Scott F. Griffiths

High-power considerations in metamaterial antennas

While many studies have been conducted on metamaterials at microwave frequencies, comparatively few have examined their use in high-power applications. Here, we perform a general study of metamaterial geometries to identify configurations that are well-suited for utilization in high-power environments. We further develop a genetic algorithm optimization scheme for synthesizing pixelized geometries with artificial magnetic conducting (AMC) properties and reduced maximum field enhancement factor (MFEF).

Mitigating Field Enhancement in Metasurfaces and Metamaterials for High-Power Microwave Applications

Metasurfaces and metamaterials have been explored extensively in recent years for their ability to enable a variety of innovative microwave devices. However, because their exotic properties often arise from resonant structures, the large field enhancements under high-power microwave illumination can lead to dielectric breakdown and damage to the device. In order to develop metasurfaces and metamaterials capable of being utilized in high-power microwave applications, this paper investigates techniques for reducing the maximum field enhancement factor (MFEF) in several types of structures from the literature. Starting with a simple Sievenpiper metasurface, this paper evaluates the dependence of MFEF on the structure design parameters. For more complex metasurface geometries, a genetic algorithm is demonstrated that can evolve structures that have minimal MFEF. In addition, negative-index and low-index metamaterials are evaluated for field enhancement. By optimizing for low loss and by operating in the resonance tails, metamaterials with low MFEF can be realized for high-power applications. To illustrate this, a quad-beam focusing metamaterial lens is presented with an MFEF less than 5 over the entire operating band.

Genetic algorithm synthesis of metasurfaces with improved similarity and robustness for high-power reflector antenna applications

A metamaterial reflector antenna is designed for robustness under high-power operating conditions by synthesizing pixelized metasurface geometries with minimized maximum field enhancement factor (MFEF) using a binary genetic algorithm (GA) stochastic optimizer. A metric is also included in the optimization to select unit cell geometries that share a high similarity among one another in order to improve the reflecting properties of the metasurface.

Metasurface for high-power reflector antenna based on counter-rotating end-loaded dipole elements

A tunable metasurface unit cell that is based on counter-rotating end-loaded dipole (ELD) elements is proposed for high-power reflector antennas. The metasurface is tuned by mechanically rotating the ELD elements and thus avoids the power handling limitations of tunable capacitors such as varactors. Simulations of the full reflector antenna show that the reflection phase can be controlled across the metasurface by properly rotating the ELDs.

High-power metamaterial phase shifter using a relay-based approach

A metamaterial design that can rapidly reconfigure its reflection phase angle and safely operate under high incident field strengths is presented for use in a high power microwave (HPM) reflectarray system. The design offers switching between three sufficiently unique reflection phase angles to allow for beam control with a stationary reflector and feed horn. The metamaterial unit cell is fairly lightweight and rigid, making it mechanically appealing. Phase reconfiguration is easily accomplished though simple relay coil operation, allowing for increased isolation between high power radio frequency (RF) energy and the electronic beam control architecture compared to traditional semiconductor biasing techniques.

Design techniques for loss mitigation in metamaterial reflector antennas

There are often discrepancies between the behavior of a metamaterial reflector unit cell in an infinitely periodic environment and in a non-ideal environment in which the reflection phase varies across a metamaterial reflector. This work considers the performance of several capacitively loaded metamaterial unit cells in order to improve the realized gain of a reflector antenna. Each structure exploits capacitive elements to achieve a desired reflection phase gradient across the surface of the metamaterial reflector. The influence of interconnectedness and surface currents on the reflector antenna performance are analyzed and compared with the antenna performance from an ideal impedance surface reflector.

A Low Cost and Highly Efficient Metamaterial Reflector Antenna

The design of an efficient, metamaterial-based reflector antenna capable of operation at high power is presented. Metamaterial unit cells are comprised of end-loaded dipoles (ELDs) with capacitive lumped elements at their center. Enhanced power handling is realized by mitigating field enhancement through rounded edges and using appropriate high voltage capacitors as loads. The metamaterial is designed to have discrete, configurable reflection phase through choice of capacitor on the ELDs. Unit cells are fabricated and patterned to a square, flat reflector surface and configured for broadside reflection with an offset-fed horn antenna. Simulated and measured radiation patterns of the complete antenna system are presented with excellent agreement. High-power testing verifies the ability of the metasurface to withstand extreme incident electric field strengths.

Extending power-handling of high-power metamaterial phase-shifters using three-dimensional counter-rotated end-loaded dipoles

Through the careful consideration of field enhancement observed in individual unit cells, metamaterials can be designed for use in high power microwave (HPM) applications. However, power handling considerations are only one aspect of any metamaterial designs performance. Presented here are two metamaterial unit cell geometries which feature reconfigurable reflection phase behaviors intended for HPM reflectarray applications. The unit cells are comprised of two end-loaded dipoles (ELD) which are counter-rotated to eliminate cross-pol while achieving the desired reflection phase angle. It is shown that by extending a planar ELD to a volumetric (i.e., three-dimensional) design, the observed field enhancement can be drastically reduced thus providing a path for improved power-handling performance.

High Power Metasurface Reflectarray Antennas Using Switched Shorted Circular Elements

A metamaterial design that can rapidly reconfigure its reflection phase angle and safely operate under high incident field strengths is presented for use in a high power microwave (HPM) reflectarray system. The design offers switching between reflection phase angles to allow for beam control with a stationary reflector and feed horn. Two variations of the design are presented which offer (a) fixed (but selectable) reflection phases with high-power operation and (b) on-the-fly reconfigurable reflection phases with low-power operation. For design (b), reconfiguration is easily and quickly accomplished though simple relay-style switching operation. The designs are developmental steps towards a fully on-the-fly reconfigurable reflectarray which can operate with several megawatts of peak input power. Fabrication and testing of the prototype antennas and metasurfaces were carried out and presented.

Metamaterials for high power reflectarray design

Most often, transmission and reflection properties and geometrical constraints (such as size) are the sole considerations in metamaterials design. Typically, power handling is of little concern as most applications are very low power. Here, we consider not only the complex transmission and reflection properties but also the magnitude of the electric fields in and around the structure when designing metamaterials for robustness in high power reflectarray applications.