By Donald Durett
A unique design allows the cable to be bent very close to the connector, resulting in a low-profile, flexible coaxial cable assembly.
A flexible coaxial cable assembly has evolved since its introduction in 1996. The connector design of the assembly eliminates the need for solder, crimp ferrules and end sleeves. This allows the cable to be bent very close to the connector, resulting in a low-cost, low-profile, flexible coaxial cable assembly that eliminates the need for custom semi-rigid cable and costly right-angle connectors (see Figure 1).
These flexible cable assemblies replace small, custom, semi-rigid cable assemblies, eliminating the need for predefined custom lengths and bend configurations. The cable exhibits comparable insertion loss and power handling as 0.047, 0.086 and 0.141 semi-rigid cable.
 Figure 1. The bend-to-the-end advantage of minibend cable assemblies. |
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For applications that require lower insertion loss than the original flexible cable assembly, other flexible cable assemblies have a microporous dielectric and compare favorably with 0.141 semi-rigid cable assemblies.
Flexible cable assemblies are also available that are an alternative to 0.047 semi-rigid cable assemblies. They feature a smaller diameter cable (0.060") than the original flexible cable assembly. They are currently available with SMP and 2.9 mm connectors.
All the cables are triple-shielded for high isolation. The outer conductor is a flatware, silver-plated copper braid that renders 99 percent coverage. A 100 percent helical wrap of aluminum polyimide offers additional support and shielding. The stainless-steel outer braid provides overall mechanical strength for increased durability. The extruded FEP jacket provides abrasion resistance and electrical insulation.
 Figure 2. The minibend V millimeter wave version of the minibend cable assembly. |
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Connectors are normally terminated to flexible cable using crimp, clamp or solder methods. These methods add overall length to the cable/connector junction and, in designs where space is at a premium, may require the system designer to specify more costly right-angle connectors. The connectors of these flexible cable assemblies use a clamp method to directly terminate the cable outer conductor to the connector body. No solder is used at the point of termination so the cable can be bent very close to the connector without fear of degradation. The cable assembly can be readily formed into any shape without special tools or predetermined bends. All that the system designer needs to specify is the overall length.
Original Development
The original flexible cable assembly was designed to meet the requirements of a customer that needed a cable assembly with similar electrical performance as 0.086 semi-rigid cable with SMA plug connectors. The customer needed cable assemblies that could be densely packaged into a system without breaking off at the cable/connector junction or causing flakes of metal to accumulate inside the system. The solder joints of the semi-rigid cable assemblies the customer was using would crack under thermal and mechanical stress when the cable was bent close to the joint.
The customer had tried "conformable-style" cable assemblies, which are essentially flexible cable with a copper outer braid that is tin-dipped to mimic the solid outer conductor of semi-rigid cable. These assemblies lose up to 30 percent of mechanical integrity if they are resoldered and can only be formed a few times until the outer braid starts to break apart. The flexible cable assembly solved the problem because the clamp design does not use solder either at the cable/connector junction or on the braids.
 A minibend cable assembly with an SMP float-mount connector. |
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Customers began to ask to have the assembly technique applied to other connector styles, such as SMA jack, SSMA plug and jack and SMP connectors. The assembly technique is applicable to any connector that can be attached to a 0.086, 0.141 or 0.047 semi-rigid cable.
Specific Application Development
The original flexible cable assembly has evolved since its introduction. A flexible cable assembly with a 2.9 mm connector has been designed for applications that require operating frequencies up to 40 GHz. A standard 2.9 mm connector has a low-temperature dielectric that limits its use between -45° and 85°C. The connector in the flexible cable assembly is designed with a high-temperature dielectric that allows it to be used from -55° to 125°C.
Another flexible cable assembly has been designed for applications that require moisture-resistance. Special gasketing is used internally at the cable/connector junction and in the interface to assure a complete environmental seal when the connector is mated. This assembly can be used in outdoor applications and has passed the temperature/humidity profile of MIL-STD-202, Method 106.
In a ruggedized version of the original flexible cable assembly, a high-temperature epoxy is used to bond the cable jacket to the connector body, which allows the cable assembly to survive repeated radial torque after the connector is mated. The design evolved from a customer that was using the original flexible cable assembly but had such dense packaging in the system that the cables were breaking off at the cable/connector junction when they were moved and twisted after the connectors were mated.
A fully captivated version of the original flexible cable assembly has been designed for use in space applications. The center contact and dielectric of the connector are epoxy-captivated to the connector body at the interface and the cable jacket is bonded with an epoxy to the connector body at the cable/connector junction. This eliminates any movement of the contact or dielectric in the connector while subjected to thermal and mechanical stress typically encountered in Earth-orbiting spacecraft.
Customers making connections between modules in fiber optic switching systems require cable assemblies with high bandwidth (40 Gbps), which equates to a 62 GHz operating frequency. A millimeter wave version of the original flexible cable assembly uses a special 1.85 mm connector that is plug-compatible with the standard 2.4 mm connector (see Figure 2). The center conductor of the assembly is also the connector contact which makes the assembly inner conductor continuous from end-to-end with no junctions and there are hardly any variations of the dielectric constant along the cable. Both of these features help eliminate reflections in the cable assembly.
Conclusion
These flexible cable assemblies replace small, custom, semi-rigid cable assemblies. The connector of the flexible cable assembly uses a clamp method to directly terminate the cable outer conductor to the connector body. No solder is used at the point of termination so the cable can be bent very close to the connector without fear of degradation. The assembly technique is applicable to any connector that can be attached to a 0.086, 0.141 or 0.047 semi-rigid cable.
DONALD DURETT is Engineering Manager, AstroLab Inc., 4 Powder Horn Dr., Warren, NJ 07059-5105; (732) 560-3800; Fax: (732) 560-9570; Web site: www.astrolab.com.
