3D printed metamaterial lenses for microwave antennas

This report presents the results from a study initialized and financed by «kompetanseområde Materialteknologi» at the Norwegian Defence Research Establishment (FFI). It is an exploratory study on metamaterials, and the work presented here is limited to materials with spatially varying permittivity and their applications to antennas. Metamaterials are materials with properties that are not readily available in nature. Usually, the special properties arise from structures in the material, which results in electromagnetic waves experiencing unusual or unnatural permittivity and/or permeability when interacting with the metamaterial. These meta properties are usually only present in limited frequency bands. The authors of this report have previous experience from antenna design using 3D printing. In this work, we use this manufacturing technology to produce blocks of plastic with varying density to demonstrate that we can produce materials with controllable permittivity. This requires a structure with a resolution significantly higher than the wavelength. For our microwave applications with wavelengths down to one centimeter, we require a structure at millimeter scale. Producing such fine structures has recently become possible with the use of modern 3D printers. Simulations and measurements on the plastic blocks show that we can achieve relative permittivity values ranging from close to one (as in air) and up towards the permittivity of 100 percent density of the material being used. To demonstrate relative permittivity, we designed a number of plastic lenses with spatially varying density and measured the effect on the radiation diagram of an antenna. The example presented here shows a 3dB increase in gain over a relative bandwidth of more than 2:1. In radio systems, the range is proportional to the square root of the transmitted power, and hence a 3dB gain results in more than 40 percent increase in range.