Considering the broad array of available flexible, semi-flexible, hand-formable, and other microwave cable assembly variants, it may seem odd that semi-rigid cable assemblies are still used. However, although their use has declined over the years, semi-rigid cables have inherent benefits that continue to make them very useful in specific high-performance applications, some of which cannot be served as well (or at all) by any other coaxial cable type. As a result, they are widely used for connecting rf and microwave components on a PC board, connecting integrated microwave assemblies, within high-performance laboratory instruments, and especially in systems operating at millimeter-wave frequencies.
From a performance perspective, semi-rigid cable comes closer than any other type of microwave cable to a pure transmission line, and it achieves this property in large measure thanks to its solid tubular outer conductor, which makes it the only cable type that achieves 100% shielding effectiveness. This ensures that both the electric and magnetic field energy remain entirely within the center and outer conductors of the cable, minimizing radiation losses and making the semi-rigid cables highly resistant to external interference.
The solid outer conductor of a semi-rigid cable assembly also provides high amplitude and phase stability, and uniformity between the center conductor, dielectric material, and solid outer conductor. Electrical characteristics such as impedance and attenuation are maintained throughout the length of the cable -- impedance variation, for example, can be as low as 0.5 ohms. Semi-rigid cables also have very low insertion loss when compared to other types of cables and very low VSWR. As insertion loss of all types of cables increases with frequency, this makes semi-rigid cables an excellent choice for use at higher microwave frequencies and mandatory at frequencies above about 30 GHz. They achieve all of these characteristics while also being very thin, which is beneficial in densely-packaged environments.
All types of cables have disadvantages and semi-rigid cables are no exception. First on this list is that while they can be bent by hand or using manual or automated forming tools, they are indeed "semi-rigid" and cannot (or should not be) re-bent after assembly. Unfortunately, designers often do this anyway to allow the cable to conform more precisely to its intended space or during removal of the assemblies to which they are attached. Long runs of semi-rigid cable must also be secured at various points along their length to prevent stress and vibration. The connectors and the areas on the cables near them are also sensitive to stress from bending, which increases with frequency. This makes these otherwise strong assemblies somewhat fragile.
Flexible microwave cables from San-tron have been enhanced over the years to reach as closely as possible to the performance attainable by semi-rigid cables while offering the significant benefits of flexibility. Nevertheless, at very high frequencies and in other situations, semi-rigid cables are still the best choice. San-tron continues to invest in this demanding product line, evidenced by the addition of new equipment for quickly and accurately forming these cables.
To learn more, watch our video detailing cable assemblies vs. standard semi-rigid assemblies as a drop-in solution.