High dielectric constant composites for high power antennas

Abstract

The volume and weight of high power antennas can be a limiting factor for compact pulsed power transmitters. Options for antenna minimization are limited due to the relationship between an antenna's physical dimensions and the frequencies which can be transmitted. Thus, low frequency antennas often require dimensions on the order of meters. An effort undertaken at the University of Missouri addresses these fundamental constraints on antenna size by developing and integrating high dielectric constant composite materials that can reduce the physical size of high power antennas. Traditional high dielectric constant materials, including sintered perovskite ceramics, are not well suited for integration in high power antennas due to their low dielectric strength, poor mechanical properties, and limitations in forming complex shapes. Therefore, a composite material is necessary for improved dielectric strength and mechanical properties over sintered ceramics while maintaining a high dielectric constant. Previous research on high dielectric constant composite materials has primarily been focused on low power applications for embedded capacitors. The development of composites for high power systems presents additional challenges, including large thickness and high dielectric strength requirements for high voltage use and the capability to be formed or machined into complex geometries. Three distinct classes of high dielectric constant composites have been developed through this effort. The three classes of composites are characterized by the range of dielectric constant values measured at frequencies from 200 MHz to 4.5 GHz. The first composite class can be manufactured with a dielectric constant of 30-50 and exhibits the best machining properties of the three classes. The second composite class has a dielectric constant of 80-100, is easily machined, and has a loss tangent of less than 0.12 up through at least 4.5 GHz. The third composite class has the highest dielectric constant at between 350 and 550, is easily machined, and is well-suited for operation at frequencies of a few hundred MHz. Three crucial dielectric measurements for high dielectric constant composites for high power components are reported. First, the complex permittivity is given in the frequency range from 200 MHz to 4.5 GHz. Second, the pulsed dielectric strength is reported as the cumulative probability of breakdown measured from a set of samples of each composite class. Lastly, the electric field dependence of each composite's dielectric constant is reported through polarization measurements taken on a custom high voltage test stand.