John P. Gianvittorio

Fractal antennas: a novel antenna miniaturization technique, and applications

Fractal geometry involves a recursive generating methodology that results in contours with infinitely intricate fine structures. This geometry, which has been used to model complex objects found in nature such as clouds and coastlines, has space-filling properties that can be utilized to miniaturize antennas. These contours are able to add more electrical length in less volume. In this article, we look at miniaturizing wire and patch antennas using fractals. Fractals are profoundly intricate shapes that are easy to define. It is seen that even though the mathematical foundations call for an infinitely complex structure, the complexity that is not discernible for the particular application can be truncated. For antennas, this means that we can reap the rewards of miniaturizing an antenna using fractals without paying the price of having to manufacture an infinitely complex radiator. In fact, it is shown that the required number of generating iterations, each of which adds a layer of intricacy, is only a few. A primer on the mathematical bases of fractal geometry is also given, focusing especially on the mathematical properties that apply to the analysis of antennas. Also presented is an application of these miniaturized antennas to phased arrays. It is shown how these fractal antennas can be used in tightly packed linear arrays, resulting in phased arrays that can scan to wider angles while avoiding grating lobes.

Self-similar prefractal frequency selective surfaces for multiband and dual-polarized applications

Frequency-selective surfaces (FSS), that have been designed using fractal iterative techniques, have been fabricated and measured. Fractals contain many scaled copies of the starting geometry, each of which acts as a scaled version of the original. A multiband FSS can be designed that uses several iterations of the geometry to form a prefractal that resonates corresponding to each of the scales present in the geometry. Minkowski and Sierpinski carpet fractals have been utilized in the design of three surfaces which exhibit two or three stopbands depending on how many iterations are used to generate the geometry of the cell. These surfaces are dual polarized due to the symmetry of the geometry. Simulation capabilities have been developed to analyze these periodic structures, including periodic method of moments (MOM) and finite-difference time-domain (FDTD) techniques which show good correlation to the measured results.

Reconfigurable patch antennas for steerable reflectarray applications

Reconfigurable reflectarrays have been designed with patch elements which can vary the reflected phase by varying the height of the patches. These patches have been designed using a periodic method of moments simulation. Reflectarrays incorporating elements of varying heights have been built and tested. The first design is a 33 element array comprised of stacked patches which operates at 7.31 GHz. The second design is a 120-element dipole array over a ground plane which operates at 5.2 GHz. Microelectrical, mechanical systems actuation technology could be used to implement these designs and a potential concept is suggested

Magnetic MEMS reconfigurable frequency-selective surfaces

A reconfigurable frequency-selective electromagnetic filter implemented by integrating hard magnetic materials with microelectromechanical systems (MEMS) provides a new variation of reconfigurable frequency-selective surfaces (FSS). By incorporating magnetically actuated dipole elements that are capable of being tilted away from the supporting surface, we can tune the FSSs operating frequency without having to physically alter the dimensions of the dipole elements. The 25/spl times/25 array of microactuators used in this work each consist of a 896/spl times/168/spl times/30 /spl mu/m/sup 3/ ferromagnetic plate made of 40Co-60Ni, layered with a 1-/spl mu/m-thick conductor (Au), attached to a pair of 400/spl times/10/spl times/1 /spl mu/m/sup 3/ polysilicon torsion beams, suspended just above the supporting substrate. The high remanent magnetization of the ferromagnetic material allows for relatively small magnetic fields (/spl sim/2.1 kA/m) to induce significant angular deflections (/spl sim/45/spl deg/). This innovative reconfigurable FSS design has successfully demonstrated electromagnetic-signal diplexing and tuning its resonant frequency over a bandwidth of 2.7 GHz at a frequency of 85 GHz.

Fractal element antennas: a compilation of configurations with novel characteristics

Fractal geometries have been used in science to mathematically define intriguing features observed in nature, from describing the formation of clouds to coastlines to tree bark. It expands the horizon of possible geometries, some of which may lead to enhanced performances in antenna designs. Fractal geometries have been studied as array elements and distributions, with the goal of finding miniaturized and efficient radiators. A compilation of simulations of several simple fractal geometries that either miniaturize or improve the input match of element antennas (including fractal loops, fractal dipoles, and multiband fractal antennas) are presented.

Fractal FSS: various self-similar geometries used for dual-band and dual-polarized FSS

In our attempt to design a frequency selective surface (FSS) that is resonant at two distinct bands, dual-polarized, and has a simple planar design, several self-similar elements based on fractal geometry have been investigated. In this paper these various geometries are presented along with their characteristics.

MEMS enabled reconfigurable frequency selective surfaces: design, simulation, fabrication, and measurement

Frequency selective surfaces have typically been static and used in applications with fixed frequencies. However, certain applications could benefit from a reconfigurable frequency response. The frequency response of an FSS can be tuned by rotating the elements off of the surface of the supporting dielectric. MEMS technology is utilized in this project to achieve this goal. Samples have been designed, simulated, fabricated and measured at UCLA demonstrating this technology.

Fractal loop elements in phased array antennas: reduced mutual coupling and tighter packing

A common goal for array antennas has been to pack the elements tighter for lower scan angles while fighting mutual coupling. The emergence of wireless networks as a rapidly expanding market has brought about a need for very small and efficient phased array antennas. An optimal design would incorporate tightly packed elements for efficient directed transmissions that radiate effectively without significant mutual coupling. This paper presents arrays for a wireless LAN configuration, designed at 2 GHz for operation at the PCS band, that scan lower angles more efficiently than conventional designs. We show the benefits of using fractal elements in linear phased arrays to achieve denser packing while minimizing mutual coupling. Previous work has shown that fractal geometries are an effective means of miniaturizing antennas. The smaller elements can work in an array in two ways: the first is to increase the gap size between the elements, thus reducing the coupling between them; the second is to allow more elements into a fixed length array for broader scan angles. Also, there are two methods for utilizing fractal in arrays. One method is fractal arrays, which arrange elements in fractal patterns. The other method, analyzed here uses fractals as elements. Linear arrays of fractal square loop elements are analyzed using a method of moments computer code developed at UCLA.

Fractal elements in array antennas: investigating reduced mutual coupling and tighter packing

Effective array design should take into account mutual coupling. This becomes even more of a troublesome area as arrays are packed tighter to increase the scan angles in phased array systems. One attempt to control mutual coupling that is utilized here is to miniaturize the elements of the arrays. The smaller elements decrease the amount of coupling by increasing the spacing between the elements, thus enhancing the array performance. To miniaturize the elements, fractal geometries are used. Fractal geometries have also been used in arrays by defining the distribution of elements.

Fractal geometry in antenna system design: miniaturized-multiband element, phased array and frequency selective surface design

Fractal geometry has been used in the design of antenna elements to achieve miniaturization, to lower the Q of the antennas and to attain multiple resonances. These benefits and design techniques have been applied to the design of scanned phased arrays and frequency selective surfaces. These designs have been simulated using the moment method and the finite difference time domain (FDTD) techniques as well as fabricated and measured.