From fiber optics to coaxial cables, common failure points in harsh environments call for various augmentations to enhance an interconnect’s operational lifetime. These modifications can range from the addition of armoring for crush/impact resistance to optimizing cable jacketing material for resistance to chemical/oil ingress. Mechanical strain is often a cause for premature failures of interconnects. Understanding the reasoning and methods behind ruggedizing commonly leveraged interconnects such as fiber optic and coaxial cables can be enlightening.
Coaxial Cable: Intrinsic and Extrinsic Sources of Signal Loss
Outside of transmission lines for microwave integrated circuit (MIC) assemblies (e.g., microstrip, coplanar waveguide, etc.), coaxial cables are arguably the go-to interconnect for high frequency transmissions. The desirable transverse electromagnetic mode (TEM) of propagation in coax offers broadband performance from DC to a specified cutoff frequency that is primarily determined from the inner cross-sectional dimensions.
Coaxial cables are a two-conductor transmission line with an inner conductor, dielectric, and shielding. The concentric inner conductor is typically constructed of either stranded or solid copper, where stranded variants typically do not function beyond 1 GHz. The dielectric material that spaces the two conductors can be a range of materials from air, to thermoplastics, to low density foams. The shielding material can be composed of a braided alloy or a foil; often both are used for added protection from electromagnetic and RF interference (EMI/RFI). Inherent attenuation of the signal is caused by the dielectric loss and resistive loss where resistive losses become more apparent at high frequencies due to the tendency of the signal to be pushed to the edges of the conductor (skin effect).
Outside of the intrinsic sources for signal loss, acquired signal loss can occur from inconsistencies in the inner geometry for both the connectors and cable. These inconsistencies are impedance mismatches that can ultimately cause a portion of the signal to reflect back at the source, degrading the signal received at the output of the coax. Inconsistencies can be generated by a large variety of stressors including excessive flexing, crushing, unwanted ingress, and poor handling.
Fiber Optic: Intrinsic and Extrinsic Sources of Signal Loss
Fiber optic assemblies are essential for high-speed, long-range data transfer and have uses in a huge range of military and industrial applications. Light-waves are sent from input to output via a glass waveguide, often fabricated from silica, as well as several layers of cladding, which is a coating with a lower refractive index that confines light within the core. In general, intrinsic attenuation is caused by the absorption, where light is converted into heat energy, as well as the scattering of light in directions outside of the waveguide.
Additional sources of signal loss occur in macro- and micro-bends in the fiber where the bend angle is too severe and causes light to pass through the interface. Similar to the coax, uniformity of the inner geometries is essential to prevent unwanted reflections of the signal. Fiber optic cables can become damaged in a number of ways including exposure to ice/wind, bending/pulling cable through ducts and under floors, and especially improper handling during the manufacturing process or any post-processing such as splicing. These failure modes, in essence, were cause for the slow propagation of cracks on the surface of the fiber core that reflect light back along the fiber.
There are generally five fundamental loading conditions: tension, compression, bending, shear, and torsion (Figure 1). Material properties such as tensile strength, elongation, and modulus of elasticity are essential parameters that determine how a cable responds to these various loads.
This basic mechanical concept can be applied to a dynamic loading scenario that would occur in real environments. A cable can be subject to a combination of all of these loads depending upon the type of stress-event, or “moment of force,” that occurs (Figure 2).
Shear: Impact and Crush Resistance
Transversal (shear) pressure can be exerted on a cable in a number of scenarios including a human stepping on a cable, a vehicle running over a cable, or even a rodent attempting to chew on a cable. A human step has an average ground pressure of around 16 psi; this number jumps up to 30 psi for a passenger car. The standard formula for pressure is as follows:
where P is pressure, F is the force perpendicular to the object, and A is the surface area of the contact. As made apparent by this equation, the pressure exerted onto the cable depends on whether the object it comes into contact with is in motion, as well as the surface area of that object. While this equation is rather basic, it illuminates the impact certain stressors can have on a cable—a rodent gnawing at a cable would produce a far more damaging shear force than a human stepping on a cable due to the smaller surface area of the contact. A more specific formula for shear stress is as follows:
where τ is torsional stress, G is the shear modulus of the material, r is the radius of the beam, L is the length under twist, and Θ is the degree of twist .
An improperly modified coaxial cable subject to shearing forces could cause either a kink in the inner conductor, dielectric, and shielding, or, a complete tear in the inner conductor and significant damage in the dielectric and shielding. Attenuation would instantly jump up and a failure would occur. More often than not, coaxial cable will handle torsional strains that come from daily use (e.g., torqueing connectors improperly when mating, accidental stepping, stored in tight bends, etc.). Outside of rodent damage, fiber optic cables are also generally not exposed to torsional and shear forces that cause irreversible failures. In most cases, coaxial and fiber optic cables will not experience enough torsional strain to cause an acute failure.