Fractal antennas: a novel antenna miniaturization technique, andapplications

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     

Seismic reliability assessment of a two-story steel-concrete composite frame designed according to Eurocode 8

Recent earthquakes have shown that some buildings designed primarily for life safety according to the seismic provisions of current building codes could be subjected to severe damage to structural and non-structural elements and building contents. In order to overcome this drawback, the scientific community is developing more accurate methodologies to assess seismic performances of structures. In this framework, the present paper aims at the evaluation of mean annual frequency (MAF) of exceeding a limit state (limit value of interstory drift ranging from 0.002 to 0.05) following the probabilistic approach introduced by Pacific Earthquake Engineering Research Center adopting (PEER) the application procedure suggested by Jalayer and Cornell. Reference is made to a single two-story steel-concrete composite frame, designed according to Eurocode 8 and tested in real scale at the ELSA Laboratory of Ispra. The reason for investigating this frame is related to the availability of results both of pseudo-dynamic tests performed on 3D steel-concrete composite frame and of monotonic and cyclic tests performed on internal and external beam-to-column joint sub-assemblages belonging to the building. As a consequence, it has been possible to assess the seismic reliability of a 3D steel-concrete composite building, designed according to Eurocode 8, taking into account both randomness and uncertainty.     

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.     

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 MEMS-enabled frequency selective surfaces

A technique for designing reconfigurable frequency selective surfaces using microelectromechanical systems (MEMS) technology is presented. A method of moments simulation technique for analysing these periodic structures has been used to predict the tunability using this technique. Fabricated and measured results verify the concept of tunability.     

Experimental analysis and modelling of bolted T-stubs under cyclic loads

Bolted connections have recently attracted new research efforts after the unexpected failures of welded connections during Northridge and Kobe earthquakes. The criteria and the formulations for predicting the rotational stiffness and the flexural resistance, under static loading conditions, of the most common connection typologies have been codified by Eurocode 3 [CEN. EN 1993-1-8 Eurocode 3: Design of steel structures — Part 1.8 Design of joints. 2005] which is based on the so-called component approach. In order to extend the component approach to the prediction of the seismic response of partial-strength connections, the modelling of the cyclic response of the joint components is necessary. Starting from the observation that the main sources of deformability and plastic deformation capacity of bolted connections can be modelled by means of an equivalent T-stub, an experimental program devoted to the cyclic response of the most important component of bolted connections has been carried out aiming at the modelling of the cyclic force-displacement curve of bolted T-stubs. In this paper, starting from the analysis of the results of the experimental tests performed, stiffness and strength degradation rules are derived as a function of the displacement amplitude required at any cycle and of the energy dissipated in the previous loading history. The combination of these rules with the theoretical prediction of the monotonic envelope leads to a proposal for predicting the cyclic behaviour of bolted T-stubs starting from the knowledge of their geometrical and mechanical properties.     

Experimental study of the thermo-mechanical properties of recycled PET fiber-reinforced concrete

This work presents an experimental study of thermal conductivity, compressive strength, first crack strength and ductility indices of recycled PET fiber-reinforced concrete (RPETFRC). We examine PET filaments industrially extruded from recycled PET bottle flakes with different mechanical properties and profiles. On considering a volumetric fiber dosage at 1%, we observe marked improvements in thermal resistance, mechanical strengths and ductility of RPETFRC, as compared to plain concrete. A comparative study with earlier literature results indicates that RPETFRC is also highly competitive over polypropylene-fiber-reinforced concrete in terms of compressive strength and fracture toughness.