Mechanical specifications are normally straightforward: size, length, and shape of the cable, connector types, materials, processes to be used, and labeling are the major requirements. The major concerns are that the mechanical requirements do not conflict with the electrical specifications, and that the overall configuration is realistic and feasible. When choosing a cable/connector combination, matching cable and connector sizes offer better results. Good matching will yield low magnitude reflections, resulting in low VSWR and low phase deviations over frequency.

Improperly controlled bending will result in ovalization or other irregular deformations of the cable, potentially increasing the insertion loss and degrading the performance. A rough rule-of-thumb is that a cable can be bent with a bend radius twice the diameter of the cable, although the use of bend dies and tooling may allow tighter bend radii. Note that using a 90-degree bend in a cable with a straight connector in lieu of a right angle connector will give better performance and lower cost. SSI uses computer controlled bending methods and equipment to insure tight bends without sacrificing quality and performance. Bending semi-rigid cable in radii less than one inch is not recommended without special equipment and knowledge.

Environmental requirements for cable assemblies depend to some extent on the application, but are usually fairly stringent. The most important requirements noted are temperature ranges (operating and non-operating), humidity or immersion, shock and vibration, and corrosion. Satisfactory solutions in these environments generally depend on materials and assembly techniques and are essentially mechanical design specifications. Note that SSI’s stainless steel coaxial cables offer a superb answer to most environmentally hazardous situations.

It should be noted that plating (normally silver, sometimes gold or nickel) of cable or connector components should always be considered to prevent loss of signal quality and strength due to oxidation. Copper oxide (CuO) has a magnetic susceptibility of +267 while silver oxide (AgO) has a value of -19; meaning that the “skin depth” for CuO is very thin and thus skin resistivity is very high in comparison to AgO. Remember also that “skin effect” becomes increasingly important at higher frequencies and thus silver plating may be accordingly more advantageous.