In the recent past, military acquisition has shifted from being locked into large long-term contracts to developing complex systems in discrete increments that can be further optimized in future design cycles. These shorter design cycles allow for equipment to be deployed more rapidly, thereby mitigating the risk of subsystems going obsolete as increasing proportions of budget dollars go towards operation and support (O&S). This transition to evolutionary acquisition (EA) leaves a major opportunity for vendors to develop commercial off-the-shelf (COTS) components that are military-compliant.

Typically, commercial-grade devices have a wide range of quality across manufacturers as the standards (or lack thereof) vary from organization to organization. Since reliability is essential for mission-critical components, it is important for companies to apply standards that are relevant to their product. This remains true for the respective interconnect between components in military systems.

Oftentimes, cables are not a significant consideration in the design process as they are far less complex than the systems they connect to. However, greater consideration may be warranted because a failing or degraded cable can cause significant loss in signal integrity and/or failures that are time consuming to troubleshoot and repair. This is especially true in aerospace applications where there are miles of cable routing in a single aircraft.

It is therefore critical to ensure that a cable assembly can tolerate the various electrical, mechanical, and environmental conditions that it may be exposed to. This involves a rigorous process of standard electrical tests, burn-in and additional relevant testing before being deemed as field deployable. RF interconnect such as coaxial cables have several primary standards that help to ensure cable performance regardless of environmental factors; these standards include MIL-DTL-17, MIL-PRF-39012, and MIL-STD-348. A more thorough listing of standards can be seen in Table 1.

The combination of standards such as these allow for reliable integration of interconnect in most military systems and mitigating the risk of failure. The MIL-DTL-17 and MIL-PRF-39012 general specifications for cables and connectors provide an outline for coax construction and testing while the J-STD and SAE standards list assembly related standards.

Figure 1. Sample incoming material flow process. Image Credit: pcb.com

Lot traceability is another important requirement for any components deployed in the military. Lot traceability can be defined as the ability for a vendor to track the materials used in a product back to their respective sources. This type of tracking must be implemented from the receiving stage to the shipping stage, and all other stages in between. Likewise, data capture of the relevant lot information is important and often best accomplished with regimented uploads to an ERP system. Figure 1 shows a sample flowchart of incoming material flow process. The assembly and test processing flow for a coaxial cable brings many more steps in this process where logging connector/soldering/heat shrink installation and verification are key for every step in cable construction.

Materials and Assembly of High Reliability Cables

By utilizing qualified (QPL'd) cable and connector materials in combination with other industry standards and process certifications, such as J-STD-001, design and manufacturing defects can be reduced, thereby making the coaxial cable assemblies better suited for the high-reliability applications where the cost-of-failure is high. While not every high-performance cable and connector are DOD qualified, it is common for these other components to at least certify to the most applicable paragraphs of MIL-DTL-17 or MIL-PRF-39012.

Table 1. Some of the major military and industry standards for hi-rel RF cable assemblies.

The J-STD standard, for instance, relates to the types of solder and methods to use when attaching center pins, outer conductors, and shielding of the coaxial cable to the connectors. The SAE International standards listed in Table 1, allow for consistency in tools, parts, and labeling between coax assemblies. The conformance testing listed in MIL-DTL-17 and MIL-PRF-39012 ensures that a cable and connectors can operate in various conditions and is therefore field deployable. Table 2 covers some of the test parameters that are listed in MIL-DTL-17 and, depending upon the military application of choice, some of these parameters are more relevant than others.

Table 2. An overview of the various measurement parameters listed in MIL-DTL-17 organized by electrical, mechanical, and environmental cable performance.

Relevant Conformance Tests for Various Military Applications

Coaxial assemblies are necessary for the connection of high frequency signals. Military applications can range from S-band Interference Friend or Foe (IFF) to millimeter-wave imaging.

Figure 2. An image of the natural and induced stressors on military equipment in various environments.

While the performing frequency band of a coax generally changes its dimensions, the environment has a significant impact upon the materials used in its construction. Some natural and induced environmental factors are shown in Figure 2, an image taken from the MIL-STD-810 standard for environmental engineering considerations of various military systems. It can be readily seen that factors such as cable size, weight, power handling, and ability to withstand certain hazardous conditions become paramount to the functionality of an entire system. The choice of materials for the dielectric, inner and outer conductors, and jacketing material can then become serious considerations.