Radar uygulamalarında vivaldi anten tasarımı ve optimizasyonu
Vivaldi antenna design and optimization in radar applications
- Tez No: 1021300
- Danışmanlar: DR. ÖĞR. ÜYESİ SEBAHATTİN EKER
- 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: İletişim Sistemleri Ana Bilim Dalı
- Bilim Dalı: Uydu Haberleşmesi ve Uzaktan Algılama Bilim Dalı
- Sayfa Sayısı: Belirtilmemiş.
Özet
Radar teknolojilerindeki minyatürizasyon yaygınlığı ve sivil/ticari insansız hava araçlarının (drone) tespiti gibi modern uygulamalar; yüksek mesafe çözünürlüğü sunan geniş bantlı, kararlı hüzme yapısına sahip ve maliyet açısından uygun olan anten sistemleri ihtiyacını artırmıştır. Radar sistemlerinde hedeflerin hassas bir şekilde birbirinden ayırt edilebilmesi, doğrudan sistemin sahip olduğu frekans bant genişliği ile ilişkilidir. Geleneksel dar bantlı rezonans antenlerinin aksine,“yürüyen dalga”(traveling wave) mekanizması ile çalışan Kademeli Yarık Antenler (TSA), enerjinin dar bir yarıktan başlayıp giderek genişleyen bir açıklığa doğru ilerleyerek uzaya yayılması prensibiyle bu gereksinimlere etkili bir çözüm sunmaktadır. Bu tez çalışmasında, 5.8 GHz Endüstriyel, Bilimsel ve Tıbbi (ISM) bandında çalışan kısa menzilli radar sensörleri ve drone tespit sistemleri için kompakt yapılı (70x40 mm), düşük maliyetli ve etkin performansa sahip bir Vivaldi anten tasarlanmış, simülasyon ortamında optimize edilmiş ve nihai hali deneysel olarak doğrulanmıştır. Anten tasarımında, üretim kolaylığı ve ekonomik avantajları nedeniyle endüstriyel süreçlere tam uyumlu olan FR-4 dielektrik alt tabakası kullanılmıştır. FR-4 malzemenin yüksek frekanslardaki doğal yalıtkanlık kayıpları, uygulanan yapısal optimizasyonlar ile en aza indirilmiş ve malzemenin literatürdeki maksimum sınırlarında bir radyasyon verimliliği hedeflenmiştir. Elektromanyetik modelleme ve parametrik analiz safhaları CST Studio Suite 2023 yazılımında tam dalga 3D zaman alanı çözücüsü (Time Domain Solver) kullanılarak gerçekleştirilmiştir. Mikroşerit besleme hattından slot açıklığına azami seviyede enerji transferini sağlamak ve dengesiz (unbalanced) quasi-TEM modunu dengeli (balanced) temel TE moduna dönüştürmek üzere, teorik hesaplamalar doğrultusunda 6.8 mm uzunluğunda dairesel kısa devre (QWS) ve 7.6 mm uzunluğunda açık devre (QWM) çeyrek dalga boyu uyumlayıcıları tasarıma entegre edilmiştir. Ayrıca, mikroşerit ve yarık hattı kesişimindeki ofset ayarı ile kapasitif ve endüktif denge kurularak maksimum kuplaj elde edilmiştir. Elektromanyetik enerjinin uzaya yayılması, log-periyodik üstel açılan (exponential taper) ışıma profili ile sağlanmıştır. Vivaldi antenin ışıma mekanizmasında, dalganın anten yüzeyinden geri yansımaması ve serbest uzay empedansına (yaklaşık 377 Ω) pürüzsüz bir geçiş yapabilmesi için üstel açılma katsayısı iteratif yöntemlerle optimize edilmiştir. Bu üstel profil, yüksek frekanslı dalgaların yarığın dar kısımlarında, düşük frekanslı dalgaların ise açıklığın daha geniş olduğu bölgelerde uzaya yayılmasına olanak tanıyarak antene arzu edilen geniş bant karakteristiğini kazandırmıştır. Bu sayede, besleme noktasındaki düşük karakteristik empedans kademeli olarak artırılarak mükemmel bir empedans transformatörü etkisi yaratılmıştır. Teorik benzetimlerin fiziksel olarak doğrulanması amacıyla antenin prototipi üretilmiştir. Ölçüm hassasiyetini artırmak ve insan kaynaklı hizalama/açı hatalarını tamamen ortadan kaldırmak maksadıyla, alıcı antenin tam 360º yörüngede 10º artış adımlarıyla otomatik döndürüldüğü Arduino ve SG90 servo motor tabanlı yarı otomatize bir ölçüm düzeneği geliştirilmiştir. E-düzlemi (elektrik alan düzlemi) radyasyon diyagramı testleri, Anritsu MS2026C Vektör Ağ Analizörü (VNA) ve yüksek frekanslı RF koaksiyel kablolar kullanılarak yarı-yankısız oda ve uzak alan (Fraunhofer) koşullarında icra edilmiştir. Elde edilen bulgular, benzetim ve fiziksel ölçüm sonuçlarının yüksek bir doğrulukla örtüştüğünü göstermiştir. Gerçek kullanım senaryoları göz önüne alındığında, sistemin çalışacağı frekans aralığında hareket serbestisi ve Elektronik Korunma Tedbirlerine (ECCM) olanak sağlaması maksadıyla antenin geniş bant karakteristiği göstermesi hedeflenmiştir. Simülasyonlarda 562 MHz olarak hesaplanmış efektif empedans bant genişliği, fiziksel ölçümlerde 705 MHz'in üzerine çıkarak (5.205 GHz - 5.910 GHz aralığında) tasarımın sağlamlığını kanıtlamıştır. Fiziksel ölçümlerde 5.615 GHz asıl rezonans frekansında -44.10 dB seviyesinde (%99,99 empedans eşleşmesi) derin bir geri dönüş kaybı (S11) elde edilmiştir. Gerçekleştirilen E-düzlemi radyasyon ölçümlerinde antenin enerjiyi istenen son-ışıma (endfire) yönünde yüksek bir hassasiyetle odakladığı doğrulanmış olup; yarı güç ışın genişliği (HPBW) yaklaşık 26 derece, ön-arka ışıma oranı ise 12 dB seviyesinde ölçülmüştür. Teorik analizlerde antenin maksimum yönlülüğü 7.62 dBi, gerçekleşen kazancı 5.86 dBi ve toplam radyasyon verimliliği %70 düzeyinde hesaplanmış; -6.0 dB yan lob seviyesi ile çevresel saçılmalardan kaynaklı yanlış alarm olasılığı minimize edilmiştir. Sonuç olarak bu çalışma, literatürde yüksek dielektrik kayıpları nedeniyle dezavantajlı kabul edilen FR-4 malzemesinin, spesifik geometrik optimizasyonlar ve yürüyen dalga prensibinin doğru modellenmesi sayesinde yüksek performanslı radar sistemleri için uygun bir anten platformuna dönüştürülebileceğini ispatlamaktadır. Geliştirilen Vivaldi anten, maliyet-etkin ve 70x40 mm boyutlarındaki kompakt yapısıyla kısa menzilli radar sensörlerinin ve drone tespit mekanizmalarının endüstriyel boyutta üretilebilmesi için yenilikçi, güvenilir bir donanım altyapısı sunmaktadır.
Özet (Çeviri)
1. Introduction and Motivation The rapid proliferation of miniaturized radar technologies and modern detection applications, such as the monitoring and tracking of civil or commercial unmanned aerial vehicles (UAVs/drones), has significantly increased the demand for high performance antenna systems. In radar systems, range resolution is inversely proportional to the operating bandwidth; therefore, distinguishing closely spaced targets with high precision inherently requires ultra-wideband (UWB) characteristics. Furthermore, industrial and short-range radar (SRR) applications demand stable beam structures, high directivity, and cost-effective manufacturing processes to ensure commercial viability. Traditional planar antennas, such as microstrip patch antennas, often exhibit narrow bandwidths and are highly susceptible to frequency shifts caused by environmental factors or manufacturing tolerances. To overcome these limitations, Tapered Slot Antennas (TSAs), specifically Vivaldi antennas, present a highly effective solution. This thesis presents the comprehensive design, parametric optimization, full-wave electromagnetic simulation, and physical experimental validation of a compact, low cost, high-performance Vivaldi antenna operating in the 5.8 GHz Industrial, Scientific, and Medical (ISM) band, tailored specifically for short-range radar sensors and drone detection systems. 2. Theoretical Framework and The Traveling Wave Concept Unlike conventional resonant antennas, which operate on standing wave principles and inherently possess narrow bandwidths, the Vivaldi antenna operates on the“traveling wave”mechanism. In this structure, electromagnetic energy is injected into a narrow slotline and propagates along an exponentially expanding flare toward free space. Radiation occurs progressively at regions where the local width of the slot is approximately half the effective free-space wavelength (Wmax ≈ λ0/2) of the operating frequency. Since this active radiation region dynamically shifts along the longitudinal axis of the antenna as a function of frequency, the Vivaldi antenna inherently achieves a remarkably wide impedance bandwidth and stable endfire radiation characteristics. A critical challenge addressed in this study is the utilization of the FR-4 epoxy glass reinforced dielectric substrate (relative permittivity εr ≈ 4.3, loss tangent tanδ ≈ 0.02). In microwave and radar engineering literature, high-frequency laminates like PTFE or ceramic-based substrates (e.g., Rogers series) are heavily favored due to their minimal dielectric losses. FR-4 is conventionally considered disadvantageous for 5.8 GHz applications due to higher attenuation. However, the core hypothesis of this thesis is that through meticulous geometric configuration, precise impedance matching, and transition optimization, the inherent dielectric losses of FR-4 can be effectively mitigated, transforming it into a highly efficient, cost-effective platform for industrial scale radar sensors. 3. Design Methodology and Parametric Optimization The antenna was modeled and simulated using the Time Domain Solver in CST Studio Suite 2023. The overall dimensions of the planar substrate were analytically calculated and optimized to 70 x 40 mm with a thickness of 1.6 mm. To ensure maximum electromagnetic energy coupling from the unbalanced quasi TEM mode of the microstrip feedline to the balanced fundamental TE mode of the slotline, an orthogonal microstrip-to-slotline transition was designed. This crucial transition region incorporates theoretical quarter-wavelength matchers to prevent energy reflection and signal degradation. Specifically, a 7.6 mm open-circuit microstrip stub (QWM) and a 6.8 mm circular short-circuit slot stub (QWS) were integrated. The circular geometry of the slot stub introduces broadband reactive loading, which stabilizes the virtual short-circuit point across the entire frequency spectrum. Additionally, a structural offset of 3.00 mm between the microstrip and the slotline intersection was determined via parametric sweeping to optimally balance the capacitive and inductive coupling, pulling the return loss down to absolute minimums. To fully comprehend the impedance matching network's behavior, the orthogonal microstrip-to-slotline transition must be analyzed through its equivalent circuit parameters. The cross-over region acts as a combination of series and parallel reactances where the quasi-TEM mode of the microstrip feedline excites the balanced TE mode of the slotline. The circular slot stub (QWS) with a radius corresponding to the optimized 6.8 mm dimension provides a broadband virtual short circuit by introducing a wideband reactive load, which effectively prevents the backwards propagation of the surface currents towards the closed end of the antenna substrate. Concurrently, the 7.6 mm microstrip open stub (QWM) compensates for the inductive reactance introduced by the slotline discontinuity. By precisely tuning the transition offset to 3.00 mm, the capacitive and inductive components of the junction susceptibility were perfectly balanced, which theoretically explains the exceptional impedance matching and the absence of high-order parasitic modes within the 5.3 GHz to 5.8 GHz operational band. 4. Mathematical Modeling of the Exponential Taper The core radiation profile of the Vivaldi antenna is governed by a precise log-periodic exponential equation. In this study, the flare rate (the“a”coefficient) was identified as the most critical parameter governing the smooth impedance transition from the tightly coupled slotline (calculated via Cohn's slotline impedance formulas) to the 377 Ω intrinsic impedance of free space. Through iterative parametric analysis, the optimal flare rate was determined to be 0.054. This specific curvature ensures that high frequency waves radiate efficiently near the narrow throat of the slot, while lower frequency waves travel further down the flare before decoupling into space. By explicitly configuring the initial slot width (s = 0.36 mm) to perfectly match the 50 Ω feedline, abrupt impedance variations were mathematically eliminated, thereby suppressing undesirable back-reflections and enhancing the total radiation efficiency to theoretical maximums for an FR-4 substrate. A rigorous investigation into the material characterization of the FR-4 substrate reveals the underlying electromagnetic interactions that enabled high-performance operation despite the substrate's high loss tangent. In standard planar microstrip configurations, dielectric attenuation significantly degrades radiation efficiency at frequencies exceeding 5 GHz due to polarization delays within the epoxy-resin matrix. However, within the Vivaldi architecture, the electromagnetic energy transitions rapidly from the microstrip line into the slotline, where the fields are tightly confined within the gap (s = 0.36 mm) and progressively couple into the surrounding air along the exponential flare profile. This rapid decoupling limits the total volume of the dielectric substrate subjected to high-density field lines, thereby maintaining the radiation efficiency at a stable 70% level. The low Quality Factor (Q-factor) resulting from the natural loss mechanisms of the material further suppresses the sharp resonant peaks typical of low loss substrates, smoothly distributing the impedance matching across a broader spectrum and contributing to the expanded bandwidth. 5. Prototyping and Semi-Automated Experimental Setup To physically validate the theoretical simulation outputs, multiple identical prototypes of the optimized Vivaldi antenna were fabricated. Physical measurements were conducted in a semi-anechoic environment to isolate the system from multipath fading and environmental clutter. A unique contribution of the measurement methodology in this thesis is the development of an Arduino-based, semi-automated mechanical test setup. To eliminate human-induced alignment errors and ensure high repeatability during spatial radiation pattern extraction, the receiving (Rx) antenna was mounted on a customized platform driven by an Arduino microcontroller and an SG90 servo motor. This setup automatically rotated the antenna in the E-plane across a full 360º trajectory with precise 10º incremental steps. High-frequency RF characteristics were captured using an Anritsu MS2026C Vector Network Analyzer (VNA) calibrated via the standard Short-Open-Load-Through (SOLT) method, utilizing high-phase-stability Huber+Suhner SUCOFLEX coaxial cables under Fraunhofer far-field conditions. The experimental validation methodology was further enhanced by strictly adhering to the Fraunhofer far-field distance criteria (R ≥ 2D2 / λ) to prevent reactive near-field coupling between the transmitting and receiving antennas. By maintaining a fixed calibration distance of R = 50 cm, the true radiation performance of the Vivaldi prototype was successfully isolated from local induction effects. The raw log magnitude data collected at each of the 36 discrete angular steps were mathematically normalized against the maximum main-lobe intensity using a custom data-processing algorithm. This meticulous process successfully minimized environmental multipath reflections caused by the non-anechoic boundaries of the laboratory space, allowing for a highly accurate reconstruction of the polar E-plane radiation diagram. 6. Comparative Analysis of Simulation and Experimental Results The comparative analysis between the CST Studio Suite simulations and the physical VNA measurements demonstrated an exceptionally high degree of correlation, proving the robustness and physical reliability of the design. In terms of impedance bandwidth, the antenna was primarily targeted to cover the 5.3 GHz – 5.8 GHz range to support Electronic Counter-Countermeasures (ECCM) and high range resolution. While simulations predicted an effective impedance bandwidth (S11 ≤ -10 dB) of 562 MHz, the physical measurements exceeded expectations, delivering a remarkably wide bandwidth of over 705 MHz (spanning from 5.205 GHz to beyond 5.910 GHz). The slight positive frequency shift of approximately 30 MHz observed in the deep resonance point (shifting from 5.585 GHz in simulation to 5.615 GHz in reality) is scientifically attributed to the minor parasitic capacitance introduced by the SMA connector soldering and minute variations in the FR-4 loss tangent during manufacturing. At the absolute resonance frequency of 5.615 GHz, an outstanding return loss of -44.10 dB was physically measured, indicating an impedance match of 99.99% and practically zero insertion loss. 7. Spatial Radiation Performance and Directivity The spatial radiation characteristics extracted from both planes fully validated the theoretical traveling wave mechanics. In the E-plane, the physical measurements successfully captured a highly focused, directional endfire radiation pattern with a Half-Power Beamwidth (HPBW) of approximately 26 degrees. This narrow beam provides the strict spatial focusing essential for accurate angular resolution in target tracking. Conversely, the H-plane analysis revealed a characteristically wider, double lobe structure with a beamwidth extending to 141.7 degrees, which is highly advantageous for sweeping broad environmental areas in short-range scanning radars. The front-to-back ratio was recorded at a solid 12 dB. Theoretical data generated by the 3D full-wave analysis confirmed a maximum directivity of 7.62 dBi and a realized gain of 5.86 dBi. Moreover, despite the utilization of an FR-4 substrate, the optimized geometric structure maintained total radiation efficiency at roughly 70%. The Side Lobe Level (SLL) was successfully suppressed to -6.0 dB, significantly minimizing the risk of false alarms (ghosting) caused by off-axis radar clutter. 8. Future Outlook on Array Synthesis and Beamforming Looking forward, the successful validation of this single-element Vivaldi antenna provides a foundational building block for advanced radar array synthesis and beamforming configurations. Due to its planar form factor and compact 70x40 mm dimensions, multiple identical elements can be arranged in a linear or planar topology with a sub-wavelength element spacing (typically d ≈ λ0/2) to prevent grating lobes. Synthesizing a 1 × 4 or 2 × 2 antenna array utilizing this cost-effective FR-4 platform would significantly narrow the spatial beamwidth in both the E and H planes, thereby elevating the maximum directivity beyond 12 dBi. Such an array configuration, when integrated with digital phase shifters or electronic steering networks, would enable dynamic beam tracking capabilities critical for real-time commercial drone monitoring and counter-UAS applications, without compromising the strict budgetary constraints of industrial mass production. 9. Conclusion In conclusion, this thesis successfully demonstrates that the high dielectric losses inherently associated with FR-4 substrates can be methodically overcome through precise geometric transition optimization and accurate modeling of the traveling wave mechanics. The developed Vivaldi antenna provides a highly stable, ultra-wideband, and highly directional RF front-end solution. With its 70x40 mm compact form factor and exceptional cost-effectiveness, it stands as a highly reliable, industry-ready hardware infrastructure for the mass production of short-range radar sensors and drone tracking systems. Furthermore, the implementation of the Arduino-based, semi-automated measurement setup proved to be a critical asset in validating the theoretical models. By eliminating human-induced alignment errors during the far-field testing, the spatial radiation characteristics were captured with exceptional precision. The remarkable correlation between the full-wave CST Studio Suite simulations and the physical Vector Network Analyzer (VNA) measurements not only confirms the robustness of the structural optimizations but also establishes a highly repeatable testing methodology for future microwave engineering research.
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