Limit Losses In Coaxial Cable Assemblies

Sept. 21, 2010
Loss is inevitable in coaxial cable assemblies. But with improved materials, refined construction techniques, and high-quality connectors, manufacturers of flexible and semi-rigid cable assemblies are continuing to meet more stringent requirements ...

Loss is inevitable in coaxial cable assemblies. But with improved materials, refined construction techniques, and high-quality connectors, manufacturers of flexible and semi-rigid cable assemblies are continuing to meet more stringent requirements for a variety of applications. They are improving performance while maintaining flexibility and reliabilitywithout inflating prices.

"Many users seem to believe that there is some magic in low-loss coaxial cables but, in reality, low loss cables are achieved by paying only slightly more attention to the basic physics of a coaxial transmission line," asserts Richard Pouliot, President of RF Depot. Similarly, there are some who think that a semi-rigid construction is the key to low loss. According to Pouliot, however, a custom flexible cable using low-density polytetrafluoroethylene (PTFE) dielectric or tape PTFE with air space can achieve lower loss than a semi-rigid cable having the same diameter and length. But developing a custom solution is not trivial. The tooling and set-up cost along with the availability of materials limits this apparent flexibility.

Offering both standard and custom solutions, RF Depot has developed triple-shielded (190 series) and doubleshielded (290 series) cable assemblies with loss of 0.49 dB/ft. at 28 GHz and 0.2 dB/ft. at 18 GHz, respectively. The firm's premium, flexible double-shielded cable assembly with a 2.9-mm male connector (1501-290290-) exhibits 0.77 dB/ft. attenuation at 40 GHz. In contrast, a tripleshielded version has 0.89 dB/ft. loss at 40 GHz.

While some believe in the custom approach, others are expanding their standard catalog lines to meet the needs of myriad applications. For example, Crystek Corp. recently expanded its low-loss RF coaxial-cable assembly line to 40 GHz with the addition of the CCK40 series. These cable assemblies are optimized for attenuation of 0.90 dB/ft. at 40 GHz. Connector-to-cable attachment has also been optimized for good voltage-standing- wave-ratio (VSWR) performance of 1.35:1 to 40 GHz. At 20 GHz, the measured VSWR is 1.10:1 (Fig. 1).

Other salient features of the RF cable assembly include rugged stainless-steel solder-clamp construction with heavyduty strain reliefs and greater than -90 dB RF shielding from -5 to +85C. (An extended range of -55 to +125C is available through special order). The assembly, which features a minimum bend radius of 1.0 in., comes with 2.92-mm (K) connectors. The cable assembly's frequency capability was recently extended to 50 GHz and plans are underway for a 60-GHz version, which is slated for introduction in 2011.

Replying to the needs of applications that demand more ruggedness at lower cost, Response Microwave has added more protective layers to its flexible cable assemblies, which include a choice of male and/or female SMA, push-on SMA, Type N, TNC, BNC, MCX, or SMC connectors in straight and rightangle configurations. Connectors can be attached by clamp or crimp method. Designed for use in production test and other cost-sensitive applications, the new TESTCABLZ family's frequency range has been extended to 20 GHz while improving attenuation, VSWR, and RF leakage performance. Typically, they offer 0.30 dB/ft. attenuation with VSWR of 1.20:1 and -110 dB RF shielding.

Likewise, Astrolab has pushed the frequency range for its newest low-loss flexible cable assembly to 110 GHz. The firm also is using microporous dielectric and 1-mm connectors in compliance with IEEE P287 specifications. With a characteristic impedance of 50 Ohms, the 32061E employs ruggedization and triple shielding similar to the firm's minibend cable products for high isolation. At 65 GHz, it offers VSWR of 1.55:1 with insertion loss of 1.42 dB/ft.

By expanding its Re-Flex cable line, IW Microwave is improving both insertion loss and ruggedness to go after semi-rigid cables (Fig. 2). Touted as an alternative to semi-rigid cables, the RF141 and RF085 feature multi-ply laminated PTFE dielectric, a silver-plated copper foil shield, and a tightly woven, tin-plated copper braid.

In addition, the supplier is improving the ruggedness of its low-loss flexible cable assembly, called Tuf-Flex. Internally ruggedized, the crush resistance for this low loss cable is rated at 180 lbs./linear inch. To replace twisted pairs in high-speed data communications market, IW Microwave also is bundling RF coaxial cables in a common jacket. In fact, the developer is planning to combine two high-frequency, highperformance coaxial cables in a single composite jacket.

Aside from cost effectively exploring new options for ruggedizing lowloss flexible cable assemblies, Carlisle Interconnect Technologies is looking to make such cables lighter in the future. For example, Carlisle is looking into weight-saving options to replace the stainless-steel arm ring that is currently utilized in the low-loss flexible assembly, AccuPhase. According to Marc Temple, Carlisle's Director of Sales for RF & Specialty Connectors, the manufacturer is planning to generate the lightweight armring option by the end of this year.

For applications that require minimum attenuation with low VSWR and repeatable performance, MegaPhase is offering its low-loss Phase3 series C coaxial cable assemblies through 67 GHz. Connector options include SMA, Type N, 3.5- mm, 2.92-mm, and 1.85-mm types. Simultaneously, the company is pursuing different dielectric materials to improve the insertion-loss characteristic of the low-loss, rugged GroveTube series 1 & 2 cable assemblies. According to Chief Engineer Bob Fisher, "We are trying to produce high-velocity-of-propagation (VP) cables without increasing the outside diameter of the cable." Using lower dielectric-constant material, the firm is looking to increase the center conductor for lowering losses without increasing the outer diameter of the cable.

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The series 1 & 2 cables offer 80 percent VP, which translates to a dielectric constant of 1.56. "We are planning to go to 90 percent VP, which will give us a dielectric constant of 1.35," explains Fisher. With this new material, the maker is expecting to improve insertion loss by 10 percent while maintaining the maximum operating frequency of 34 GHz. Additionally, the connectors are designed and optimized for the cables to ensure minimal loss. To minimize cost, the company is trying to improve its manufacturing process to increase yield.

Although MegaPhase is currently using tape wrap material, it is trying to move toward a proprietary Teflon-based dielectric. If all goes well, the improved cable assembly with new dielectric material will be ready before the end of the fourth quarter.

A program to develop cable-assembly designs using a variety of new cables is also in place at Pasternack Enterprises. It is currently combining air-filled foam dielectric with a novel design that incorporates an air core between the dielectric and the center conductor. In doing so, Pasternack is looking to improve both the phase stability and insertion loss of upcoming cable assemblies. While phase stable assemblies operate to 50 GHz, insertionloss improvements will be offered in 26.5- and 40-GHz assemblies.

By utilizing software models, Rosenberger North America continues to economically improve the loss, flexibility, and ruggedness of its cable assemblies. Utilizing an overmolded strain relief for ruggedness and durability, the RFlex family operates to 18 GHz with -100 dB minimum RF shielding. In addition, Rosenberger's SMA+ line has been extended to 36 GHz with minimum return loss when mated with SMA, 3.5-mm, or 2.92(K)-mm connectors.

Likewise, Mini-Circuits continues to streamline its low-loss flexible cable assemblies. The maker's newest QBL3-SMQ+ offers a double shield for shielding effectiveness beyond -100 dB with superior insertion- loss stability, VSWR, and phase versus flexing. The 3-ft. QBL3-SMQ-NM+ exhibits typical insertion loss of 0.72 dB/ ft. from 12 to 18 GHz.

Others in the race to deliver lowloss coaxial cable assemblies include Advanced Technical Material, Huber+Suhner, RF Industries, Semflex, Times Microwave, and W.L. Gore.

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