Cables for microwave applications come in many lengths and sizes, and with a variety of connector options. Suppliers offer everything from spools of unterminated cables to specific lengths of assemblies with connectors, and even phase-matched sets of cable assemblies. What follows is a brief review of microwave cable assemblies for general-purpose system and measurement applications.

In their excellent tutorial text, Practical RF Circuit Design for Modern Wireless Systems (ISBN: 1-58053-521-6, Artech House, www.artechhouse.com), authors Les Besser and Rowan Gilmore use the analogy of rice traveling through a blowgun to represent the behavior of electromagnetic (EM) waves propagating through a transmission line, such as a cable. In a relatively narrow tube, the rice can only travel in one form. But if the diameter of the tube is increased, the rice can travel in more random patterns, slowing the speed of propagation. The effect is much the same for microwave cables, where the diameter of the cable will determine the frequency of propagation. Larger-diameter cables may support a greater flow of energy (higher power), but will be limited in frequency compared to smaller-diameter cables. For that reason, raw-cable and cable-assembly suppliers offer products with a variety of different diameters, based on the upper-frequency requirements of their customers' applications.

For example, LMR-240 cable from Times Microwave Systems is a low-loss flexible microwave cable designed for communications applications. The cable consists of a copper inner conductor with aluminum tape outer conductor, tinned copper outer braid, and ultraviolet (UV) resistant polyethylene jacket. It has a diameter of 0.24 in. and has a cutoff frequency of 31 GHz. The cable, which is available with a wide range of coaxial connectors, is rated for peak power of 5.6 kW. Compare this to the larger-diameter 0.4-in. LMR-400 cable, which has a cutoff frequency of 16.2 GHz but is rated for power levels to 16 kW. Since these are flexible cables, they are also characterized in terms of minimum bend radius, which is 0.75 in. for the LMR-240 and 1.0 in. for the LMR-400.

Cable assemblies are also rated in terms of attenuation (usually per foot or per 100 feet of cable), average power-handling capability, velocity of propagation, time delay, voltage withstanding capability, and shielding effectiveness (SE). The attenuation plot for most cables resembles a straight line with positive slope in the direction of increasing frequency, since loss tends to increase as a linear function of frequency. Conversely, a plot of average power-handling capability will resemble a straight line with negative slope in the direction of increasing frequency, since the average power-handling capability diminishes with increasing frequency. Using the LMR-240 cable as an example, the attenuation per 100 feet is 1.7 dB at 50 MHz, rising to 7.6 dB at 900 MHz, and 20.4 dB at 5.8 GHz. The average power-handling capability is 1.15 kW at 50 MHz, dropping to 0.26 kW at 900 MHz, and 0.1 kW at 5.8 GHz. The company's larger-diameter LMR-400 cables exhibit attenuation per 100 feet of 0.9 dB at 50 MHz, rising to 3.9 dB at 900 MHz, and 10.8 dB at 5.8 GHz. The average power-handling capability is 2.57 kW at 50 MHz, dropping to 0.58 kW at 900 MHz, and 0.21 kW at 5.8 GHz.

While the LMR series cables are nominally aimed at communications systems applications, the company's SilverLine™ test cables have been developed for measurement use. Test applications, of course, bring demanding requirements for cable assemblies, since accuracy and repeatability must be maintained over time and environment conditions, such as temperature. Cable assemblies developed for testing, in particular flexible cable assemblies, are usually characterized in terms of how performance holds up after repeated mating and unmating cycles and with cable flexure. Test cables typically include two or three shielding layers in order to achieve high SE performance.

Numerous suppliers offer flexible cables, semirigid cables, and a third category of cables known as conformable or hand-formable cables, which approach the electrical performance of semirigid cables while maintaining some of the formable convenience of flexible cables (see table). For example, aluminum cables, such as the Easy Bend II cables from Haverhill Cable and Manufacturing Corp., can be hand formed with minimal spring back. The Easy Bend II cables are qualified to MIL-C-17 requirements for military and space applications.

Insulated Wire offers a number of different series of cables, from the low-frequency 480 series (with maximum frequency of 11 GHz) to the high-frequency 125 series cables (with maximum frequency to 60 GHz). GrooveTube cables from MegaPhase are flexible test cables fabricated with copper outer conductors. Available for applications from DC to 50 GHz, the cables can be supplied with metal braid over metal armor for additional ruggedness.

Some firms specialize in raw cables or cable assemblies but not connectors. Others provide connectors but not cables. For example, Semflex offers both bulk cables and finished cable assemblies, including HPT series high-performance test cable assemblies to 40 GHz and HPI Series high-performance interconnect cable assemblies to 40 GHz. Harbour Industries, a supplier to many cable-assembly manufacturers, produces a wide range of cable types, including flexible and semirigid cables, but not connectors. Amphenol RF, although strongly associated with its extensive lines of coaxial connectors, also offers cable assemblies. Firms such as Florida RS Technology provide essentially custom cable-assembly solutions. The firm, a contract manufacturer specializing in cable assemblies and wire harnesses, boasts a long list of commercial and military customers including BAE Systems, Northrop Grumman, IBM, and Intel. Some companies, such as Anritsu Co., Applied Engineering Products, Maury Microwave, Sabritec, San-tron, Southwest Microwave, SRI Connector, SV Microwave, and Trompeter, specialize in connectors. And some firms, such as Federal Custom Cable, offer not only raw cables, but also connectors and cable assemblies, in addition to a lineup of coaxial components such as adapters, attenuators, and bias tees.

The largest number of suppliers in the table provides cable assemblies based on either standard commercial coaxial connectors or proprietary connector designs. EZ Form Cable Corp., for example, offers extensive lines of miniature and military-grade copper- and aluminum-jacketed semirigid coaxial cables as well as delay lines. The firm also supplies EZ Flex™ flexible cables and EZ Formable™ hand-formable cables in 0.086-, 0.141-, and 0.25-in. diameters for applications through 30 GHz. Astrolab's Minibend^® and Microbend™ flexible coaxial cable assemblies are promoted as cost-effective alternatives to 0.086-in. and 0.047-in.-diameter semirigid cable assemblies, respectively. These flexible cables eliminate the need for predefined custom lengths and bend configurations. The new, smaller-diameter cables feature a frequency range to 65 GHz with triple shielding for high SE. The firm offers assemblies with a wide range of connectors, including SMPM, SMP, 1.85-mm, and 2.9-mm connectors. All Microbend assemblies are also available in a ruggedized version (Microbend R) for use in severe stress environments.