Cables and connectors work best when noticed least. Ideally, they should remain electrically invisible with minimal impact on system performance. Understanding a few simple performance parameters for each type of interconnection can help when searching for cables and connectors.
Coaxial cables come in many forms, including flexible, semi-rigid, and handformable varieties. Many suppliers, such as Teledyne Storm Products, provide both semirigid and flexible cables, while some add ruggedized variants such as corrugated coaxial cables.
Coaxial cables consist of two conductors separated by dielectric material. The outer conductor often includes a shield and is also protected by a wrap. Most coaxial cables are formed with center conductors of copper, tinned or silver-plated copper (for improved solderability), copper-clad steel, and copper-clad aluminum. Flexible cables use stranded center conductors, with typically seven strands, while rigid cables are formed with solid center conductors. The outer conductor is wrapped around a coaxial cable's dielectric core, which is commonly formed of a dielectric material such as fluorinated ethylene propylene (FEP) or polytetrafluoroethylene (PTFE). A jacket material, such as FEP or polyvinyl chloride (PVC), may be added for environmental protection.
Different cables can be compared in terms of their highest operating frequency, outside diameter, insertion loss (usually presented in terms of a value per 100 feet of cable), velocity of propagation (the speed of RF signals through a cable relative to their speed in a vacuum), power-handling capability, VSWR (with connectors), shielding effectiveness (SE), and minimum bend radius (for flexible cables).
For example, the 1151 series of cable assemblies from Insulated Wire operate to 18 GHz with a variety of different connectors, including Type N and SMA connectors. They have an outside diameter of 0.11 in. and achieve velocity of propagation of 71 percent. They exhibit insertion loss of 30 dB/100 ft. at 2 GHz and 90 dB/100 ft. at 16 GHz. The VSWR for an 18-GHz assembly with two straight connectors is 1.35:1. Capable of SE performance to 100 dB, the flexible cable assemblies boast a 0.5-in. bend radius.
Many cable providers, such as Belden, offer products with a wide range of shielding options to fit the SE requirements of different applications. Belden's coaxial products include cables with shields formed of metallic foil or shielding tape, outer braid, and extra foil layers.
Micro-Coax characterizes its coaxial cables by means of extensive test data, including information on the capacitance per foot of its different UtiFLEX flexible cables. Its model UFA147A cable, which is usable to 40 GHz, features capacitance of 26.2 pF/ft.
W.L. Gore has developed cables for use with a vector network analyzer (VNA) and cables capable of operating through 110 GHz. The firm's PHASEFLEX microwave test cable assemblies are designed for minimal changes in signal phase from DC to 67 GHz. The flexible cables allow for a minimum bend radius of 1 in. without affecting performance. To test reliability, test cables were bent 90 deg., and then reverse bent 180 deg. The stranded center conductor cables in these assemblies performed beyond 100,000 cycles (or more than 200,000 total cable bends) while still meeting electrical specifications. The PHASEFLEX test cable assemblies are available with a wide range of connector options (which will determine the ultimate highfrequency limit) including 3.5-mm (to 26.5 GHz), 2.4-mm (to 50 GHz), and 1.85-mm connectors (to 67 GHz).
Gore also offers PHASEFLEX cable assemblies with 1-mm coaxial connectors for use to 110 GHz. The typical insertion loss is 1.34 dB for a 10-cm section and 2.67 dB for a 20-cm section, both with VSWR of 1.50:1.
Electronic Assembly Manufacturing, Inc. (www.eamcableassemblies.com) recently announced the availability of low-loss flexible cable assemblies for applications to 18 GHz. The cables have 0.335-in. diameter and exhibit insertion loss of just 0.048 dB/ft. at 1 GHz and 0.22 dB/ft. at 18 GHz. The cable assemblies can handle 1800 W CW input power at 1 GHz and 600 W CW input power at 10 GHz. They offer SE of better than 95 dB. EAM's low-loss cable assemblies feature a minimum bend radius of 1.7 in. and are available in custom lengths with a wide range of connector choices, including SMA, Type N, and TNC connectors.
SSI Cable Corp. creates its Stainless Steel Advantage coaxial cable assemblies using stainlesssteel- jacketed coaxial cable with welded cable/connector interfaces for applications requiring the highest reliability albeit with limited flexibility. The cable assemblies employ a stainless-steel jacket over a thin copper outer conductor, providing excellent SE performance with high corrosion resistance. A copper outer conductor is used for optimum conductivity. The cables are available with outside diameters of 0.093, 0.145, and 0.240 in. with solid, medium-loss, and low-loss PTFE dielectrics.
EZ Form Cable Corp. produces the EZFlex 141 handformable cable as a compromise between the high performance of semi-rigid cables and the versatility of flexible cables. It is a 0.141-in.-diameter coaxial cable with silver-plated, copper-clad steel center conductor, solid PTFE dielectric, and tin-plated copper braid. It uses a silverplated copper-clad steel wire inner conductor and 0.117-in. nominal diameter solid PTFE dielectric and weighs 2 lbs. per 100 ft. The cable has a bend radius of 0.25 in. minimum for a 360-deg. excursion. The velocity of propagation is nominally 69.5 percent while insertion loss is typically 12 dB/100 ft. at 1 GHz and 29 dB/100 ft. at 5 GHz.
A number of firms offer coaxial cable assemblies for use with vector network analyzers (VNAs) and other microwave test equipment, including Mini-Circuits, MegaPhase, Pasternack Enterprises, Micro-Coax, Teledyne Storm Products, Agilent Technologies, and Anritsu. For example, Huber + Suhner, a leading provider of both RF/microwave and optical cables, has developed the SUCOFLEX 307 and RG393 cables for RF power measurements. Designed to handle high power levels with low insertion loss and high stability, the firm supports the cables with power terminations for use at power levels to 100 W and frequencies to 6 GHz and attenuations for use at power levels to 300 W and frequencies to 6 GHz. Florida RF Labs offers its Lab-Flex line of high-reliability flexible coaxial cable assemblies for measurement applications. Available with diameters from 0.135 to 0.335 in., the 0.312-in.-diameter cables can be used to 31 GHz and can handle as much as 740 W at 1 GHz. They have a minimum bend radius of 1.50 in. and feature 39.5 dB insertion loss per 100 ft. of cable at 18 GHz. Mini-Circuits, one of the newer players in the test cable assembly market, offers standard cable lengths from stock. Its CBL-series test cables, which are available with a wide range of connectors, are usable from DC to 18 GHz and are rated for 20,000 flexures without failure.
Although connectors would appear to be fairly mundane, enhancements occur on a regular basis to improve reliability or electrical performance. For example, last year Tyco Electronics announced a snap-lock SMA connector designed to mate with standard SMA connectors for simple, reliable links. Suitable for use through 6 GHz, the connector's inner fingers slide over the threads of a conventional SMA connector and lock into place by engaging the other connector's outer collar. The connector pair can be disengaged by pulling back on the snap-lock connector's outer collar, making them ideal for measurement applications. The snap-lock SMA connectors are available in a variety of configurations, including in straight and right-angle forms.
Radiall grew its QMA line of connectors with the addition of high-density waterproof QMA (WQMA) connectors with snapon Quick-Lock technology. They are rated at 200 W power at 1 GHz and temperatures from -40 to +105C as well as for at least 100 mating cycles The RoHS-compliant connectors feature enhanced corrosion resistance to weather hostile environments.
RF Connectors, a division of RF Industries, supplies a wide variety of coaxial connectors, including 1- , 2.9-, and 3.5-mm connectors, 7/16 connectors, BNC, Type N, SMA, and TNC connectors. Another division of RF Industries, Aviel Electronics (www.avielelectronics.com), has improved the SMB connector by adding a push-lock feature to the standard MILSTD- 348 snap-on coupling mechanism increasing the disengagement force to at least 5 lbs. A locking shell pushes over the mated interfaces, locking the interface while allowing full rotation of the mated pair. Pulling back the shell allows the interfaces to be disengaged. The mating pairs include a right angle plug with crimp attachment for RG-174U cable. These enhanced SMB connectors are usable to 10 GHz.