Dielectric properties of polythene for submarine telephone cables

Increases in the signal-carrying capacity of submarine telephone cables by extensions of the frequency bandwidth have led to progressively tighter specifications for the relative permittivity and losses of the polythene dielectric. A study of the factors determining the dielectric losses of low-density, i.e. high-pressure-process, polythene has resulted in a low-loss material, of precise relative permittivity, for which comprehensive data are presented, and has brought about improved techniques of low-loss measurement. An attempt is made to distinguish the losses of pure polythene from those due to additives and process residues, and an ultimate loss/frequency profile is estimated. Lower losses can be achieved with polythenes of higher density, but this is at the expense of higher relative permittivity, and hence of higher overall cable attenuation; a continuous gradation in these properties is possible.