Bir dairesel polarizasyonlu mikroşerit antende eksenel oran frekans bant genişliğinin mantar biçimli bir elektromanyetik bant aralığı yapısı ile artırılması
Axial ratio frequency bandwidth enhancement in a circularly polarized microstrip antenna using mushroom-like electromagnetic band gap structure
Yazar: ALİCAN UYSAL
Danışman: DOÇ. DR. CEVDET IŞIK
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Elektronik ve Haberleşme Mühendisliği Ana Bilim Dalı / Telekomünikasyon Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering
Dizin: Mikroşerit antenler = Microstrip antennas

Özet:
Bu çalışmada ISM 2.4 bandında çalışan dairesel polarizasyonlu bir mikroşerit antenin eksenel oran bant genişliğinin, etrafına yerleştirilen polarizasyon bağımlı mantar biçimli düzlemsel elektromanyetik bant aralığı yapıları ile artırılması hedeflenmiştir. Bilindiği üzere tek beslemeli mikroşerit antenlerin dairesel polarizasyon bant genişliği çok dardır. Eksenel oranın 3 dB'in altında kaldığı bant aralığı göz önüne alındığında bu bant genişliğinin yüzde 2-3 civarında olduğu görülmektedir. Çoklu besleme ve birden fazla eleman kullanımı gibi teknikler ile bant genişliği artırılabilse de bu yaklaşım, genellikle antenin besleme hattının karmaşıklaşması veya anten toplam boyutunun artması problemini beraberinde getirmektedir. 
Elektromanyetik bant aralığı yapıları, metamalzeme olarak adlandırılan yapay malzemelerin bir alt dalıdır. Mantar biçimli elektromanyetik bant aralığı yapıları ise daha özel bir kavramdır. Bu yapıların üst kısmı iletken bir şerit halinde kare, altıgen gibi düzgün bir geometrik şekle sahiptir. Yapının alt kısmı elektriksel iletken bir toprak yüzeyi ile kaplıdır ve yapının ortasında üst kısmı ile alt kısmını birbirine bağlayan iletken yollar bulunur. Bu yapılar, elemanlar arasındaki boşluk ve üst kısmı alt kısma bağlayan iletken yollar nedeniyle bir LC devresi ile modellenebilir ve rezonans devresi gibi davranır. Bu özelliği sayesinde bu yapıların yasak bant oluşturduğu bilinmektedir.
Polarizasyon bağlımlı elektromanyetik bant aralığı yapıları ise dalganın farklı polarizasyonuna farklı tepkiler veren yapılardır. Mantar biçimli polarizasyon bağımlı yapılar ise genellikle üst iletkeninin şeklinin farklı polarizasyonlarda farklı tepkiler verecek şekilde dikdörtgen yapı olarak veya iletken yolun yerinin değiştirilmesi ile gerçeklenir. Bu şekilde farklı polarizasyonlu dalgaların göreceği endüktans değeri  değişeceğinden rezonans frekansı da değişecektir.
Bu tezde polarizasyon bağımlı elektromanyetik bant aralığı yapılarının uygun konfigürasyonda dairesel polarizasyonlu bir mikroşerit antenin eksenel oran bant genişliğini artırabildiği gösterilmiştir. Anten etrafına yerleştirilen 12 adet dikdörtgen şekilli mantar biçimli elektromanyetik bant aralığı yapısı sayesinde %2-3 civarında olan 3dB eksenel oran bant genişliği %6-7 civarına çıkarılmıştır. Bu da ISM 2.4 bandının tek beslemeli bir mikroşerit anten ile düşük profilli bir yapı kullanılarak kapsanması anlamına gelmektedir. Tasarımın boyutu, geleneksel kare yama antene göre bir miktar daha büyük olsa da bu bant genişliğini elde etmek için tasarlanan dizi yapılarından daha az yer kapladığı söylenebilir.

Abstract:
Wireless communication using circularly polarized electromagnetic waves becoming popular since such waves are beneficial in some cases. They are used in especially deep space communication where polarization mismatch losses can be higher due to the imperfect alignment of linearly polarized transmitter and receiver antennas. Some other benefits of circularly polarized waves include multipath fading rejection and eliminating the effect of Faraday rotation through ionized media. 
In order to obtain circularly polarized electromagnetic waves it is necessary to use antennas capable of radiating circularly polarized waves i.e. circularly polarized antennas. Since most of the antennas radiate electromagnetic waves with linear polarization related to their structural nature, circular polarization operation requires care and specific designs different from some canonically shaped structures.
Antenna studies in the literature mostly related to linearly polarized antennas. Main purposes on these studies usually are improvement their features such as expanding impedance bandwidth, dual or multi band operation, improvement in gain and efficiency etc. Usually having acceptable return loss within a frequency band is adequate to conclude that the structure is radiating electromagnetic waves in this band. Radiation characteristics are further concern, which gives the idea of spatial power distribution (antenna pattern). In these studies, mostly, since antenna polarization is presumed as linear, polarization characteristics of the antenna are not investigated deeply, it is enough to compare co-pol and cross-pol linearly polarized components of the radiated power from the antenna.
A circularly polarized antenna has a different feature, which is one of the most important parameters as well as its impedance matching. Axial ratio is a term which is mostly defined and used with elliptic and circularly polarized antenna. Being a radiation characteristics, it is defined as the ratio of the major and minor axis of the polarization ellipse of the radiated field. Since polarization ellipse is very narrow for linearly polarized antennas, they do not need this definition (or axial ratio is very high).
Obtaining a high quality circular polarization requires antenna to have very low axial ratio, ideally 1 (0 dB). This situation corresponds to polarization ellipse to be pure circle. Since it is not possible to achieve 0 dB axial ratio exactly, in literature axial ratio below 3dB is accepted to show an antenna to be circularly polarized. Thus, axial ratio bandwidth is defined as the frequency band where axial ratio of the antenna remains under 3 dB in a specified direction usually in the direction of main beam.
Microstrip antennas are the antennas with a dielectic insulator material. Due to their benefits and production easiness, they attracted much attention since they were discovered. Along with any other advantages, these antennas can have circular polarization operation mode when designed properly. In literature, so far, some antennas have been introduced that can radiate circularly polarized waves with single patch structure. Having such an advantage, however, axial ratio bandwidth of such structures are usually very narrow which is not adequate for use within an application. Depending on the substrate quality factor and thickness, mostly 2-3 percent relative bandwidth where the axial ratio is below 3 dB. Such a bandwidth is usually narrower than antenna impedance bandwidth, thus being limiting factor to the antenna operation frequency band. In literature, also there are some approaches to extend its circular polarization bandwidth. Most known and used approaches are use of multiple feeding with single element and sequentially rotated antennas in which antenna elements are in an array configuration, with rotated orientations and fed with a phase difference. These techniques shows great improvement in frequency dependency of axial ratio, widening bandwidth significantly. On the other hand, since the phase difference is provided by quarter wave transmission line, such designs are still frequency dependent. Another problems such as being bulky, lossy and more complicated arises with the use of power splitters and quarter wave microstrip lines. Port isolation is also a problem when multiple feed is used.
Electromagnetic band gap structures are branch of metamaterials, the structures designed artificially to provide electromagnetic behavior that naturally does not exist. Electromagnetic band gap structures are the structures, which provide a forbidden gap and does not pass electromagnetic waves within this band. Such behavior can be described as a spatial filter, which can be used for several purposes. These structures mainly gain their behavior with periodicity of a cell structure and the structure of the cell itself. Along with one and three dimensional variants, most popular electromagnetic band gap structures are built in two dimensions, which are periodic in two dimensions. This allows to use printed circuit board manufacturing techniques, thus provides easy realization. 
Mushroom-like electromagnetic band gap structures are specific form of two dimensional electromagnetic band gap structures. They consist of a perfect conductor bottom plate, conducting top side, which can be a square, circle etc. and one or more conducting vias, which connect top and bottom sides. Between top and bottom sides, a dielectric insulator, especially a printed board material is placed. Since the shape of conducting parts is similar to mushroom, the structure is called mushroom-like electromagnetic band gap structure.
Being a spatial filter, mushroom-like electromagnetic band gap structures are becoming popular for antenna applications. Such structures are mainly used for two different purposes in an antenna application. Since the structure has a forbidden band a coplanar configuration with a microstrip antenna is beneficial to suppress surface waves, therefore antenna directivity, gain and efficiency can be improved. This ability also improves antenna radiation pattern by lowering side and back lobes. Another configuration of this structure is to use with wire antennas. According to image theory, image of a horizontal dipole on a ground plane cancels main source if the antenna is placed very close to ground plane. For efficient radiation, antenna must be placed on quarter wave above the ground plane. This causes the antenna height to be increased. Mushroom-like electromagnetic structure can be used to overcome this problem, by providing an artificial magnetic plane. It was shown that, this structures show zero phase reflection coefficient at a certain frequency. This ability allows these structures to be used as ground plane. Since the structure provides in phase reflection coefficient, it is possible to realize low profile antennas with these structures.
Research on mushroom-like electromagnetic structures show that these structures can be polarization dependent in terms of reflection coefficient. It was made possible by using shapes, such as rectangular. It was realized that frequency dependency of reflection coefficient phase differs for different polarization. However, in literature, research on these structures mainly focuses on reflection coefficient behavior. Their coplanar configuration with microstrip antennas can be a probable research topic.
In this work coplanarly placed polarization dependent electromagnetic band gap structure with a circularly polarized microstrip antenna is investigated. Rectangular shaped electromagnetic band gap structure is used as unit cell. Main purpose of this study is to extend axial ratio bandwidth of circularly polarized microstrip antenna. By using 12 rectangular mushroom-like electromagnetic band gap structure surrounding a truncated corner patch antenna with coaxial feed, it was shown that a secondary minimum point can be obtained in the frequency dependency of axial ratio at broadside direction. Taking advantage of this, by selecting dimensions properly, this second minimum point can be brought close to the minimum caused by patch antenna itself. This approach introduced a ripple in axial ratio graph, therefore extending its bandwidth. At the end of the study, a final design is produced and tested. Both simulation and experimental results show that axial ratio bandwidth is extended from 2-3 percent to 6-7 percent when 3 dB axial ratio limit is considered. It was also observed that this structure also causes the antenna gain to be increased about 1 dB in the operation band. Since this method uses single coaxial feed and there is no distribution network antenna can be realized with low loss without complex feeding networks. Although antenna size is also increased along its performance, it is still smaller to that of four element sequentially rotated arrays. Having such features, designed antenna is a candidate for middle gain broadband applications.