Changes in signal transmission speed in coaxial cables through regulating the foam structure of the polyethylene dielectric section

Polyolefins have been widely used in the electrical insulation and cable industry in recent years. One of the main usages of these materials is dielectric insulation in coaxial cables. Low attenuation and high signal transmission speed are among the desired features in coaxial cables. The role of polyethylene foam on signal transmission speed in the coaxial cables is the main focus of this study. In the present study, the velocity factor of coaxial samples with different dielectric structures is investigated in both theoretical and experimental approaches. In theoretical formulation, only the void fraction of foam is taken into consideration and other foam properties such as cell density, cell size, and foam structure are neglected. This is the reason for the difference observed between theoretical and experimental results. In theoretical results, a linear increase in the velocity factor is witnessed with the increase of the void fraction while in experimental results there are some exceptions. The foaming degree of the samples was reached 63% causing a 37.7% decrease in theoretical relative permittivity and consequently a 26.8% increase in theoretical velocity factor. On the other hand, up to 36% increase is observed in the experimentally measured velocity factor of foamed dielectric samples compared to the samples with solid polyethylene dielectric.