Avionics systems provide electronic guidance, navigation, and communications through harsh and hostile environments for a wide range of airframes. Operating environments present elevated levels of shock and vibration; vacuum-like conditions of high altitudes; corrosive effects of hydraulic fluids, fuels, and other chemicals; and the effects of wide temperature ranges. Avionics systems must handle such challenging environments even as they are being designed with greater functionality into smaller payload spaces.

The high-frequency RF/microwave coaxial interconnections within those systems are essential components that must perform repeatably and reliably while also meeting modern military electronic systems requirements for reduced size, weight, and power (SWaP). RF/ microwave coaxial cables and connectors for military avionics systems must perform according to many electrical, mechanical, and environmental demands, still they must be accessible when avionics systems maintenance or troubleshooting is needed, so finding the “best fit” RF/microwave cables and connectors for these systems often involves balancing tradeoffs.

High-frequency 50-Ω RF/ microwave coaxial interconnection solutions for military avionics systems must be light in weight and small to fit the high-density requirements of modern airframes and avionics systems. Avionics applications typically range in frequency from DC to 12 GHz, including such applications as 4-GHz radar altimeters, distance measuring equipment (DME), a satellite-based Global Positioning System (GPS) receiver, a microwave landing system (MLS), and a Traffic Alert and Collision Avoidance System (TCAS). Driven by demands for smaller SWaP equipment requirements, avionics systems are being mounted within smaller airframes and equipment housings, requiring coaxial assemblies to maintain reliable electrical and mechanical interconnections in tight spaces and under the most severe operating conditions.

For example, the limited physical room for military avionics systems results in tight spacing that invites unwanted coupling between RF/microwave transmission lines such as coaxial cables. If not adequately shielded, RF cable assemblies within an aircraft’s avionics systems can function as antennas and generate electromagnetic interference (EMI) within the system, affecting components or subsystems such as receivers susceptible to excess levels of EM emissions. RF/microwave coaxial cables and connectors for military or civil avionics applications should provide minimum required levels of EM compatibility (EMC) and EMI suppression to enable interference-free operation of avionics systems within tightly packed housings.

Tracking Trends

Modern military avionics systems with greater functionality from smaller spaces reach higher frequencies to provide the bandwidth for more subsystems. A military avionics range that once reached 12 GHz is being extended to 18 GHz and higher, even into the millimeter-wave (mmWave) frequency range (30 GHz) and beyond. Tight spaces for multiple avionics subsystems often require highly flexible RF/micro-wave coaxial cable assemblies for signal interconnections. High cable flexibility allows a minimal bend radius exiting a coaxial connector to accommodate tight space requirements within an avionics frame. Still, some amount of space is usually required between the connector interface and the cable to allow for any movement in the physical parts due to the high shock and vibration levels that an avionics system may endure.

Higher frequencies call for smaller diameter cables and connectors to support the smaller wavelengths of those higher frequencies. Unfortunately, the smaller cable dimensions have increased insertion loss for the same signal frequency handled by a larger-diameter cable. Although with higher insertion loss, smaller cable diameters result in lower weight for a given cable length. For frequencies that larger-diameter cables can handle, long cable runs in an avionics system are often best accomplished with a series of different cable assemblies with different diameters, selected to balance the size and weight and loss at a required signal frequency. Interfaces between the different cable assemblies must be closely matched in VSWR to maintain impedance continuity and preserve phase characteristics throughout the cable run length.

Coaxial cables and connectors for avionics systems incorporate dielectric materials and conductive metals to meet those systems’ electrical, mechanical, and environmental requirements. Temperature extremes for avionics cable assemblies can be quite broad, in part due to the rapid changes in altitude endured by avionics systems. Cable assemblies must maintain consistent performance over wide temperature ranges and during rapid changes in temperature known as thermal shock. They are evaluated in terms of a set of performance parameters that include insertion loss, VSWR, and phase stability over temperature ranges as wide as -50 to +200°C. All materials undergo a certain amount of expansion and contraction with temperature changes. Different materials’ thermal behavior must be considered when developing cable assemblies and connector interfaces. To maintain the reliability of the connectors’ 50-Ω interfaces with changing temperatures, forces, such as a spring force, may need to be applied.

For systems in which phase is a crucial parameter, such as radars, RF/microwave cable assemblies must provide good phase stability. Phase behavior is affected by temperature and other environmental conditions, such as shock and vibration. Because some amount of material expansion and contraction with changing temperatures is almost inevitable, those mechanical changes must be considered as part of the mechanical structures of the cables and connectors. By constructing the cables and connectors with the appropriate materials, phase variations can be minimized even across the wide temperature ranges specified for avionics applications.

High humidity is another environmental condition that can impact the electrical performance of RF/microwave connectors in avionics systems. A coaxial connector with a hermetically sealed interface may be required for operating environments with high humidity, to guard against degradation of electrical performance, such as increased insertion loss and VSWR. Moisture can modify the electrical behavior of the connector’s interface, resulting in variations in amplitude and phase that increase with increasing frequency.

Seeking Solutions

Faced with the challenges of developing reliable, high-performance coaxial cables and connectors for military avionics interconnections that fit shrinking airframe spaces, Times Microwave Systems developed several RF/microwave interconnection solutions. MilTech® Lightweight (MTL) hermetically sealed cables (Figure 1) are light and flexible but with low insertion loss (Figure 2). These RF/microwave coaxial transmission-line assemblies feature a proprietary spiral strip conductor for excellent electrical performance, meeting the needs for high-frequency interconnections in modern military avionics systems.

Figure 2. The loss of flexible coaxial cables such as the MTL cables increases with frequency and decreases with larger diameter.

MTL cable assemblies are designed to match the performance levels of the company’s well-established MilTech® cables but with lighter weight. They are built for high reliability and a long operating lifetime even in the hostile operating conditions typically faced by military and civil avionics systems. Engineered and manufactured with tight integration between cables and connector interfaces, they maintain a vapor seal of 1 × 10-5 cm3/s/ft. for protection against humidity and harmful chemicals. With tight bend radii, the lightweight and flexibility also makes them candidates for ground-based and shipboard system applications where interconnection space is at a premium.

Figure 3. These different M8 connector shells provide examples of how multiple coaxial connector assemblies can be mounted into tight spaces such as in military avionics systems.

As Times Microwave Systems learned from analyzing the MilTech® cables to create the lighter MTL cable assemblies, the company’s engineers also studied the modular Multiport (Figure 3) to develop a smaller modular connector assembly in the Mini Multi-Port (MMP) connector system. These multiport connector systems integrate multiple coaxial connector contacts into a single housing for much higher interconnection density than possible with standard coaxial connectors. The single connector interface also reduces system installation time and, assuming that the multiport connector housing is accessible, eases system maintenance and testing. Mating a single multiport connector rather than multiple separate pairs of cable assemblies reduces interconnection time while increasing reliability.

The mini-multiports are constructed from stainless steel and titanium for long lifetimes. They ease interconnections to 20 GHz in many avionics systems (and are usable to 40 GHz) while meeting MIL-T-81490 and MIL-C-87104 assembly requirements. They feature a well-shielded structure for good EMI/EMC performance and a design that allows for blind-mating interconnections for ease of installation. They are spring-loaded to ensure reliable interconnections and as much as 100-dB isolation between channels under changing temperatures and other environmental conditions

The compact MMP modular connector system has been developed to provide smaller, lighter interconnections supporting the trends for denser, more tightly packed avionics systems. MMP connector interfaces and their cable assemblies also meet MIL-T-81490 and MIL-C-87104 requirements with good shielding via a jackscrew mating system to meet the EMI/EMC requirements of densely packed military and civil avionics systems. They are usable to 40 GHz with low insertion loss and VSWR. As with their larger M8 counterparts, the MMP connector interfaces can be specified for phase-matched sets of coaxial cable assemblies when multichannel control of phase is needed.

This article was written by Ted Prema, Director of Aerospace Programs, Times Microwave Systems (Wallingford, CT). For more information, click here .