Braid construction and attenuation of coaxial cables at microwave frequencies

The attenuation performance of flexible coaxial cables in the microwave region has been measured for a number of different braid designs in order to elucidate the phenomenon of attenuation instability and to achieve a design for a cable of predictable performance. The variables investigated were braid-wire size and tension, braid angle and filling factor, type of weave, and, for the first time, gold plating as an alternative surface finish to the usual silver plating. A simplified theoretical treatment is given in support of the previously held view that attenuation instability results from dissipation of energy in the contact resistances between braid wires, and order-of-magnitude agreement is obtained with measured values. Aging tests for up to 8000h at 85°C showed that, for cables which are not subjected to flexing in service, the best performance is given by a short-lay gold-plated copper-wire braid of adequate tension. Silver plating is preferable in cables where flexing is adequate to clean the contacts between braid wires.