Recent years have seen a burgeoning interest in the physics and applications of metamaterials. Briefly, metamaterials consist of a periodic array of subwavelength structures that produce an electromagnetic response not typically available in nature. Notably, composites of these structures in general exhibit both frequency and spatial dispersion, leading to a rich variety of physical phenomena such as backward wave propagation and a negative refractive index.

The most common form of Metamaterial unit cell is formed through a split-ring resonator on a standard printed circuit board (PCB), with a single strip line on the reverse side of the PCB (PCB grade FR4).
Metamaterials offer two key advantages for High-Power Microwave (HPM) source design. First, they offer the possibility of exploiting new forms of interaction previously unavailable for microwave source design. Additionally, they offer a path to reduce the dimensions of standard HPM sources and components, gaining significant size advantages for the source designer. Both issues, however, hinge on the ability to find sub-wavelength, resonant structures in configurations that can withstand the harsh operating environment of an HPM device.

A configuration was developed in which an array of split-ring resonators, forming a “mu-negative” structure, allows transmission of power in a waveguide well below the cutoff frequency. This configuration would not be used in an actual HPM device, but explores the methods and considerations that might be required for developing a metamaterial structure for either making HPM sources more compact, or developing new types of interaction at these high powers. For any HPM application, a microwave structure must be able to sustain high electric and magnetic fields, as well as high peak and possibly average power. The challenge for metamaterials consists of devising the sub-wavelength structures that can sustain such fields.

In particular, one must understand the sensitivity of any metamaterial system to changes in the individual elements, which in the case of high power, pertains mainly to the loss of an individual resonator element. As such a sample system, this work explored the physical operating characteristics of the waveguide system loaded with an array of split-ring resonators (SRR), with particular emphasis upon the role of defects on its properties. Such defects would form an important feature in any high-power application in which sub-wavelength structures can be damaged by high field stresses. The physics of microwave propagation were examined below cutoff using a splitring resonator array, via both numerical models and experiments.

This work was done by Rebecca Seviour of Lancaster University (UK) for the Air Force Office of Scientific Research. AFRL-0214

This Brief includes a Technical Support Package (TSP).
Fundamentals of Metamaterials for High-Power RF Applications

(reference AFRL-0214) is currently available for download from the TSP library.

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This article first appeared in the August, 2012 issue of Defense Tech Briefs Magazine.

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