Low Loss Allows Connections To Varied Applications

April 6, 2009
Requirements in the defense, test, and aerospace markets have undergone major changes in recent years. They also have become more demanding. For defense applications, for example, enormous importance is placed on aspects like vibration, shock, ...

Requirements in the defense, test, and aerospace markets have undergone major changes in recent years. They also have become more demanding. For defense applications, for example, enormous importance is placed on aspects like vibration, shock, minimal phase change with temperature, RF leakage, moisture, chemical resistance, and weight. In contrast, the test and measurement market calls for long flexure life, minimal phase change versus bending and torsion, good amplitude stability with bending and shaking, and more. For space applications, these requirements are combined with the added rigors of testing in a thermal vacuum chamber. As a result, even more requirements are added to the mix, such as low outgassing and venting. Despite the vast differences in these applications, however, low-loss microwave cable assemblies offer an ideal solution for all of them.

In electronic defense systems, low-loss coaxial cable assemblies are primarily used in tactical and strategic communications as well as electronic warfare. Radar applications specifically call for phase-matched cable assemblies. Some companies offer complete sets of cable assemblies matched in relative or absolute phase, time delay, or amplitude characteristics. High-performance cable assemblies for satellite applications must withstand rigorous thermal vacuum tests. Such high-performance cable assemblies may also find use in medical system applications like high-frequency ablations of cancerous ulcers or connections between high-frequency generators and probes.

Such demanding applications require coaxial cable assemblies that exhibit low insertion loss with superior phase stability versus temperature and bending. In addition, excellent return loss and mechanical stability are of the utmost importance. Cables like SUCOFLEX 400 meet these requirements through the use of an extruded low-density polytetrafluoroethylene (PTFE) dielectric with a dielectric constant ( r) of 1.26. The high proportion of air in the dielectric material accounts for light weight without compromising mechanical stability and reliability. Table 1 provides an overview of 5.5-mmdiameter SUCOFLEX 404 cable, which is usable to 26.5 GHz.

The cable's low dielectric constant and low loss enable robust continuous-wave (CW) power capability ( Table 2). At the same time, phase stability versus temperature benefits from the new dielectric formulation (Figs. 1 and 2). These aspects offer an advantage in applications like phased-array antenna systems. They also help to simplify adjustment on radars.

Space applications demand a high level of testing on individual components as well as on a complete satellite. A satellite's transmit and receive componentsas well as the cables and connectors that link themmust work under extreme environmental conditions, such as in a vacuum and with rapid changes in temperature. For instance, satellites must endure sunlight (temperatures to +150C) and darkness (temperatures as low as -100C). These conditions have to be simulated on Earth to guarantee suitability for space.

Such simulation has led to the development of hermetically sealed RF adaptors as well as flexible microwave cable assemblies for thermal vacuums (TVACs). A device under test (DUT) in a thermal vacuum chamber must be connected to test equipment outside the chamber. In addition, satellites under test in vast vacuum test chambers must be connected with RF and low-frequency (LF) assemblies for testing. During the preparation of the DUTs in the thermal vacuum chambers, all of the components utilized are at ambient temperature and pressure.

Pressurization And Depressurization
Another important factor in preparation for space applications is the pressurization and depressurization of all components in a vacuum chamber including cable assemblies. This pressurization and depressurization, which takes place rapidly and within well-defined parameters, requires venting holes for non-hermetic components. With such holes in the connectors and defined evacuation paths in the cable and connector interface, the best venting characteristics can be achieved. The result is fast and non-material- stressing adaptation to the applicable pressure. Table 3 outlines the features required by assemblies and adaptors for TVAC.

To satisfy demanding space applications, one range of RF assemblies and hermetically sealed RF adaptors features low outgassing characteristics designed to meet European Space Agency (ESA) requirements. The cable assemblies maintain their mechanical and electrical properties under vacuum and over a wide range of environmental conditions. Although cable assemblies designed for space applications need not be light in weight, the assemblies must be robust in order to operate efficiently and effectively in the rough space and space-test environments.

In addition, cable assemblies for space applications must meet outgassing requirements according to ESA-PSS-01-702. Contamination of the DUTs from solvents of plastic materials during environmental testing must be avoided, as it can lead to corrosion of the content of the thermal vacuum chamber. Thus, all connectors in these TVAC series assemblies are vented to ensure fast pressure adaptation, gentle treatment of the assembly, and a resulting longer lifetime.

Considerable investment is needed in testing and environmental equipment to ensure the performance of high-quality components for space applications. Such equipment includes temperature cycling chambers, thermal shock chambers, climatic chambers, a salt-spray cabin, vibration/shock equipment, RF-leakage equipment, and flex-life test equipment as well as test stations capable of performing hermetic leak testing, tensile load testing, cable-retention force testing, and burn-in processing. The test capabilities must be supported by high-frequency measurement tools including vector network analyzers (VNAs) and spectrum analyzers for all necessary RF verification measurements. When properly equipped, cable-assembly suppliers can meet the increasingly difficult demands of military, space, and test applications.

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