Browsing by Subject "Electromagnetism."

Now showing 1 - 4 of 4

Open Access
Application of iterative techniques for electromagnetic wave scattering from dielectric random rough surfaces
(2005) İnan, Kıvanç
Mobile radio planning requires accurate prediction of electromagnetic field strengths over large terrain profiles. However the conventional method of moments (MoM) becomes unsuitable for electrically large rough dielectric surfaces, because of the O(N³) computational cost due to the large number of surface unknowns N. Iterative Methods are beneficial methods for faster electromagnetic problem solutions. By using such methods, very accurate results can be achieved, causing a computational cost of O(N²). In this work, among the stationary iterative methods; Forward-Backward Method (FBM), and among the nonstationary iterative ones; Conjugate Gradient Squared (CGS), BiConjugate Gradient Stabilized (Bi-CGSTAB) and Quasi Minimal Residual (QMR) Methods are presented to investigate the electromagnetic wave scattering from dielectric random rough surfaces. These techniques are compared to each other over various kinds of surface models that reflect the real terrains to find out the best solution methodologies. Furthermore, efficiency of the methods are assessed by comparing the obtained scattering results, normalized radar cross sections (NRCS) of the surfaces considered, with the numerically exact ones computed by employing the MoM.
Open Access
Chiral metamaterial and high-contrast grating based polarization selective devices
(2013) Mutlu, Mehmet
The utilization of purposely designed artificial media with engineered electromagnetic responses enables the obtaining of intriguing features that are either impossible or difficult to realize using readily available natural materials. Here, we focus on two classes of artificial media: metamaterials and high-contrast gratings. Metamaterials and high-contrast gratings are designed within the subwavelength periodicity range and therefore, they are non-diffractive. We exploit the magnetoelectric coupling effect in chiral metamaterials to design several structures. Firstly, we design a linear to circular polarization convertor that operates for x-polarized normally incident plane waves. Then, we combine the chirality feature and the electromagnetic tunneling phenomenon to design a polarization insensitive 90◦ polarization rotator that exhibits unity transmission and crosspolarization conversion efficiencies. Subsequently, we combine this polarization rotator with a symmetric metallic grating with a subwavelength slit for the purpose of enabling the one-way excitation of spoof surface plasmons and achieving a reversible diodelike beaming regime. Then, we exploit the asymmetric transmission property of chiral metamaterials and show that a polarization angle dependent polarization rotation and a strongly asymmetric diodelike transmission is realizable. Afterwards, a brief waveguide theory is provided and eventually, the dispersion relations for a periodic dielectric waveguide geometry are derived. Then, using these relations and considering the finiteness of the waveguide length, we show the theoretical description of high-contrast gratings. Finally, we theoretically and experimentally show that the achievement of a broadband quarter-wave plate regime is possible by using carefully designed high-contrast gratings.
Open Access
The left hand of electromagnetism : metamaterials
(2010) Alıcı, Kamil Boratay
Metamaterials are artificial periodic structures whose electromagnetic response is solely dependent on the constituting unit cells. In the present thesis, we studied unit cell characteristics of metamaterials that has negative permeability and permittivity. We investigated negative permeability medium elements, especially in terms of their electrical size and resonance strength. Experimental and numerical study of µ-negative (MNG) materials: multi split ring resonators (MSRRs), spiral resonators (SRs) and multi-spiral resonators are presented. The resonance frequency of the structures is determined by the transmission measurements and minimum electrical size of λ0/17 for the MSRRs and of λ0/82 for the SRs observed. We explain a method for tuning the resonance frequency of the multi-split structures. We investigated scalability of MNG materials and designed a low loss double negative composite metamaterial that operates at the millimeter wave regime. A negative pass-band with a peak transmission value of -2.7 dB was obtained experimentally at 100 GHz. We performed transmission based qualitative effective medium theory analysis numerically and experimentally, in order to prove the double negative nature of the metamaterial. These results were supported by the standard retrieval analysis method. We confirmed that the effective index of the metamaterial was indeed negative by performing far field angular scanning measurements for a metamaterial prism. Moreover, we illuminated the split-ring resonator based metamaterial flat lens with oblique incidence and observed from the scanning experiments, the shifting of the beam to the negative side. The first device was a horn antenna and metamaterial lens composite whose behavior was similar to Yagi-Uda antenna. We numerically and experimentally investigated planar fishnet metamaterials operating at around 20 GHz and 100 GHz and demonstrated that their effective index is negative. The study is extended to include the response of the metamaterial layer when the metamaterial plane normal and the propagation vector are not parallel. We also experimentally studied the transmission response of a one dimensional rectangle prism shaped metamaterial slab for oblique incidence angles and confirmed the insensitivity of split-ring resonator based metamaterials to the angle of incidence. After the demonstration of complete transmission enhancement by using deep subwavelength resonators into periodically arranged subwavelength apertures, we designed and implemented a metamaterial with controllable bandwidth. The metamaterial based devices can be listed under the categories of antennas absorbers and transmission enhancement. We studied electrically small resonant antennas composed of split ring resonators (SRR) and monopoles. The electrical size, gain and efficiency of the antenna were λ0/10, 2.38 and 43.6%, respectively. When we increased the number of SRRs in one dimension, we observed beam steerability property. These achievements provide a way to create rather small steerable resonant antennas. We also demonstrated an electrically small antenna that operates at two modes for two perpendicular polarizations. The antenna was single fed and composed of perpendicularly placed metamaterial elements and a monopole. One of the metamaterial elements was a multi split ring resonator and the other one was a split ring resonator. When the antenna operates for the MSRR mode at 4.72 GHz for one polarization, it simultaneously operates for the SRR mode at 5.76 GHz, but for the perpendicular polarization. The efficiencies of the modes were 15% and 40% with electrical sizes of λ/11.2 and λ/9.5. Finally, we experimentally verified a miniaturization method of circular patch antennas. By loading the space between the patch and ground plane with metamaterial media composed of multi-split ring resonators and spiral resonators, we manufactured two electrically small patch antennas of electrical sizes λ/3.69 and λ/8.26. The antenna efficiency was 40% for the first mode of the multi-split ring resonator antenna with broad far field radiation patterns similar to regular patch antennas. We designed, implemented, and experimentally characterized electrically thin microwave absorbers by using the metamaterial concept. The absorbers consist of i) a metal back plate and an artificial magnetic material layer; ii) metamaterial back plate and a resistive sheet layer. We investigated absorber performance in terms of absorbance, fractional bandwidth and electrical thickness, all of which depend on the dimensions of the metamaterial unit cell and the distance between the back plate and metamaterial layer. As a proof of concept, we demonstrated a λ/4.7 thick absorber of type i), with a 99.8% absorption peak along with a 8% fractional bandwidth. We have also demonstrated experimentally a λ/4.7 and a λ/4.2 thick absorbers of type ii), based on SRR and MSRR magnetic metamaterial back plates, respectively. The absorption peak of the SRR layout is 97.4%, while for the MSRR one the absorption peak is 98.4%. We conveyed these concepts to optical frequencies and demonstrated a metamaterial inspired absorber for solar cell applications. We finalized the study by a detailed study of split ring resonators at the infrared and visible band. We studied i) frequency tuning, ii) effect of resonator density, iii) shifting magnetic resonance frequency by changing the resonator shape, iv) effect of metal loss and plasma frequency and designed a configuration for transmission enhancement at the optical regime. By using subwavelength optical split ring resonator antennas and couplers we achieved a 400-fold enhanced transmission from a subwavelength aperture area of the electrical size λ2 /25. The power was transmitted to the far field with 3.9 dBi directivity at 300 THz.
Open Access
A performance-enhanced planar schottky diode for terahertz applications : an electromagnetic modeling approach
(2014) Ghobadi, Amir
Today’s state-of-the-art search engines utilize the inverted index data structure for fast text retrieval on large document collections. To parallelize the retrieval process, the inverted index should be distributed among multiple index servers. Generally the distribution of the inverted index is done in either a term-based or a document-based fashion. The performances of both schemes depend on the total number of disk accesses and the total volume of communication in the system. The classical approach for both distributions is to use the Central Broker Query Evaluation Scheme (CB) for parallel text retrieval. It is known that in this approach the central broker is heavily loaded and becomes a bottleneck. Recently, an alternative query evaluation technique, named Pipelined Query Evaluation Scheme (PPL), has been proposed to alleviate this problem by performing the merge operation on the index servers. In this study, we analyze the scalability and relative performances of the CB and PPL under various query loads to report the benefits and drawbacks of each method.