By Maria Calia
Originally published with Aerospace & Defense Technology
RF cable assemblies might appear to be a minor component in system design, but they can make all the difference between success and failure, especially in mission-critical industries such as defense and space. The RF interconnect is the vital bridge between many critical systems, including payload, communications, signal transport, and processing. This article will primarily focus on hypersonic missile systems and satellites to illustrate these concepts, as they jointly highlight the importance of RF cable assembly design in extreme environments.
Designing an interconnect system in the extreme environments these applications have in common is not the same as creating an ordinary RF interconnect. For instance, modern hypersonic missiles and satellites require compact and ruggedized RF cable assemblies to support high-density communication systems within confined spaces. These assemblies must withstand high-shock and vibration conditions, extreme temperatures, and other demanding requirements while maintaining high reliability and continuous functionality without hands-on maintenance.
Following is a comprehensive guide on the critical performance parameters to consider when selecting an RF cable for hypersonic missile systems and satellites, as well as related defense and space applications.
Once the correct RF cable is identified and meets the application’s minimum mechanical, electrical, and environmental requirements, selecting the optimum design according to multiple performance measures is the next step. For hypersonic missile and satellite systems, these include extreme temperatures, attenuation, and phase stability.
During their journey, hypersonic missiles may reach skin temperatures ranging from 2,000-2,500°C. This means internal temperatures can be as high as 600-1,000°C. Furthermore. a hypersonic missile utilizes multiple antennas and sensors, which must survive at speeds that can exceed Mach 5, at times topping 5,000 miles an hour. At that rate, it is possible for the temperature on the surface and in the boundary layer of the missile to exceed 2,500°C.
The temperature conditions of a satellite traveling through space can also vary significantly depending on its position in relation to the sun or shadows, as well as other factors. For example, a satellite facing the sun can reach high temperatures on a planet’s day side to cold at night based on its orbit.
This creates an unprecedented material challenge as extreme temperatures melt the plastics and polymers typically used in coaxial cables.
After determining the temperature range the RF system will need to operate in, the next consideration when specifying electrical performance for the cable is attenuation or insertion loss. Three properties define an RF cable’s attenuation: diameter, the conductivity of the conductors, and dielectric constant.
In general, larger-diameter cables provide lower attenuation per unit length than comparable smaller-diameter cables, but this comes at the cost of the increased mass and an increased 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, worse, to kink, causing an impedance mismatch and excessive return loss. Designers must balance cable loss contribution to the RF link budget along with the mechanical considerations for system mass and size.
High-conductivity materials such as silver and copper provide low attenuation per unit length but are often heavy or expensive. Conversely, lighter-weight materials such as stainless steel and aluminum reduce overall mass but are poor conductors. As a result, 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.
The third contributor to cable performance is the dielectric constant. A lower-loss dielectric generally will be lighter because it incorporates more air into the media. More air in the dielectric material lowers the total media’s effective dielectric constant, meaning the transmitted signal will encounter less resistance or loss.
Next, in terms of electrical length, it should be determined if the cables in the system need to be phase matched. In either of the application types discussed, as the cable is routed across areas that can range from extreme cold to scorching temperatures, the phase matching between cables must track, regardless of environmental factors. However, temperature variations degrade the electrical match between coaxial cable assemblies. That small amount of degradation, known as its phase tracking characteristic, can adversely affect system performance.
The ability to match multiple microwave cables to each other so the signal takes the exact same amount of time to travel through, as well as controlling how that phase relationship changes over temperature, are absolutely essential properties in the type of RF cables used in hypersonic missile and satellite applications. For example, the electronically steered antennas used in numerous RF applications employ radiating elements to steer antenna beams rather than physically moving the antenna. Beam steering for reception or transmission is performed by adjusting the phase of the individual antenna elements in the array. High-frequency transmission lines feed each antenna array element. The accuracy of the signal phase presented to each array depends on the phase stability and accuracy of the cable assemblies. Therefore, cable assemblies that are optimized for phase performance typically exhibit minimal changes in phase with temperature.
Unfortunately, the commonly used dielectric PTFE exhibits an electrical property known as the “knee” at approximately +19 C, at which point the electrical length per unit temperature undergoes a non-linear transition. The basis of this property is the nature of PTFE itself. PTFE is a long-chain molecule with crystalline sites connected by amorphous chains arranged in a helical fashion. Below +19 C, there are 13 CF2 groups per 180-degree twist of the molecule. At the +19 C transition point, sufficient energy is imparted to the molecule to unwind it slightly, leading to 15 CF2 groups per twist. Unwinding the molecule makes it longer, reducing the volume. Electrically, this leads to a higher velocity for the signal in the cable and a smaller electrical length per unit of mechanical measurement. Graphically, this non-linear change in electrical length vs. temperature looks like a “knee.”
Semi-rigid cables have previously been the standard cabling solution for these applications because their solid copper outer conductor protects the dielectric material inside and provides superior shielding performance to a comparable flexible construction. However, RF cables constructed using a silicon dioxide dielectric are increasingly used throughout the microelectronics industry for their excellent insulating properties, low-loss, and high velocity, and to offer semi-rigid cable solutions that are highly temperature and radiation resistant.
They also provide superior phase and loss versus temperature performance as the dielectric does not produce a non-linear change. In addition, silicon dioxide cables provide exceptionally low hysteresis, with phase and loss values returning to the same values at a given temperature even after extreme excursions.
The construction of the silicon dioxide coaxial cable begins with a solid oxygen-free copper center conductor, a silicon dioxide insulating dielectric, and a stainless-steel jacket with copper cladding to act as the outer conductor. Connectors in silicon dioxide cable assemblies should utilize a crack-free, fired glass seal to provide optimum microwave performance and hermetic sealing.
Silicon dioxide excels in severe environments and can perform at temperatures ranging from just above absolute zero to above 1,000°C. In addition, the metal and silicon dioxide dielectric construction naturally makes the cable resist radiation of more than 100 megarads.
In fact, this cable type has been qualified and is currently used on numerous high-profile satellite programs, including technologies created by NASA and the European Space Agency in applications seeing exposures in the 10s of Gigarads. It has also been successfully employed in numerous cutting-edge hypersonic missile designs.
In conclusion, RF cable assemblies are crucial in mission-critical applications such as hypersonic missiles and satellites. These systems operate in extreme environments and require compact, rugged, and reliable RF interconnects to support communications equipment within a highly confined space.
When selecting an RF cable for these applications, it is essential to consider critical performance parameters such as extreme temperatures, attenuation, and phase stability. Attenuation depends on the cable size selected, the conductivity of the conductors, and the dielectric constant. Phase stability is critical and requires compensation for temperature variations that can degrade the electrical match between coaxial cable assemblies. Overall, selecting the right RF cable for applications that must excel in extreme environments without the ability for routine maintenance requires a balance between electrical, mechanical, and environmental requirements and optimal cable design to ensure reliable and consistent performance over extended periods.
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