Michael J. Wilhelm

The design synthesis of multiband artificial magnetic conductors using high impedance frequency selective surfaces

This paper introduces several different design methodologies for multiband artificial magnetic conducting (AMC) surfaces. The paper begins by investigating the multiband properties exhibited by a conventional electromagnetic bandgap (EBG) AMC that consists of a frequency selective surface (FSS) on top of a thin dielectric substrate with a PEC back plane. The higher-order resonances associated with these surfaces have not been discussed in detail to date, as previous research has been concerned only with exploiting the primary resonant frequency. However, it will be shown that by understanding and making appropriate use of these higher order resonances, it is possible to design multiband AMC surfaces that work for nearly any desired combination of operating frequencies. The first multiband AMC design approach that will be considered is based on the introduction of FSS screens that have fractal or nearly fractal unit cell geometries. This is followed by a more general and robust genetic algorithm (GA) technique for the synthesis of optimal multiband AMC surfaces. In this case, a GA is used to evolve multiband AMC surface designs by simultaneously optimizing the geometry and size of the FSS unit cell as well as the thickness and dielectric constant of the substrate material. Finally, several examples of multiband AMC surfaces are presented, including some practical dual-band and tri-band designs genetically evolved for operation at GPS and cellular frequencies, as well as an example illustrating the success in creating a multiband AMC surface with angular stability.

Active negative impedance loaded EBG structures for the realization of ultra-wideband Artificial Magnetic Conductors

A new design methodology is introduced for an ultra-wideband Artificial Magnetic Conductor (AMC) that is based on loading the elements of Electromagnetic Bandgap (EBG) structures with active devices. The types of active loads used for this application belong to the class of devices known as Negative Impedance Converters (NICs). NICs are active two-port networks for which the impedance looking into one port is the negation of the impedance connected to the other port scaled by the impedance conversion coefficient of the device. Several design examples will be presented that demonstrate the considerable enhancement in bandwidth that can be achieved, compared to conventional passive AMC surfaces, by using EBG structures actively loaded with NICs.

Novel design techniques for miniature circularly-polarized antennas using genetic algorithms

We introduce new methodologies employing a genetic algorithm (GA) to evolve classes of antenna shapes that are circularly polarized and offer optimal performance characteristics such as input impedance, VSWR, bandwidth, and/or gain. These new methodologies are also capable of producing designs that are significantly smaller when compared to a conventional crossed-dipole antenna. It is demonstrated that, by utilizing these design approaches for miniature, circularly polarized, stochastic, crossed-dipole antennas, size reductions of over 40% are typically achieved, and in some cases, it is even possible to realize designs with size reductions up to 86%. An important feature of these antennas is that the size reduction achieved is in terms of area, which differs from the case of stochastic linear dipoles where a reduction in length is achieved at the expense of an increase in width.

Printed electronic processes for flexible hybrid circuits and antennas

Next generation electronics will be painted onto plastic substrates providing both functionality and physical flexibility. The corresponding manufacturing process will lead to significantly reduced cost; however the performance of the resulting circuits will not match many existing high-performance electronic circuits fabricated with traditional technology today. High performance is not always necessary, but when required, Printed Electronics may still be a viable solution. The future of Printed Electronics is bright and will eventually include sophisticated function, but until that time, a Printed Hybrid option will simultaneously provide the cost savings of printing and the high performance of silicon and printed circuit boards. An interim contribution is to use state-of-the-art silicon devices and print the supporting conductive patterns and passives on substrates for flexible systems integration. This paper describes and demonstrates a hybrid circuit technology, which can manufacture circuits more easily adapted to complex shapes and diverse sizes. Additionally, a number of printed components and devices are reviewed and a demonstration of two circuits is provided: an RF antenna and the supporting substrate design of a Field Programmable Gate Array.

Direct-Write Processes as Enabling Tools for Novel Antenna Development

Significant research effort is regularly applied to the goal of reducing the size of radio-frequency antennas while maintaining the entire set of positive attributes of proven but relatively large antennas. Such parameters as frequency response (multiple or single), bandwidth, and complexity of the antenna-driver balun structures require iterative optimization. The direct-write processes now available have enabled the insertion of reactive-loading elements as integral parts of the antenna structure, especially into new conformal designs. These reactive-loading elements were used in conjunction with modern design techniques to achieve antenna devices that were reduced in size to as much as half that of traditional counterparts. The performances of the miniaturized antennas constructed by direct-write methods were evaluated and compared to those of traditional antenna structures.

Direct-write processes as enabling tools for novel antenna development

Significant research effort is regularly applied to the goal of reducing the size of radiofrequency antennas while maintaining the entire set of positive attributes of proven, but relatively large, antennas. Such parameters as frequency response (multiple or single), bandwidth, and complexity of the antenna-driver balun structures require iterative optimization. The direct-write processes now available enable the insertion of reactive-loading elements as integral parts of the antenna structure, especially into new conformal designs. These reactive-loading elements are used in conjunction with modern design techniques to achieve antenna devices that are reduced in size to as much as half that of traditional counterparts. The performances of the miniaturized antennas constructed by direct write methods are evaluated and compared to those of traditional antenna structures.