RF cables and connectors for avionics balance size, materials

By David Kiesling

Originally published in Military Embedded Systems

Today’s complex frequency-band requirements – including 5G, new and upgraded satellite communications (SATCOM) systems, instrument flight procedure (IFP) systems using Bluetooth, and more – are creating the need for additional lightweight, small, high-precision radio frequency (RF) solutions. These systems operate at frequencies up to 90 GHz in some cases. The avionics industry needs low-loss, high-temperature, high-flexibility cable for navigation, collision avoidance, and communication systems in applications such as GPS, Automatic Dependent Surveillance-Broadcast (ADS-B), SATCOM, and air-to-ground comms.

Avionics applications also have limited space, as they accommodate more application needs throughout the vehicle. As frequencies increase and interconnect dimensions decrease to accommodate the smaller wavelengths, semi-rigid solutions have traditionally been used for these applications. However, these assemblies in very small sizes become fragile, making installation difficult and troubleshooting impossible.

Use of highly flexible, high-performance cable can be used in densely packed in-the-box applications; such flexible assemblies can be bent around tight corners and very closely behind the connector to minimize footprint, save space, and simplify cable routing in tight spaces. The flexible cable eliminates the need to protect the back of the connector and simplifies maintenance.

In flight, extraordinarily high vibration puts stress on the board hardware. Minimizing space between the cables and connectors is necessary for the interconnect system to survive the high vibration.

Taking a cue from commercial air innovations

Urban air mobility (UAM) is a concept for a safe and efficient aviation transportation system that will use highly automated aircraft to operate and transport passengers or cargo at lower altitudes within urban and suburban areas. Most UAM vehicles will operate below 10,000 feet, with battery power limiting their altitude and endurance. As the technology evolves, the density of UAMs in motion will increase, and in a scenario called “swarming,” their ADS systems will need to be in constant communication to avoid collisions. These systems are largely dependent upon antennas and high-frequency transceivers to enable the sensors to talk to each other, so a low-loss, reliable, low-weight cable is critical for reliable communications.

Further developments include advanced air mobility (AAM) solutions, which build upon the UAM concept with applications that will operate beyond urban environments, ranging from delivery drones to electric vertical takeoff and landing (eVTOL) applications. Like electric vehicles, eVTOL controls run on onboard electric power, and batteries are heavy, so weight is a substantial consideration. As a result, these technologies require advanced solutions to meet new requirements while adhering to the size and weight principles that have governed their predecessors for decades.

Avionics for UAVs

Another rapidly growing area of avionics are those for unmanned aerial vehicles (UAVs). Some UAVs are fairly simple and use a single data link, which requires a relatively simple RF interconnect solution. However, as hypersonics are introduced, some of these UAVs are as much as Mach 5, which adds high-temperature requirements into the mix. The higher the altitude, the higher the speed, the higher the frequency, the more complex the problem from a materials point of view. These issues can be addressed with dielectrics; moreover, there is the possibility of using quartz materials as dielectrics for hypersonic applications.

Ultimately, whoever is designing the structure needs to understand the environment, the UAV’s altitude requirements, what sort of longevity is required, how often maintenance is allowed, and what kind of test environment is available. For example, will the UAV carry electronic warfare (EW) systems or electronic intelligence systems, or is the payload  purely data links and video?

The right interconnect

A key consideration for avionics RF interconnections is the dielectric and how it behaves in that environment as it reaches a state of equilibrium with the surrounding environment. For example, an aircraft on the ground is fully loaded with air, while at flight altitude, more of a vacuum environment is created, with outgassing conditions for dielectric and other electronic materials. When the aircraft returns to the ground, the cable is fundamentally a vacuum. Any fluids or gases surrounding it will be absorbed by the dielectric, and they will recondense within it. At that point, the dielectric acts like a sponge: The only way to remove contaminants is to bake it. This is obviously not feasible inside an aircraft, so vapor sealing is also critical for high-end, high-performance, high-altitude applications.

So many factors must be considered in the design of the right RF interconnect solution. For example, polyethylene improves fire retardancy and flame resistance; it’s a really good cabling option for outdoor environments or for indoor environments with a fire-retardant, UL-listed jacket. A caveat: Polyurethanes can be significantly more flexible, but they cannot be used in a manned environment.

On the airframe side, PTFE (synthetic fluoropolymer) dielectric cables that tolerate high temperatures are typically used and can be combined with a higher-than-standard PTMP (polyester plastic) tape wrap that is very low loss. This type of solution can be used all the way up to 50 gigahertz or more. At the high end of the temperature spectrum, some silicon dioxide cables that are rated for up to 800 °C or more.

Connector optimization 

The ultimate goal is to optimize connectors for mechanical, environmental, and electrical performance, and make it very easy to install. A number of aluminum and plastic connectors are available to keep it lightweight.

Development work is being done on plastics, including threaded connectors, snap-line connectors, bayonet-style connectors and more, with materials like Delrin. Additional research and development is underway on an ultralight solution, including a modular connector solution with a plastic coupling and tooling solutions. This approach would help with maintenance, enabling users to fix problems directly in the field.

Tooling can ease assembly

When there is a need to install the RF interconnect solution aftermarket or perform maintenance work in the field, tooling is available that eliminates the need to use razor blades in the field, making on-site assembly safer and easier in what could be a contested area. This approach from companies like Times Microwave Systems^® enables everyone to use the same tool on the cable for the same application as opposed to the alternative, multiple technicians using different types of tooling and techniques.

Times Microwave Systems offers InstaBend^® (IB) 047, a compact, phase stable, highly flexible, micro coaxial cable. Originally designed for space flight applications, this high-performance cable easily accommodates densely packed in-the-box applications. For more information, visit www.timesmicrowave.com.