Ultra geniş bant radar görüntüleme sistemleri için metamalzeme tabanlı kompakt vivaldi anten tasarımı
Design of a metamaterial-based compact vivaldi antenna for ultra-wideband radar imaging systems
- Tez No: 1024238
- Danışmanlar: PROF. DR. ÖZGÜR ÖZDEMİR
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2026
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Lisansüstü Eğitim Enstitüsü
- Ana Bilim Dalı: Elektronik ve Haberleşme Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Telekomünikasyon Mühendisliği Bilim Dalı
- Sayfa Sayısı: Belirtilmemiş.
Özet
Dielektrik ve manyetik geçirgenlik parametreleri olan, ε ve μ, elektromanyetik dalga yayılım davranışını belirlemektedir. Bu nedenle doğadaki malzemelerin elektromayetik olarak sınıflandırılmasında ε ve μ parametreleri etkilidir. Bu parametrelerin işaretine göre, doğadaki malzemeler dört gruba ayrılmış durumdadır. ε ve μ parametresinin ikisininde pozitif olduğu maddeler, doğada kolaylıkla bulunabilen yalıtkan ortamları ve dielektrik tabakaları temsil etmektedir. Sağ elli malzemeler olarak adlandırılmaktadır. ε<0 ve μ>0 olduğu durum, belirli frekanslarda yayılımın olmadığı, plazma benzeri ortamları temsil etmektedir. ε>0 ve μ<0 olduğu durum da belirli bir mesafeden sonra dalga yayılımın olmadığı ortamları temsil etmektedir. Gyrotropik maddeler, belirli frekanslarda gösterdiği manyetik tepki ile bu grup içerisinde yer almaktadır. ε ve μ parametrelerinin her ikisinin de 0'dan küçük olduğu senaryoda, sol elli olarak olarak adlandırılan, doğada kendiliğinden bulunmayan, negatif indisli malzemeler ortaya çıkmaktadır. Snell yasasına göre, ters kırılma davranışı gibi, geleneksel malzemelerin sahip olmadığı olağan dışı elektromanyetiksel özellikler sergilemektedir. Bu malzemelerin, elektromanyetiksel dalga yayılımını kontrol etme özellikleri nedeniyle metmalzemeler, son yıllarda yoğun ilgi duyulan araştırma ve uygulama konularından biri olmuştur. Metamalzemelerin elektromanyetik dalgayı, kontrol etme yeteneği sayesinde antenlerde, ışıma paterni üzerinde etki göstererek, hüzme şekillendirme, yönlendirme ve daraltma, kazanç ve verim arttırımı, özellikle dizi antenlerde, anten birim elemanları arası kuplaj etkisinin azaltımı gibi birçok uygulamada kullanılmaktadır. Böylece yüksek kazanç ve yönlülüğe ihtiyaç duyulan anten sistemlerinde ışıma performansının iyileştirilmesine yönelik etkili ve alternatif bir çözüm sunmaktadır. Bu çalışma kapsamında 3.1-4.8 GHz frekans bandında çalışan ultra geniş bantlı bir radar sistemi için metamalzemelerle desteklenmiş Vivaldi anten tasarımı önerilmiştir. İlk olarak, uygulamaya yönelik, istenilen frekans bandında ışıma yapan, FR-4 dielektrik tabakası üzerinde, Vivaldi anten tasarımı yapılmış ve metamalzeme enegrasyonu için referans olarak alınmıştır. Ardından ayrık halka rezonatörü (Split-Ring Resonator (SRR)) tabanlı, istenilen frekans bant aralığının belirli bölümünde çift negatif (Double Negative (DNG)) karakteristik gösteren üç farklı birim hücre tasarımı gerçekleştirilmiştir. Tasarlanan birim hücreler MTM_1, MTM_2 ve MTM_3 olarak adlandırılmıştır. Birim hücrelerin ε_eff, μ_eff ve n_eff parametreleri NRW (Nicolson-Ross-Weir) algoritması ile elde edilmiştir. Tasarımı yapılan birim hücrelerin rezonans frekansları ve DNG karakteristik gösteren frekans bant genişliği karşılaştırılmış olup, MTM_3 birim hücresi, referans anten üzerine entegre edilmiştir. Referans anten ile MTM_3 birim hücresi ile desteklenmiş olan Vivaldi antenin ışıma performansı karşılaştırılmıştır. Farklı olarak, 3.1-4.58 GHz'de, sıfıra yakın indisli (Near Zero Index (NZI)); 4.58-4.8 GHz'de, DNG karakteristik gösteren U şekilli metamalzeme birim hücre tasarımı gerçekleştirilmiştir. DNG karakteristik gösteren frekans bant genişliğinin arttırılması için, daha önce tasarımı yapılan birim hücrelerden farklı olarak, RT5880 dielektrik tabakası üzerinde oluşturulan, U_2 isimli, yeni bir birim hücre tasarımı önerilmiştir. Tasarımı yapılan birim hücreler, Vivaldi antenin açıklık bölümüne ve üst kısmına entegre edilmiştir. U_1, U_1 ve U_2 birim hücreler ile desteklenmiş Vivaldi anten tasarımları ve referans antenin çalışma performansı karşılaştırılmıştır. Son olarak, U_1 metamalzeme birim hücresi ile desteklenmiş Vivaldi antenin ışıma performansının daha da iyileştirilmesi için, antenin her iki yanına, dikdörgen şekilli oluklar eklenmiştir. Elde edilen anten tasarımının performans analizleri gerçekleştirilmiştir. Metamalzeme birim hücrelerinin entegrasyonu sonucu, referans antenin ultra geniş bantlı çalışma karakteristiği korunmuş olup, daha yönlü ve kazançlı yeni Vivaldi anten tasarımları elde edilmiştir. U_1 metamalzeme birim hücresi ve dikdörtgen olukların eklenmesiyle elde edilen anten tasarımlarının üretimi gerçekleştirilmiş olup, S11 parametrelerinin VNA (Network Ağ Analizörü (Vector Network Analyzer)) cihazı ile ölçümü gerçekleştirilmiştir. Ölçüm sonuçlarında simülasyon değerlerine göre bir miktar sapma olduğu gözlemlenmiş olsa da, genel eğilim ve rezonans frekansların simülasyon sonuçları ile tutarlı olduğu görülmüştür.
Özet (Çeviri)
The dielectric permittivity and magnetic permeability parameters, denoted by ε and μ, determine the propagation behavior of electromagnetic waves. For this reason, the parameters ε and μ are effective in the electromagnetic classification of materials found in nature. According to the signs of these parameters, materials in nature can be divided into four groups. Materials for which both ε and μ are positive represent insulating media and dielectric layers that can be easily found in nature. These materials are referred to as right-handed materials. The case ε<0 and μ>0 represents plasma-like media in which wave propagation does not occur at certain frequencies. The case ε>0 and μ<0 represents media in which wave propagation ceases after a certain distance. Gyrotropic materials, due to the magnetic response they exhibit at specific frequencies, are considered to belong to this group. In the scenario where both ε and μ are less than zero, materials that do not occur spontaneously in nature and that have a negative refractive index, referred to as left-handed materials, arise. According to Snell's law, they exhibit unconventional electromagnetic properties that are not possessed by conventional materials, such as a reversed refraction behavior. Owing to their ability to control electromagnetic wave propagation, metamaterials have become one of the most intensively studied research and application topics in recent years. Thanks to their ability to control electromagnetic waves, metamaterials are used in many antenna-related applications, such as influencing the radiation pattern, beam shaping, steering and narrowing, enhancing gain and radiation efficiency, and particularly in antenna arrays, reducing the mutual coupling effect between individual antenna elements. In this way, they offer an effective and alternative solution for improving the radiation performance of antenna systems that require high gain and high directivity. Within the scope of this study, a metamaterial-assisted Vivaldi antenna design operating in the 3.1–4.8 GHz frequency band is proposed for an ultra-wideband radar system. The main objective is to improve the radiation performance of a conventional Vivaldi antenna by integrating appropriately designed metamaterial unit cells and geometrical modifications, and to demonstrate the potential of such structures for remote sensing and radar applications that demand wide bandwidth, high gain, and stable directional characteristics. As a first step, a Vivaldi antenna was designed on an FR-4 dielectric substrate to operate and radiate within the desired frequency band for the intended application. In designing this reference antenna, attention was paid to ensuring that the antenna exhibits sufficient impedance matching over the 3.1–4.8 GHz band, acceptable return loss levels, and a stable radiation pattern suitable for ultra-wideband radar operation. This antenna configuration, realized on the FR-4 substrate, is taken as the reference design for the subsequent metamaterial integration process. The reference antenna serves as a baseline structure against which the improvements achieved by the proposed metamaterial unit cells can be quantitatively evaluated in terms of parameters such as gain, directivity, side-lobe levels, and radiation efficiency. Following the realization of the reference Vivaldi antenna, three different SSR-based (Split-Ring Resonator) metamaterial unit cell designs were developed. These unit cells were specifically tailored to exhibit double-negative (DNG) characteristics, i.e., simultaneously negative effective permittivity and effective permeability, within certain portions of the targeted frequency band. Each unit cell was carefully modeled and optimized so that, in addition to achieving DNG behavior, the corresponding resonant frequencies and operational bandwidths would be compatible with the 3.1–4.8 GHz antenna operating range. For an accurate characterization of the electromagnetic behavior of the designed cells, the effective parameters ε_eff, μ_eff and n_eff were retrieved using the Nicolson–Ross–Weir (NRW) algorithm. This well-known parameter extraction method allowed the determination of the frequency intervals where both ε_eff and μ_eff become negative, thus confirming the DNG nature of each unit cell and enabling a systematic comparison between them. The designed unit cells were then compared in terms of their resonant frequencies, the width of the frequency bands over which they exhibit DNG characteristics, and their potential impact on antenna performance. The designed unit cells are referred to as MTM_1, MTM_2, and MTM_3. As a result of this comparative analysis, the unit cell denoted as MTM_3 was identified as the most promising candidate for integration, as it provided a suitable DNG band overlapping with the operating band of the Vivaldi antenna and offered favorable resonance behavior. Subsequently, the MTM_3 unit cell was integrated into the reference Vivaldi antenna structure in an appropriate configuration. The metamaterial-loaded Vivaldi antenna obtained in this way was then analyzed and its radiation performance was compared with that of the reference antenna in detail. In this comparison, key performance criteria such as reflection coefficient (S11), impedance bandwidth, realized gain, beamwidth, and radiation pattern shape were taken into account, and the advantages of using the MTM_3 metamaterial unit cell were clearly demonstrated. In addition to the MTM_3-based design, a U-shaped metamaterial unit cell was developed to provide a more versatile electromagnetic response over different subbands of the operating frequency range. In particular, a U-shaped unit cell was designed that exhibits a near-zero refractive index in the 3.1–4.58 GHz band and DNG characteristics in the 4.58–4.8 GHz band. The near-zero-index (NZI) behavior at the lower portion of the band allows the phase of the propagating wave to vary very slowly inside the metamaterial, which can be used to manipulate the phase distribution across the antenna aperture. On the other hand, the DNG behavior at the upper portion of the band offers additional control over wave propagation, enabling further shaping and focusing of the radiated fields. In this way, by combining NZI and DNG responses within different frequency regions, the U-shaped unit cell provides a richer set of tools for controlling the electromagnetic field distribution in and around the Vivaldi antenna. To further increase the bandwidth over which DNG characteristics are observed and to obtain a more efficient metamaterial response, a new unit cell, referred to as U_2, was proposed. Unlike the previously designed unit cells, the U_2 structure was implemented on an RT5880 dielectric substrate, which has lower dielectric losses and a lower relative permittivity compared to FR-4. The use of RT5880 helps to reduce dielectric losses, especially at higher frequencies, and improves the quality factor of the resonant behavior of the unit cell. As a result, the U_2 unit cell offers a broader and more pronounced DNG frequency band, thereby providing a more effective interaction with the electromagnetic fields generated by the Vivaldi antenna. The designed metamaterial unit cells, namely U_1 (the first U-shaped design) and U_2, were integrated into different regions of the Vivaldi antenna to investigate their impact on the antenna performance. In particular, these unit cells were placed around the aperture region and on the upper part of the antenna structure, where they can strongly interact with the fields radiated by the antenna. By carefully choosing the positions and configurations of these metamaterial inclusions, it is possible to alter the current distribution on the antenna conductors and to shape the near-field distribution at the aperture, which in turn affects the far-field radiation pattern. Several Vivaldi antenna configurations supported by U_1 and U_2 metamaterial unit cells were designed and simulated, and their performance was compared with that of the reference Vivaldi antenna. In these comparisons, the effects of metamaterial loading on parameters such as impedance matching, gain, directivity, side-lobe levels, and beamwidth were examined in detail. The obtained results show that Vivaldi antenna designs supported by U-shaped metamaterial unit cells achieve noticeable improvements in terms of radiation performance, especially in the upper portion of the operating band. In particular, it was observed that the gain and directivity values increase, the main beam becomes narrower and more focused, and the radiation pattern becomes more suitable for applications requiring high directivity. Furthermore, in certain configurations, the side-lobe levels are reduced, which is advantageous for radar and remote sensing systems that demand high angular resolution and low interference from undesired directions. Finally, in order to further enhance the radiation performance of the Vivaldi antenna supported by the U_1 metamaterial unit cell, an additional geometrical modification was introduced. Rectangular-shaped slots were added along both sides of the antenna. These slots effectively modify the surface current distribution on the metallic parts of the antenna and thereby provide an additional degree of freedom for controlling the aperture field distribution. By optimizing the dimensions, positions, and number of these rectangular slots, improvements in key performance parameters such as realized gain, and side-lobe suppression were achieved. In this way, the combination of metamaterial loading and geometrical shaping allowed the proposed antenna structure to reach superior radiation characteristics compared to the initial reference design. The performance analyses of the resulting antenna designs were carried out comprehensively. For each configuration, the reflection coefficient (S11), radiation patterns in the principal planes, realized gain, and radiation efficiency were evaluated across the 3.1–4.8 GHz frequency band. The comparison between the reference Vivaldi antenna and the metamaterial-assisted designs clearly demonstrates that the proposed metamaterial-based approaches provide an effective and practical solution for improving the radiation performance of ultra-wideband antennas used in radar and remote sensing applications. Thus, the metamaterial-supported Vivaldi antenna designs presented in this study constitute a strong and competitive alternative to conventional antenna design methods, particularly in systems where high gain, high directivity, and broad bandwidth are simultaneously required.
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