A comprehensive test regimen can help determine the suitability of microwave coaxial cable assemblies for the final frontier.
By Aidan Doran, Senior Product Manager, Space, and John Lewis, Director of Product Management, Defense and Space, Carlisle Interconnect Technologies test
Reliability can be defined as performance over time, and in deep-space applications, nothing is more important than reliability. Achieving reliability in deep space with microwave coaxial cable assemblies is extremely difficult due to inherent instabilities that can permanently damage critical mechanical and electrical properties. While cable assemblies are often considered the last piece in a system design, most systems will fail if assembly performance is compromised.
Since failure in deep space is impossible to repair, system designers must have 100% confidence in the stability of their coaxial cable assemblies. The stability of cable assembilies is critical to a successful mission.
Unfortunately, coaxial cable assemblies — even when fully qualified for space — are constructed from various layers of metals, plastics, and fibers with very different coefficients of thermal expansion. This translates to material movement directly related to thermal extremes and time. Resolving this challenge begins with the dimensional stability test from MIL-DTL-17, the industry’s most expansive coaxial cable standard.
Dimensional stability testing exposes products to extreme temperatures and then measures mechanical change. While this is an excellent starting point for deep-space validation, the test should be expanded to include the measurement of both mechanical and electrical characteristics. Stability parameters such as voltage standing wave ratio (VSWR), radio frequency (RF) shielding effectiveness, and interface gauging are so critical to success that they sit at the very top of the coaxial cable assembly performance pyramid, as maintaining them throughout mission life is a top priority.
A typical deep-space thermal cycling validation regimen consists of 400 or more cycles over temperature extremes spanning -150°C to +165°C with at least 30-minute dwells at each extreme. Fortunately, mission life performance in coaxial cable assemblies correlates directly with successfully passing this extended dimensional stability testing.
The validation data detailed here represents the performance of Carlisle Interconnect Technologies’ MCJ311A space-grade coaxial cables assembled with extreme temperature, subminiature version A (SMA) connectors. These coaxial cable assemblies exhibit the same VSWR, RF shielding, and connector interface control at the beginning and end of the test regimen.
Of these attributes, VSWR is the most critical. VSWR measures the reflections that result from impedance transitions along a cable, the cable/connector interface, and finally, along the connector’s own internal geometric structure. Said another way, if someone stands in a pool of still water and creates a wave with one hand, that wave will travel to the wall of the pool and bounce back. The pool water is the transmission path and can be compared to the cable, the wall can be compared to steps in the connector, and the wave equates to VSWR. If that wall were to become dimensionally unstable (i.e., move or change), then the wave bouncing back would change as well. This is why VSWR should remain as low and consistent as possible.
Consider the VSWR responses below. Performance remains measurably unchanged after 400 cycles of the extreme thermal exposure described herein.
Two additional characteristics critical to mission success are RF shielding effectiveness and connector interface gauging. A coaxial cable assembly may exhibit acceptable VSWR performance, but significant changes in shielding and connector interface control can predict stability failures. Staying with the pool metaphor, this would equate to gaps or holes in the wall enabling water leakage or disruption, which no one desires. The same MCJ311A assemblies with Extreme Temperature SMA connectors exhibit very solid shielding effectiveness exceeding -90dB through 18GHz and unchanged connector interface measurements throughout the cycling regimen, exactly as desired.
Combined, the consistent performance of these characteristics assures that all materials within the coaxial cable and connectors have remained dimensionally stable with temperature and time.
While focusing on results at the beginning and end of the thermal cycling regimen provides strong correlation to mission success, more granular data can also be achieved by measuring test assemblies at various intervals throughout the cycling (e.g., after every 50 cycles). In this way, slight changes in performance may appear as trends over time, leading to the need for more analysis or even adjustments to the coaxial cable assembly manufacturing process.
In summary, the test validation protocol consisting of 400 cycles from -150°C to +165°C, plus pre- and post-measurements for VSWR, RF shielding, and interface gauging, is an excellent method for validating the suitability of microwave coaxial cable assemblies for completing successful missions in deep-space environments.