Foamed dielectric material

Foam-dielectric cables are ideal for use as feeders with antennas that do not require pressurization. The center conductor is completely surrounded by foam-dielectric material, resulting in a high dielectric breakdown level. The dielectric materials are polyethylene-based formulations that contain antioxidants to reduce dielectric deterioration at high temperatures. 

The other approach to the reduction of the power loss to the dielectric material is by reducing the amount used. This is done by replacing part of the dielectric by air, an inert gas, or by vacuum. As examples there are three cable constructions in common use which employ these approaches to minimize dielectric loss. The first is the use of a foamed dielectric PS plastic that is commonly used in either twin lead transmission lines or in coaxial cables used for antenna lead-in wires in the UHF-TV antenna applications. The second system, which is illustrative of several sectional spacers, is used widely in communications cables of the coaxial type to minimize losses to the dielectric by reducing the amount of dielectric material in the cable. 

FMCW radar level gauges are the primary tank farm radar devices because of the inherent accuracy. Pulse transmitters are not suitable for tank farm inventory and custody transfer applications. The combination of foam and low-dielectric material can cause errors or provide an insufficient signal strength on the return signal. 

Electrical Insulation. The substitution of a gas for part of a soHd polymer usuaUy results in large changes in the electrical properties of the resulting material. The dielectric constant, dissipation factor, and dielectric strength are aU generaUy lowered in amounts roughly proportional to the amount of gas in the foam. 

Syntactic foams are composites made of micro-balloons or hollow macrospheres bound into a plastic matrix. The polyimide syntactic foams are used in specific electronic equipment for microwave and RF applications. They act as structural, dielectric and heat-barrier materials. Micro-balloons can be made of polyimides for special applications. 

Foams have a large variety of applications. Solid foams are widely used as insulating materials. Due to the presence of air bubbles they have a low thermal conductivity. Polyurethane foams and Styrofoam are examples. Styrofoam is also used as a packing material. The light weight of polymer foams makes them attractive as filling materials to stabilize otherwise hollow structures. A natural solid foam is pumice stone. Metal foams are used in the automotive and aerospace industry as light and stable materials [567], Ceramic foams are developed for electronic applications as piezoelectric transducers and low dielectric constant substrates [568],... 

Polyethylene foams are used extensively in buoyancy applications because of their excellent water-resistant properties. These basically closed-cell foams absorb less than 0.5% by voliune of water after being immersed for 24 hours. The low density of the foams also contribute to their buoyancy. The excellent dielectric characteristics of polyethylene are retained when it is expanded into foams. Polyethylene foam is a candidate for many electrical-material uses requiring good properties of dielectric strength, dielectric constant, dissipation factor, and volume resistivity (6). 

Applications are for use as lightweight filler materials for syntactic foam shapes or for parts for dielectric applications. 

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