Theses supervised by Prof. Dr. Ayhan Altıntaş
13 theses · İhsan Doğramacı Bilkent University, Anadolu University
Elektriksel olarak küçük kapasitif antenlerin non-foster empedans uyumlandırması
Device scaling and component-miniaturization are the main drivers of the development of electronic technology. In time, electronic devices have become smaller in size and hence, the scaling down of antenna dimensions has come to be not only an interesting but also substantial areas of research. The gain - bandwidth product of an antenna is limited by its electrical size, therefore reducing the size of an antenna narrows the bandwidth or lowers the gain. The work presented in this thesis contributes to the existing body of research on the structure of electrically small antennas and complications of its design with regard to the fundamental limitations. The large input reactance of electrically small antennas (ESA) are conventionally matched with passive circuits, however, the matching works at a single frequency which shrinks the bandwidth. In previous studies, non-Foster impedance matching which employs active networks of negative inductors and capacitors to overcome the restrictions of gain-bandwidth theory has been examined. In this study, the origins and development of Non-Foster impedance matching is reviewed and its stability issues are discussed. The design and simulation of a negative impedance converter circuit and together with an electrically small disk loaded dipole are presented. In this research, the designed matching circuit is fabricated, measured and its results are analyzed. Additionally, promising future studies and their possible effects in the antenna field are reviewed.
Yayın kule anten sistemlerinin çözümleme ve tasarımı
In broadcasting, the coverage and interference requirements of antennas are designated in terms of Effective Radiated Power (ERP), tower location and height, and antenna patterns. When antennas are tailored to satisfy the requirements, transmitter powers, cable losses, and phase adjustments for antenna elements must be taken into account in addition to the parasitic effect of the supporting and nearby towers on characteristics of antenna. Digital video and audio broadcasting requires antennas to be at least 95 % efficient, wideband, low weight and high gain with enhanced radiation performance. For this reason many wire and planar antennas have been designed to meet the requirement of Digital Video Broadcasting-Terrestrial (DVB-T) and Terrestrial Digital Audio Broadcasting (T-DAB). We used a tool NEC (Numerical Electromagnetic Code) to model antenna on top of a tower structure. Simulation results for parasitic effect of the tower on characteristics of antenna such as impedance, return loss, gain, front-to-back ratio and radiation patterns are reported. In addition, the effect of nearby tower on antenna characteristics is studied. We designed a broadband antenna in the UHF (470 - 860 MHz) band that works for both digital and analog TV with return loss ≥10 dB, fractional bandwidth of 85% and gain at center frequency 12.5 dB. The ERP is calculated by mounting the antenna at each face of the tower to give a satisfactory coverage to a region around the antenna. Moreover, the design of a transmitter antenna for DVB-T and T-DAB is presented with Stacked Suspended Plate Antenna (SSPA) structure. Considering the similar products in the world, antennas have been designed in (174-254)MHz for DVB-T and (174-230)MHz for T-DAB with horizontal polarization and return loss ≥ 10 dB. The antenna gives an average gain of 8.5 dB and cross polarization isolation is 31 dB. Contrary to conventional dipole structures, we have employed SSPA structure with two plates for wideband matching and design flexibility. Radiating primary plate has been excited by novel wideband modified inverted L-type probe in a capacitively coupled manner. Vertical plate between primary plate and ground plane also provides wideband matching and adjustment of half power beam width properly. Parasitic secondary plate has been used for further matching and tuning. Together with the parametric study of designed antenna, simulation and measurement results for the input impedance, S11, gain and radiation patterns are presented.
Sabit karasal noktadan-noktaya sistemlerinin tasarımı için tahmin modellerinin analizi ve uygulanması
In this study, propagation link analysis and implementation of prediction models for the design of fi xed terrestrial point-to-point systems are aimed. Di fferent propagation models in the literature are examined as case studies and comparisons are made. Rec. ITU-R P.530 Model is analyzed in detail. The worst month link availability is investigated for terrestrial microwave LOS/NLOS radio links operating in NATO Band 3+ (1350-2690 MHz) and NATO Band 4 (4400-5000 MHz) frequency bands. The calculation of Bullington model of di ffraction loss is extended for LOS path case and determination of reflection points on the terrain pro file is improved. Several terrestrial microwave LOS/NLOS radio links are analyzed using the propagation parameters such as TX (transmitter) and RX (receiver) station coordinates, path length, frequency, antenna heights above ground level, antenna gains, polarization, radio refractivity gradient, time percentage, target SNR (Signal to Noise Ratio), bandwidth, digital terrain elevation and climate data. The calculation of the received power with the e ffect of the ground reflection is developed to calculate the fade margin in the de fined microwave LOS/NLOS radio links. Received power is calculated by taking into consideration the attenuation due to rain and atmospheric gases, di ffraction loss and the e ffect of multipath fading due to reflection. The validity of the implementation of link analysis is justi fied by comparison with the commercial ATDI ICS telecom software and the measurement data existing in the literature over sample microwave LOS/NLOS radio links.
Mükemmel iletken ince tellerin Radar Kesit Alanı (rka) ve bunun radar karşı tedbiri olarak uygulaması: dipol bulutu
In electronic warfare, active and passive countermeasures are used to jam threat RF radars. While electronic jamming pods are accepted as an active one, chaff is accepted as a passive countermeasure that consists of millions of perfectly conducting thin metallic wires, dipoles. The aim of this thesis is to first implement Van Vleck's Methods A and B [1], Tai's Variational Method [2] and Einarsson's Direct Method [3] to get radar cross section (RCS) of a dipole and then apply the results to calculate RCS of designed chaff cartridges. The ultimate goal is to suggest more effective passive countermeasure system than commercially available ones. In this thesis, performance of these methods are evaluated. According to these evaluations, Van Vleck's Method B and Einarsson's Direct Method are selected for calculating RCS of chaff cartridges. Performance of RR-178 (XN-2) commercial chaff cartridge is compared with three different suggested designs. For each of these designs, 2 to 20 GHz frequency interval is divided into three or six equal sub-frequency intervals and for these intervals particular chaff cartridges with different dipole lengths and numbers are proposed. In terms of total dipole length in the cartridges, instead of 88775 meters dipoles that is used in RR-178 (XN-2), by using chaff cartridges of third proposed design, in average only 25300 meters dipoles are used while providing more flat and equal average RCS value for 2 to 20 GHz frequency interval. Moreover, for the stated frequency interval, if total dipole length for the chaff cartridges of RR-178 (XN-2) and third proposed design keep equal, about 5.2 dB increase in average RCS value is obtained. Analysis of these results shows that designed chaff cartridges are more effective than commercial ones if the designed ones are used together with compatible Radar Warning Receiver (RWR) and Dispensing System.
Yer bademinin farmakognozik açdan incelenmesi
Cyperus esculentus Eski Dünya'ya özgü, yenilebilir, saz ailesine ait çok yıllık, çimen benzeri bir bitkidir. Bu tür, Kuzey Amerika'nın tropik ve subtropiklerinde yaygın olarak bulunmaktadır. Cyperus esculentus, Afrika'da popüler bir atıştırmalık olarak tüketilmektedir. Toprak bademi veya kaplan fıstığı olarak adlandırılan yenilebilir yumruları için, İspanya'da ve diğer Avrupa ülkelerinde yaygın olarak tüketilen süt benzeri tatlı bir içecek olan horchata de chufa yapmak için yetiştirilmektedir. Horchata de chufa zengin protein ve mineral (fosfor ve potasyum) içeriğinden dolayı son derece besleyici bir içecektir. Farmakolojide hipertansiyon, diyabet, varis tedavi ve profilaksisinde kullanılmaktadır. Yağ, nişasta, protein, mineral ve vitamin içeren küçük yumru köklerden dolayı potansiyel bir ekonomik üründür. Geniş kullanım olanakları ve yeni bitki arayışı nedeniyle, kaplan somunu son zamanlarda çiftçiler ve gıda endüstrisi için cazip bir alternatif ürün haline gelmektedir. Türkiye'de iki tescilli çeşidi olmasına rağmen, ülke çapında bilinmemesi ve hasat mekanizasyonunun yetersizliği nedeniyle yeterince yetiştirilememiştir. Cyperus esculentus'un kaliteli bir yağ kaynağı olması, nişasta açısından zengin oluşu ve ayrıca barındırdığı antioksidanlar nedeniyle kanserden koruyucu özelliklere sahip olduğu belirtilmektedir. Cyperus esculentus sütünün düşük glisemik karbonhidrat içeriği nedeniyle diyabet hastalarında, kilo vermek isteyenlerde, ayrıca çölyak hastaları ve laktoz intoleransı olanlar için de uygun bir içecek olduğu kayıtlıdır. Katalaz, amilaz, lipaz gibi enzimleri içerdiğinden sindirim, şişkinlik ve ishal sorunları olanlara da önerildiği belirtilmektedir.
Saçılma ve RKA Problemleri için SIY Tekniğinin Geliştirilmesi ve Uygulanması
Full wave solution techniques are preferred to calculate the electromagnetic characteristic of an object. These methods give very accurate results. However, when the object is electrically large regarding wavelength, computation time may take very long, or required computer resources may not be provided. In this case, high-frequency approximation techniques are employed to solve electromagnetic scattering problems involving electrically large objects. Shooting and Bouncing Ray (SBR) method is a high-frequency technique that combines Geometric Optics (GO) and Physical Optics (PO). It can be used for the solution of electrically large objects. Multiple reflection feature of SBR makes it possible to calculate current distribution accurately on the target for the complex objects. In this study, an SBR code is developed in MATLAB which solves di fferent kinds of problems that involve electrically large objects. Required formulas are derived and implementation procedure of the code is discussed. Radar Cross Section (RCS) of some basic shapes is calculated using SBR. An RCS reduction technique is explained and implemented. As the antenna-platform interaction, di fferent kind of antennas are placed on a tank-like and a ship-like object to observe the e ffect of a scatterer on antenna pattern. An antenna coupling formula is derived. This formula is used to calculate coupling between two antennas in diff erent environments. Results show fairly good agreement with Method of Moment (MoM) solutions.
Aronia melanocarpa (MICHX) elliot türü üzerine farmakognozik incelemeler
Bu tez çalışması kapsamında Kırklareli bölgesinde kültür olarak yetiştirilmekte olan Aronia melanocarpa (Michx.) Elliot bitkisinden toplanan meyvelerden metanol, %70 etanol, etanol, etil asetat, hekzan ve su ekstreleri hazırlanmıştır. Elde edilen ekstrelerin verim hesapları yapılmış, yüksek performanslı sıvı kromatografisi ile kimyasal içerikleri aydınlatılmış, toplam fenolik, toplam flavonoit ve toplam antosiyanin madde miktarları spektrofotometrik olarak ölçülmüş ve sonuçlar değerlendirilmiştir. Ekstrelerin antioksidan aktivitelerini belirlemek için DPPH•, ABTS•+, Metal şelatlama, CUPRAC ve Lipid peroksidasyon yöntemleri kullanılmış ve sonuçlar değerlendirilmiştir. Antimikrobiyal aktiviteleri için MİK değerleri bakteri ve maya mikroorganizmalarında belirlenmiştir. Antikanser aktivite tayini için meme kanseri (MCF-7) ve serviks kanseri (HeLa) hücre hatları üzerinde, sağlıklı hücrelerde sitotoksik etki tayininde ise insan endotelyal (HUVEC) ve fare fibroblast (L929) hücre hatları üzerinde MTT yöntemi kullanılmış ve ekstreler etkinlik açısından karşılaştırılmıştır. Ekstrelerin DNA hasarını önleyici etkileri pBR322 plazmit DNA'sı üzerinde incelenmiş ve sonuçlar kıyaslamalı olarak değerlendirilmiştir.
Fiziksel kırınım teorisi ile büyük plakalarla modellenmiş iletken ve empedans cisimlerin radar kesit alanı hesaplanması
Calculation of Radar Cross Section (RCS) of arbitrary bodies using PhysicalOptics (PO) Theory and Physical Theory of Diï¬raction (PTD)is considered. Forbodies with impedance surface boundary condition, only PO is used. Analyticalapproach to PO integral is used to achieve faster computations. A computerprogram has been developed in Fortran in order to calculate the Radar CrossSection (RCS). Arbitrary shape is modeled as triangular facets of any area bythe help of graphical tools. Given the triangular meshed model of an arbitrarybody, Physical Optics(PO) surface integral is numerically evaluated over thewhole surface. There is no limitation on the size of the triangles, as soon as thetotal surface does not retire from the original one.Shadowing algorithm has been used in order to have more accurate solutions.Additionally, ï¬ash points of PO are visualized over the surface of targets, hencelocal nature of high-frequency phenomena is proved.Induced surface currents, edge currents and RCS have been calculated forsome basic shapes and the fuel tank model of F-16 airplanes. Induced surfaceiiicurrents have been visualized over the surface of the particular targets usingMatlab.Keywords: Physical Optics (PO), RCS, Radar Cross Section, Impedance Sur-face, Flat Plate Modeling, Triangular Flat Plates, Physical Theory of Diï¬raction(PTD).iv
İki fiziksel optik integrali yaklaşımının elektromanyetik saçınım açısından karşılaştırılması
A computer program which uses two different Physical Optics (PO) approaches to calculate the Radar Cross Section (RCS) of perfectly conducting planar and spherical structures is developed. Comparison of these approaches is aimed in general by means of accuracy and efficiency. Given the certain geometry, it is first meshed using planar triangles. Then this imaginary surface is illuminated by a plane wave. After meshing, Physical Optics (PO) surface integral is numerically evaluated over the whole illuminated surface. Surface geometry and ratio between dimension of a facet and operating wavelength play a significant role in calculations. Simulations for planar and spherical structures modeled by planar triangles have been made in order to make a good comparison between the approaches. Method of Moments (MoM) solution is added in order to establish the accuracy. Backscattering and bistatic scattering scenarios are considered in simulations. The effect of polarization of incident wave is also investigated for some geometry. Main difference between approaches is in calculation of phase differences. By this study, a comprehensive idea about accuracy and usability due to computation cost is composed for different PO techniques through simulations under different circumstances such as different geometries (planar and curved), different initial polarizations, and different electromagnetic size of facets.
MIMO sistemler için polarizasyon içeren geometrik kanal modellemesi
Most of the studies in the literature about channel modeling do not include the polarization. Aiming to develop a more realistic geometric model including polarization, the channel characteristics are examined using measurement data. Each multipath in the measurement data is modeled with a scatterer. Locations of scatterers are determined in the geometry based single bounce model. Then, each scatterer is replaced by a thin impedance disc. Electrical properties, sizes and orientations of discs are obtained using physical optics approximation. Using the channel model, XPD characteristics of the environment are examined. As a result of this study, a channel model for characterizing the general scenarios as much as possible is developed.
Kanal parametrelerinin kestirilmesi için SAGE algortiması uygulaması ve geliştirilmesi
In recent years, Multiple Input Multiple Output (MIMO) systems have gainedimportance due to the improvements on the performance of radiosystems. Channel parameter estimation is an important factor in thedesign and optimization of MIMO systems. In this thesis, channelparameters such as delay, angles (azimuth and elevation) of arrival(AoA) and departure (AoD), Doppler frequency and polarization areestimated from measurement data using Space Alternating GeneralizedExpectation-Maximization (SAGE) algorithm. One of the focuses ofthis thesis is to reduce the computational complexity of thealgorithm by using Particle Swarm Optimization (PSO) technique.Additionally, a new initialization procedure is proposed to getbetter estimates and to improve the processing time of thealgorithm. Moreover, performance of the SAGE algorithm andimprovements on the algorithm are tested via extensive measurementdata. It is found that SAGE algorithm is a powerful tool for channelestimation and it can further be improved by the aforementioned propositions.
MIMO kablosuz haberleşme sistemleri için anten analizi/tasarımı ve yayılım kanalı modellemesi
Multiple-input-multiple-output (MIMO) wireless communication systems have been attracting huge interest, since a boost in the data rate was shown to be possible, using multiple antennas both at the transmitter and receiver. It is obvious that the electromagnetic effects of the multiple antennas have to be included in the wireless channel for an accurate system design, though they are often neglected by the early studies.In this thesis, the MIMO channel is investigated from an electromagnetics point of view. A full-wave channel model based on the method of moments solution of the electric field integral equation is developed and used in order to evaluate the MIMO channel matrix accurately. The model is called the channel model with electric fields (MEF) and it calculates the exact fields via the radiation integrals, and hence, it is rigorous except the random scatterer environment.The accuracy of the model is further verified by the measurement results. Thus, it is concluded that MEF achieves the accuracy over other approaches which are incapable of analyzing antenna effects in detail.Using the technique, MIMO performance of printed dipole arrays is analyzed. Effects of the electrical properties of printed dipoles on the MIMO capacity are explored. Appropriate dielectric slab configurations yielding high capacity printed dipole arrays are presented. The numerical efficiency of the technique allows analyzing MIMO performance of arrays with large number of antennas, and high performance array design in conjunction with well-known optimization tools. Thus, MEF is combined with particle swarm optimization (PSO) to design MIMO arrays of dipole elements for superior capacity. Freestanding and printed dipole arrays are analyzed and optimized.MIMO performance of printed rectangular patch arrays is analyzed using a modified version of MEF. Various array configurations are designed, manufactured, and their MIMO performance is measured in an indoor environment. The channel properties, such as the power delay profile, mean excess delay and delay spread, are obtained via measurements and compared with MEF results. Very good agreement is achieved.
Mükemmel iletken bir kama üzerindeki empedans cisimlerden saçılma
In this study, scattering from impedance bodies positioned at the edge of a perfectly conducting (PEC) wedge is investigated. In the treatment of the problem, eigenfunction expansion in terms of spherical vector wave functions is employed. A complete dyadic Green?s function for the spherical impedance boss at the edge is developed and through decomposing the dyadic Green?s function, it can be observed that the contribution of the scatterer is separated from the wedge. It is shown that the scattering is highly enhanced by the edge guided waves. For the general case of irregularly shaped scatterer the solution is extended using T-matrix method. The method is implemented by replacing free space Green?s function with the dyadic Green?s function of the PEC wedge. The solution is verified by applying it to the case of spherical scatterer and results are compared with the dyadic Green?s function solution. The T-matrix solution is generalized for the multiple scatterer case. Numerical results are obtained for two impedance scatterers at the edge and compared with the PEC case.