Mikrodalga görüntüleme uygulamaları için ultra geniş bant anten tasarımı
Design of ultra wide band antenna for microwave imaging applications
Yazar: EMRE KIZILAY
Danışman: PROF. DR. İBRAHİM AKDUMAN
İstanbul Teknik Üniversitesi / Bilişim Enstitüsü / İletişim Sistemleri Ana Bilim Dalı / Uydu Haberleşmesi ve Uzaktan Algılama Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering

Özet:
MİKRODALGA GÖRÜNTÜLEME UYGULAMALARI İÇİN ULTRA GENİŞ BANT ANTEN TASARIMI    
                                              
                                              ÖZET

Mikroşerit antenler boyutları, hafiflikleri ve kolay uygulanabilir olmaları nedeniyle sıklıkla tercih edilir.Yüksek performanslı uçak, füze ve uydu uygulamalarında rahatlıkla kullanılabilirler. Mikroşerit antenler düşük profilli küçük ve hafif yapıları ile mikrodalga tümleşik devreler ile uyumludurlar.Bu avantajlardan dolayı mobil cihazlarda yaygın olarak kullanılmaktadır.Fakat Birçok avantajının yanı sıra mikroşerit yama antenlerin düşük verimlilikleri, düşük güç kapasiteleri, düşük bant genişlikleri gibi dezavantajları bulunmaktadır.
Kanser tüm dünyada giderek artan sağlık sorunlarının en başında gelmektedir. Mikrodalga görüntüleme sistemleri tehşiste sağlıklı hücreler ile kanser hücrelerinin arasındaki yüksek  dielektrik sabiti farkından faydalanmaktadır. Ayrıca x-ışını görüntülemenin aksine iyonlaştırıcı olmayan radyasyon kullanmasından dolayı sağlıklı dokular için zarasızdır. Tümörlü dokunun büyüklüğünün ve yerinin yüksek doğrulukta tespitinin yapılabilmesi için  tasarımı yapılacak olan antenlerin istenilen uygun kazanç değerlerinde, yüksek verimlilikte, geniş bantta, uygun ışıma örüntüsünde olması istenir. 
Ultra geniş bant antenler yüksek yönlülükleri, yüksek kazançları, geniş bandda çalışmaları kolay üretimlerinden dolayı mikrodalga görüntüleme uygulamarında sıklıkla kullanılırlar. Ultra geniş bant antenlerin çoğu  3.1-10.6 GHz kominikasyon bandında düzenlenir ama mikrodalga görüntüleme uygulamaları için çalışma frekansının, spektrumun düşük ve yüksek taraflarından yayılarak bant genişliğinin artırılması hedeflenir. Yukarıda bahsedilen uygulamar dolay, ilk olarak Gibson tarafından sunulan konik oluklu (Vivadi) antenler mikrodalga görüntüleme uygulamalarında sıklıkla tercih edilir.
Bu çalışma kapsamında öcelikle antenler, anten tipleri, temel anten parametreleri genel olarak tanıtılmıştır. Ayrıca tasarımı yapılacak antene ilişkin temel bilgiler olması açısından mikroşerit antenlerin genel yapısı,  özellikleri ve türleri ile ilgili genel bilgiler verilmiştir. Bu genel bilgilerden sonra antenin  tasarımı yapılmış ve elde edilen simülasyon sonuçları ve ölçüm sonuçları karşılaştırılarak yorumlanmıştır.

Abstract:
DESIGN OF ULTRA WIDE BAND ANTENNA FOR MICROWAVE IMAGING APPLICATIONS
                                      
                                            SUMMARY

Microstrip antennas are frequently used because of their size, lightness and easy application. They can be used in high performance aircraft, missile and satellite applications. Microstrip antennas are compatible with low-profile small and lightweight structures and microwave integrated circuits. They are widely used in mobile devices due to these advantages.
Cancer is one of the leading health problems in the world. Microwave imaging systems benefit from the high dielectric constant difference between healthy cells and cancer cells. Unlike x-ray imaging, it is harmless for healthy tissues as it uses non-ionizing radiation. The antennas to be designed in order to be able to determine the size and location of the tumor tissue with high accuracy are required to be in the appropriate radiating pattern in the desired efficiency values, high efficiency, wide band.
Ultra-wide band antennas are often used in microwave imaging applications because of their high versatility, high gain and easy to operate in wide band. Most of the ultra- wide band antennas are regulated in the 3.1-10.6 GHz communication band, but for microwave imaging applications, the frequency of operation is aimed at increasing the bandwidth by extending both on the lower and higher side of the spectrum. Due to the above mentioned applications, the Vivadi antennas first offered by Gibson are often preferred in microwave imaging applications.
Within the scope of this study, antennas, antenna types and basic antenna parameters are introduced. In addition, general information about the general structure, properties and types of microstrip antennas is given in order to provide basic information about the antenna to be designed. After this general information, the antenna was designed and the results of the simulation were compared with the results of the measurements and results were commented.
Antennas are one of the most important parts of microwave imaging systems both as receiver and transmitter. The antenna designed for this is required to have a high efficiency, suitable radiation pattern and gain value. In this study, ultra wideband Vivaldi antenna design is used for microwave imaging applications. The design was realized in two stages and the first step was to design the classic Vivaldi antenna. In the second step, slots are added to the antenna designed to increase the bandwidth of the antenna.



In the second part, it is aimed to give general information about antennas. General information about antenna definition, historical development, antenna types and basic antenna parameters. 
In the third section, it is aimed to give general information about microstrip antennas. General structure and properties of microstrip antennas, feeding methods and analysis methods will be given.
The main goal of the fourth refinement study is to design the corrugated ultra-wide band Vivaldi antenna. Firstly, general information about CST simulation program used in antenna design and simulation will be given and two-stage design will be introduced. We will then examine the measurement results obtained from the laboratory experience of the purification antenna.
In the fifth chapter, the simulation results obtained with the cst simulation program will be compared with the measurement results obtained in the laboratory environment and the experiences obtained by examining these comparison results will be explained.
Summary of the fourth section where the design is made and the antenna is realized and the fifth section where the results are compared are given below.
It will first design the UGB vivaldi antenna that can operate on the 3.1 -10.6 GHz communication band. Later, the transverse slots will be placed on the edges of the antenna in the exponential contraction structure of this designed antenna and these slots are expected to be positive and oriented. 
The basic parameters in UGB vivaldi antenna design are bandwidth, dielectric property of the base material, and height of the base material. In addition, the antenna to be designed must be of a size that can be used in microwave imaging devices, should be easy to install and have high durability. It is highly effective in terms of dielectric constant system performance of the selected base material. In general, the materials used in microstrip antenna design are dielectric constants in the range of 2.2-12. The structural size of the antenna varies according to the dielelectric constant of the substrate. Generally, the larger the dielectric constant, the smaller the antenna size. However, in designs made with materials with low dielectric constant, antennas with higher bandwidth and efficiency are obtained despite increasing antenna sizes. Increasing the height of the dielectric material also increases the bandwidth and efficiency. However, unwanted surface waves are generated by increasing the height. these superficial waves cause disturbances in polarization and antenna pattern. Bandwidth value is maintained by eliminating the surface waves by various methods.
Vivaldi antennas are widely used in microwave imaging methods due to their large bandwidth, high directivity, symmetrical antenna patterns and low side lobe levels. In Vivaldi antennas, there is an exponential constricted radiation region on one side of the base material, and on the other side there is a feed line and a passage from the feed line to the groove line. Vivaldi antennas are used in radar systems because of their light weight, small size and ultra wide frequency band. Rogers RT 6002, which is 0.762 mm high εr = 2.94, was preferred as the dielectric base material due to its ability to cover the 3.1-10.6 GHz frequency band, performance and ease of production. The designed antenna must meet the requirements of S11 ≤ -10 dB and VSWR ≤ 2 for efficient radiation.

Looking at these conditions, the antenna we design is able to radiate efficiently between the frequencies of 2.25 - 14.59 GHz. The antenna radiates to cover the 3.1 - 10.6 GHz frequency band that we are aiming for. In addition, VSWR takes very small values in the bandwidth that the antenna radiates, and it is observed that reflections and standing wave ratios at the antenna input have minimum values.
The transverse slots of the antenna will be placed in the exponential contraction structure of the designed UGB vivaldi antenna operating in the 2.25 - 14.6 GHz frequency band and the positive and negative effects of these slots will be examined. The purpose of this design is to extend the antenna's bandwidth by lowering the lower limit of the operating frequency band and to obtain better directivity at low frequencies.
The designed antenna must meet the requirements of S11 ≤ -10 dB and VSWR ≤ 2 for efficient radiation. When the simulation data obtained under these conditions were obtained, an UGB antenna was obtained which gives the desired gain and directivity values in the 1.63-14.54 GHZ frequency band of the corrugated vivaldi antenna.
When the results of the measurement in the laboratory were examined, it was observed that the antenna radiated in the frequency range of 1.74-12.84 GHz. During the design, we can explain the difference between the S11 parameter obtained from the simulation results and the results of the S11 parameter change obtained as a result of the measurement of the antenna, as the error rate of the simulation program and the errors that occur during the production phase.
Consequently, UGB conventional vivaldi antenna design has been designed to be used in microwave imaging which can radiate between 2.25 - 14.6 GHz frequencies. Then, slots were added to this antenna to gain greater bandwidth and higher directivity and gain at frequencies below 6 GHz. In the studies carried out with the CST simulation program, the bandwidth (1.63-14.54 GHz) increased as the antenna started to radiate at a smaller frequency thanks to the slots added to the antenna. In addition, gain and  directivity were  increased at  frequencies of 5 GHz and below. At the larger frequencies, no significant change was observed in terms of gain and directivity. The reasons for the differences between the simulation results obtained in the design of the antenna and the measurement results made in the laboratory can be said as the errors in the production process and the error rate of the simulation program.
In studies using CST simulation program, dielectric constant and height change of dielectric base material causes major changes in directionality and gain in radiation bandwidth. In addition, changes in the exponential structure and the feed line affect the bandwidth gain and directivity. It has been found that with the slots added to the antenna, better gain and directivity of radiation at lower frequencies can be achieved. In later studies, constructions of conical corrugated antennas with different contraction structure can be made by adding grooves of different sizes and shapes and designs with better features can be made.