Reduction of Dielectric Loss in Polyethylene

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Abstract
A mechanism contributing to the dielectric loss at 55.2 Gc/sec in highly crystalline polyethylene has been identified. The manufacturing process introduces imperfections in the form of vinyl unsaturation which can orient in the presence of a microwave field. Hinderance to orientation, in similar cases, by neighboring atoms is known to produce losses. The dielectric loss tangent of the polyethylene was found to decrease by a factor of 1.5 from 4.5×10−5 when the vinyl groups were destroyed with a small amount of gamma irradiation. Furthermore, the dielectric loss attributable to the vinyl unsaturation was found to vary linearly with the vinyl concentration as indicated by the associated infrared absorption coefficient of the 11.0-μ vinyl line, and to be consistent with the view that the unsaturation is homogeneously distributed in the polyethylene. A technique for the measurement of the complex dielectric constant of isotropic solids at millimeter wave frequencies is described which utilizes a unique property of the TEono modes in circular waveguide. For these modes the tangential electric field vanishes at all points on the waveguide wall. This gives rise to the important results that wall loss corrections to the complex dielectric constant are relatively minor and that the effects of the air space at the junction between sections of waveguide are also minor provided the length of the space is less than about λ/6.