radars that have multiple antenna elements rely on coaxial cables having the same electrical lengths between the transmitter-receiver and antenna. This poses challenges on how to match the cables.

Transmission lines feed the arrays; beam accuracy depends on those cables’ phase relationships. Phase is also responsible for precision in some more time-sensitive satellite applications like GPS and radar. The phase must be accurately controlled in the components within those systems, and phase-stable cable assemblies are essential in today’s increasingly sophisticated electronics.

After that initial matching, the coaxial cables must also stay matched over varying temperatures. As temperatures change, coaxial cables do not precisely track together; the phase match degrades just slightly. That small amount of degradation can adversely affect system performance. Therefore, some cable assemblies must be optimized to minimize phase change over temperature.

The concept of phase starts with the fact that a microwave signal propagates in the form of a sine wave. For every sine wave cycle, 360 deg. of electrical length will accumulate. Millions or billions of cycles per second will accumulate in the higher frequency range of 5G applications. The wavelengths are very short, so maintaining phase accuracy across a cable length becomes exponentially more challenging as frequency increases.

Frequency, time delay, and physical properties like length, dielectric constant, and propagation velocity affect electrical length. Coaxial cables contain a consistent dielectric material throughout the length of the cable and hence have a constant velocity factor. Even though the material is consistent, environmental factors can alter the electrical properties of the cables, including temperature fluctuations, flexure, handling, twisting, pulling, and crushing that can happen to a cable during installation and maintenance.

A cable assembly also gets electrically longer as it gets colder and shorter as it gets warmer. Electrical length is proportional to physical length. Metals expand as they get warmer and contract as they get colder, but as that happens the dielectric constant expands and contracts and its density changes, altering the velocity. The dielectric effects of the plastic offset and dominate the metal effects.

RF coaxial cables often use PTFE dielectric because it can operate across a broad temperature range of 50o C to 150o C) and it has a low dielectric loss. The challenge with PTFE is that the material goes through a phase transition around room temperature, causing the phase length to vary non-linearly with temperature and introducing significant hysteresis as the temperatures vary up and down. Controlling phased array antennas with PTFE-based cables is challenging in varying temperature environments. Times has experience designing cable assemblies to minimize temperature effects on phase through the use of special materials, such as cables made from silicon dioxide and TF4 and TF5 proprietary foam polyethylene blended dielectrics.

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. PhaseTrack InstaBend 047 incorporates this proprietary TF4 dielectric material for superior phase stability over temperature, making it another excellent option for small spaces that require phase stable solutions.

Conclusion

Searching for and qualifying an RF interconnect supplier for space applications can be lengthy and costly. Some suppliers offer standard qualifications and documentation for basic products to simplify this process. But nothing is standard or easy in space.

Custom designs, special testing and qualifications, and new product development for space applications require experience and commitment. “Standard” RF systems are not good enough in space. It is important to thoroughly evaluate the capabilities of an RF supplier to ensure a positive outcome.

In the design phase, consider the key considerations to be factored into any RF system that will operate in space: high-density equipment installation for minimal footprint; materials selection to withstand environmental challenges and optimize fit, weight, and durability; and phase stability for optimal signal transmission. Work with a reputable RF solutions provider who understands space’s unique requirements and can customize components to meet your application needs. A qualified provider will have experience designing custom solutions for defense, military, avionics and aerospace installations.