Cable suppliers continue to find new solutions based on ingenious combinations of materials, such as the Strip Flex Taped (STF™) series of cables and cable assemblies from Times Microwave Systems. Although they are flexible, they can also handle high power levels with good phase stability at cutoff frequency as high as 63 GHz, depending upon diameter (see figure). They are suitable for ground-based and airborne applications.

The STF™ series of cables and cable assemblies operate as high as 63 GHz with high power-handling capability, low loss, and good phase stability. (Photo courtesy of Times Microwave Systems.)

Connectors intended to fit on the end of high-frequency cables should be dimensionally similar to those cables, so as to minimize impedance discontinuities between the connectors and cables as much as possible. The physical size of coaxial connectors generally shrinks with increasing frequency, with larger connectors such as BNC and Type-N connectors cutting off at lower frequencies than smaller connectors such as SMA types. Every connector type will exhibit a moding frequency, which is its upper-frequency limit, with Type-N connectors initially developed for use through 12 GHz.

Coaxial connectors were initially designed with screw-on type interfaces and for particular applications, such as the Type-N connectors developed for US Navy shipboard applications. But for some applications requiring faster interconnections between pairs of similar connectors—such as in test-and-measurement applications—screw-on connectors were supplemented by connector types with push-on interfaces, such as SMA connectors.

Over time, the moding frequencies of many connector types has been extended, with some Type-N connectors now operating as high as 18 GHz and some SMA connectors working past 30 GHz. Quite simply, smaller connector dimensions mean higher moding frequencies. Pioneering work by such innovators as Bill Oldfield at Anritsu/Wiltron and Julius Botka at Agilent/Hewlett-Packard Co. have pushed coaxial connectors well into the millimeter-wave frequency range.

Of course, this short piece can barely scratch the surface of important details on specifying coaxial cables and connectors. The requirements of a particular application—including temperature, vibration, power-handling capability, intermodulation distortion (IMD), frequency range, and phase stability—will help establish the performance minimums for the cables and connectors in that application, and those requirements will vary widely for test and system applications. Many of the leading cable and connector suppliers offer excellent white papers and application notes that can help simplify the selection process.