A viable alternative to braided shielded cable

Any engineer with hands-on experience is familiar with cables which are shielded with a woven, all-metal braid, seen in Figure 1. Braided shielding has been used for decades. It is widely available, has modest cost, and it works—at least up to a point.

Figure 1 This schematic of a cable using conventional braided shielding illustrates the RF barrier it provides; the illustration shows multiple basic-wire conductors, but the same concept is used with coaxial cables and twisted-pair cabling. Source: Stecker Express

The braid serves two complementary EMI functions in most cases. It attenuates emissions of electromagnetic energy from the cable, and, at the same time, it prevents interfering energy from reaching the cable. It’s used for cables carrying DC all the way to gigahertz signals. Shielding is used with coaxial cable, twisted pairs, multiconductor cable assemblies, and more.

Engineers have a love/hate relationship with cables that use a tightly woven conductive braid for shielding. On one side, it’s an essential passive component for maintaining signal integrity, and how can you put a price on that, especially when there seems to be no viable alternative. On the other side, it adds bulk, weight, cost, and routing challenges to the end-product.

Shielded cable also has termination challenges. Every hands-on engineer can tell “horror stories” of trying to cleanly terminate the shield to the connector, especially if it’s being done by hand without the right tools or jigs. Those tiny cut shards of braid that “fall” when trimming it can easily short the cable, and nearby circuit board, and cause general troubleshooting havoc.

The shielded-cable dilemma increases in today’s world of complex cabling in small spaces, with design emphasis on cable size, weight, and performance. Cables with conventional braided shields—almost always made of copper, but sometimes other metals such as aluminum are used—are relatively heavy and bulky.

Furthermore, the relative proportion of their overall weight and bulk increases compared to core cable size as the core that is being shielded shrinks down. That creates a dilemma for designers as gigahertz cables go down to millimeter-levels diameter, and the weight and bulk of the shielding soon become a significant design issue.

There’s even another insidious concern: performance. As frequencies reach in the multi-gigahertz range, the shielding performance of the braid decreases. The tiny voids in the woven braid become “open windows” to the short-wavelength RF energy. These shielded cables also restrict movement, can be difficult to handle and install, and their “shape memory” makes routing and rerouting more difficult.

Tighter weaves improve the situation somewhat, but at a cost in materials, flex, weight, and other factors.

Some cables use a solid foil as a first-layer shield, which is then covered with a braided shield. This improves RF shielding to nearly 100%, but further increases cost and weight and adds to termination complexity.

What can be done?

New shielding technologies and approaches are needed as cable applications shift. A vendor-authored article I saw highlighted the ARACON shielding system from Micro-Coax, a division of Amphenol. It’s based on a very different approach to creating the RF cage around the signal-carrying conductors.

Instead of woven metal braid, it uses lightweight, ultrathin, and extremely strong DuPont Kevlar fibers to create a textile-like material that then is coated with either nickel or silver (for solderability). It forms a composite shielding braid that is up to 80% lighter than copper, while still providing strong EMI/RFI protection, lightning protection, and mechanical robustness.

The conducting coating adheres to the fabric-like fibers to form a “wrapper” against electromagnetic energy. The result is a thin braid-equivalent that slips onto the cable core like a thin, open-ended, super-flexible sock. It can be used with coaxial cable, twisted pair, and even wire bundles, shown in Figure 2.

Figure 2 The ARACON shielding system uses a thin but super-tough fabric sock that slips over the conductor or wire bundle to be shielded. Source: Micro-Coax

To get a sense of the bulk differences, the density of aramid fibers is only 1.44 grams per cubic centimeter (g/cc), compared to copper at 8.9 g/cc. Even with the addition of the metal coatings, the density of fibers ranges from 3 g/cc to 5 g/cc. At the same time, the tensile strength of the aramid core, at 350 kilopounds per square inch (Ksi) is 3 to 10 times higher than that of traditional or high-strength copper cores (35 Ksi to 95 Ksi).

Its other attributes are flexibility and longevity. It has been tested to withstand over 50,000 flex cycles over a 180-degree arc; installs easily, as it cuts like fabric with standard tools; and it offers corrosion resistance (important in many real-world settings). Termination techniques are similar to those for wire-braided shields.

It meets or exceeds the many stringent mil/aero and space-related electrical, mechanical, and outgassing performance standards for shielding (it’s a long list), but with far less bulk and weight. Micro-Coax says it can be braided and fabricated by wire/cable vendors using standard metal-wire equipment and it supports crimping or banding, as well as soldering (if it’s silver plated).

The high surface-to-volume ratio of ARACON fibers contributes to effective RF EMI shielding, exceeding the performance of conventional shielding, as seen in Figure 3.

Figure 3 Despite its reduced bulk and weight, the Aracon approach offers approximately 20 dB greater attenuation compared to conventional copper braid (here, industry standard AA59569). Source: Micro-Coax

There’s a lot that is attractive about this shielding approach, which is not just a lab development but has been in use for over 10 years. It has been proven “in the field” in aircraft, space vehicles, and more.

While I couldn’t get comparative pricing—and direct one-to-one comparisons are difficult—I suspect it’s more costly than conventional metal-braided shielding. But I can see where the extra cost is justified in applications where lower weight and bulk, along with performance and flexibility, are more critical priorities than cost.

It feels weird to say “we’ve been a team all these years, but things have changed” to braided shielding. Still, sometimes we must acknowledge reality and move on in a somewhat different direction to meet new design demands and constraints. Perhaps that’s what will happen with the availability of this alternative to braided shielding.

Have you ever found the realities of standard braided shielding to be inadequate for your project, or that it imposed too many burdens? Did you look for alternative cable-shielding solutions? Was your search successful?

Bill Schweber is a degreed senior EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features. Prior to becoming an author and editor, he spent his entire hands-on career on the analog side by working on power supplies, sensors, signal conditioning, and wired and wireless communication links. His work experience includes many years at Analog Devices in applications and marketing.

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