Microwave cable assemblies have an electrical length that varies with temperature. Often it is required that this length vary minimally or linearly with temperature. The electrical length of a cable assembly depends primarily on the dielectric material, and the dielectric of choice for many microwave applications is a form of polytetrafluoroethylene (PTFE). While PTFE generally exhibits excellent electrical and mechanical properties, at around 19°C it has a structural phase change, resulting in dramatic dimensional changes that affect the electrical length of the cable assembly (see Figure 1). This type of abrupt phase length with temperature change can be challenging for systems requiring linear or minimal phase change over temperature.
Figure 1 Typical phase change vs. temperature for cables with an ultra-low density PTFE dielectric.
Figure 2 UTiPHASE cable construction: center conductor (1), dielectric (2), outer conductor (3), outer shield (4) and outer jacket (5).
UTiPHASE CABLES
The UTiPHASE™series of flexible cables developed by Carlisle Interconnect uses a proprietary microporous dielectric technology. The UTiPHASE cable consists of a center conductor (1), the dielectric (2), outer conductor (3), outer shield (4) and outer jacket (5), as shown in Figure 2. The center conductors are either silver-plated copper (SPC) or silver-plated copper-weld steel (SPCW) and meet the requirements of MIL-DTL-17. As noted, the dielectric is microporous to improve phase linearity and stability with temperature change. The outer conductor is SPC, per ASTM B-298, and the outer shields are either SPC, per ASTM B-298, or ARACON®(silver-plated polyparaphenylene terephthalamide), which offers up to a 15 percent reduction in weight. The outer jacket may be extruded fluorinated ethylene propylene (FEP) or ethylene tetrafluoroethylene (ETFE), in accordance with ASTM D-3159. Eight part numbers in the UTiPHASE series offer combinations of these options. The cable sizes were chosen to be consistent with Carlisle’s UTiFLEX™ flexible cable product line, so they use the same connectors (see Table 1).
PERFORMANCE
Figure 3 Phase change of the UTiPHASE vs. ultra-low density PTFE cables.
Figure 4 Phase change of the UTiPHASE cable vs. cables fabricated with three PTFE dielectrics.
Multiple lots of UTiPHASE cable assemblies have been tested, measuring the electrical length from -65°C to 100°C to demonstrate the microporous dielectric eliminates the abrupt phase change caused by the PTFE dielectric. Figure 3 compares the typical phase change for a UTiPHASE cable with a flexible cable fabricated with the ultra-low density PTFE dielectric. Figure 4 extends the comparison to add two additional cables fabricated with PTFE dielectrics: low density and solid PTFE. The UTiPHASE cable assembly has the most stable electrical length with temperature variation.
The UTiPHASE series of cables can be used wherever flexible cables are needed with minimal or linear phase change versus temperature. Using a proprietary microporous dielectric flattens the phase change versus temperature response curve.
Carlisle Interconnect Technologies
St. Augustine, Fla.
www.carlisleit.com