A SECOND LOOK AT FOAMED DIELECTRIC CABLES FOR MICROWAVE USE

For the past decade, foamed materials saw limited use as the dielectric in RF coaxial cables. Foamed dielectrics are extremely attractive because their inherently low dielectric constant and dissipation factor make possible the design of cables with 40 to 50% lower attenuation than solid dielectric cables of comparable size. The cost of foamed dielectric cables is as much as 50% lower than other flexible cable with similar electrical characteristics. Such cables would seem to fill the need for low-loss, lightweight, flexible coaxial transmission lines for retractable antenna systems. In the early experimentation with such constructions, the attenuation of these cables at microwave frequencies increased drastically in the presence of moisture. This effect was attributed, at various times to absorption of water in the voids of the dielectric, thus increasing the effective dielectric constant. Consequently, the application of foamed cables was confined to semi-rigid constructions for microwave use and to frequencies below 100 Mc in flexible constructions. No further serious attempts were reported to utilize flexible foam dielectric cables at microwave frequencies. (Author)