Orta menzilli otomotiv radarları için çift eğik polarizasyonlu mikroşerit yama anten dizisi tasarımı
Dual slant polarization microstrip patch antenna design for medium range automotive radars
Yazar: FURKAN MUHAMMED TAYDAŞ
Danışman: PROF. DR. SELÇUK PAKER
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Elektronik ve Haberleşme Mühendisliği Ana Bilim Dalı / Telekomünikasyon Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering
Dizin: Anten = Antenna ; Anten dizileri = Antenna arrays ; Mikroşerit antenler = Microstrip antennas ; Polarizasyon = Polarization ; Radar = Radar

Özet:
Sürüş ortamını daha güvenli ve kolay kullanılabilir hale getirmek için çeşitli görüntüleme ve algılama teknolojileri geliştirilmektedir. Milimetre dalga uygulamalarının popüler ve tüketici odaklı bir uygulaması olan otomotiv radarları, güvenlik önlemleri için kullanılan teknolojilerin ilk örneklerindendir. Yüksek çözünürlük, düşük parazit etkisi, artan frekans ile anten ve ön RF devrelerinin boyutunun küçülmesi gibi avantajlarla gelen milimetre dalga radarları, otomotiv uygulamalarının ilgi odağı haline gelmiştir.
Araç çevresinin algılanması, hedeflerin tespiti ve görüntülenmesi ile gerektiği zaman araca müdahalede bulunmak amacıyla kullanılan otomotiv radarları, sürücü destek sistemlerinde en çok tercih edilen sensör sistemidir. Farklı amaçlar için kullanılan radar sensörleri çalıştıkları mesafeye göre Kısa Menzilli Radar (SRR), Orta Menzilli Radar (MRR) ve Uzun Menzilli Radar (LRR) olmak üzere üç grupta sınıflandırılabilir. Uyarlanabilir hız sabitleyici, uzun menzilli radarlar gibi sürüş yardım sistemleri; kör nokta uyarısı, şerit değiştirme yardımı ve çapraz trafik uyarı sistemleri orta menzilli radarlara örnek olarak verilirken, park etme veya yaya algılama sistemleri kısa menzilli radar uygulamalarıdır.
Otomotiv radarlarının performansını belirleyen en önemli bileşenlerinden biri ise antenlerdir. Hüzme genişliğini daraltmak, kazancı arttırmak ve açısal çözünürlüğü yükseltmek amacıyla radarlarda sıklıkla tercih edilen anten türlerinden biri dizi antenlerdir. Radarların gelişmesiyle antenlerden beklenen performanslar da değişmektedir. Radar sensörleri ile yüksek doğruluklu verilerin toplanmasını sağlamak ve hedef tespit ve sınıflandırma performansını iyileştirmek amacıyla polarizasyon çeşitliliği günümüz otomotiv radar antenlerinden beklenen önemli bir parametredir. Ayrıca kullanılan dizi antenlerde girişim etkisinin oluşmaması amacıyla yan kulakçık seviyesinin ana hüzmeden mümkün olduğunca düşük olması sistemin yanlış alarm olasılığını da azaltacaktır.
Tez kapsamında, orta menzilli otomotiv radarları için seri beslemeli, çift eğik polarizasyonlu ve yan kulakçık seviyesi bastırılmış bir dizi anten tasarımı gerçekleştirilmiştir. Paralel bağlantılı seri besleme ağı ilk olarak eş genlik dağılımlı anten dizisi için tasarlanmıştır. Daha sonra Taylor genlik dağılımı eşitlikleri kullanılarak -20 dB yan kulakçık seviyesi elde etmek amacıyla uygun genlik katsayıları hesaplanmış ve besleme devresine uygulanmıştır. 77 GHz frekans bandında çalışacak anten dizisi için eleman boyutları belirlenmiş ve besleme ağı ile bağlantısı gerçekleştirilmiştir. Polarizasyon çeşitliliğinin sağlanması amacıyla, ilkine eş bir besleme hattı daha uygun açıyla anten elemanlarına bağlanarak analizler gerçekleştirilmiştir. Elde edilen sonuçlar grafikler halinde verilmiştir.

Abstract:
Various imaging and sensing technologies are being developed to make automotive safer and easier to use. Automotive radars, a popular and consumer-oriented implementation of millimeter-wave applications, are among the first examples of technologies used for security measures. Millimeter wave radars, which come with advantages such as high resolution, low interference effects, and decreasing the size of antenna and front RF circuits with increasing frequency, have become the focus of attention for automotive applications.
In the early days of automotive radars, they were used only in driver support systems for information and warning purposes, which do not directly interfere with the vehicle, such as giving an audible warning during parking. Later, with the development of the systems, they were used for comfort and safety purposes, such as adaptive cruise control, line-keeping assistant, and automatic braking in the event of a possible accident, which can directly intervene in the vehicle. Since the data collected from the environment of the vehicle are used together with complex algorithms and signal processing applications in these systems, these methods are called advanced driver support systems (ADAS), which is a sub-title of driver support systems. With the increase in the usage areas of automotive radars and their ability to interfere with the vehicle, semi-autonomous and fully autonomous automotives, where systems take on more tasks than the driver, are also developing rapidly. Driver support systems can be classified according to its different characteristics. If the systems intervene directly in the vehicle, they are called as active driver support system, and they do not intervene, called passive driver assistance system.
Although different frequency bands such as 10 GHz, 16 GHz and 35 GHz have been used in automotive radars since their early days, the dominant frequencies are the 24 GHz and 77-78 GHz frequency ranges. With the use of the 77 GHz frequency band, the required high bandwidth and angular resolution have been achieved, the transmission power has been increased and the antenna dimensions have been reduced. The usage rate of the 24 GHz frequency band has started to decrease with the decrease in the production costs and electronic components used in high frequencies.
Radar sensors used for different purposes can be classified into three groups as Short-Range Radar (SRR), Medium Range Radar (MRR) and Long Range Radar (LRR) based on the distance they work. Driving assistance systems such as adaptive cruise control, long range radars; blind spot warning, lane change assist and cross traffic warning systems are examples of medium range radars, while parking or pedestrian detection systems are short range radar applications.
The performance of these sensors varies according to the type and characteristics of the antenna used. For antennas used in mid-range automotive radars, half-power beamwidths in the azimuth and elevation axes are required to be a maximum of 10° and 80°, respectively.
In addition to the operating frequency and half-power beamwidth (HPBW) of antennas used in automotive radars, two other important features are polarization and low side-lobe level. In radar scattering problems, polarization can be used to distinguish between different targets or target and chaos when target separation cannot be achieved using the range difference. For example, by using antennas with slant polarization, the polarizations of vehicles coming from the opposite direction and from the same direction will be perpendicular to each other, and thus target detection will be facilitated by reducing the interference between signals. In cases where dual polarization is used, higher target detection and estimation rates can be achieved. The high side lobe level increases the probability of false alarms in collision detection systems. For this reason, these levels are desired to be low in automotive radar systems.
In the thesis, important antenna parameters are briefly mentioned. Among these parameters, the important ones for automotive radars were determined and the necessary solution was presented to improve antenna designs in order to meet the needs of developing automotive radars. A microstrip antenna array design, which uses a series feeding circuit with amplitude distribution applied in order to provide the low side lobe level required by automotive radars, and has dual slant polarization for polarization diversity, has been proposed in detail within the scope of the thesis.
Antenna arrays are basically excited using parallel and series feeding networks. The advantages and disadvantages of these two feeding networks relative to each other are examined. It has been seen that it would be appropriate to use the serial feeding circuit in the antenna array for automotive applications. In order to design the feeding network, the number of elements to be used in the array antenna must be determined. The number of elements was calculated by using the half-power beam width defined for mid-range radars with driver assistance systems such as collision detection and lane change assistant, and it was found to be 10. The formulas used to calculate the thickness of the transmission lines used in the serial feeding network are explained with examples over a four-element feeding network. Then, a 10-elements, uniform-amplitude series feeding circuit is designed. As a result of the analyzes carried out using the three-dimensional simulation model, it has been seen that the power supplied from the input port of the supply network is divided equally to each output port.
In an array antenna, the power ratios transferred to the antenna elements can be varied using certain coefficients. When these coefficients are selected appropriately, the side lobe level of the antenna array are suppressed. The amplitude coefficients used in suppressing the side lobe level can be obtained with the help of different functions. These coefficients for the Taylor, Chebyshev and Kaiser functions were calculated and applied to a sample array antenna. The performances of the functions were evaluated by considering the effects of the gain and half power beamwidth on the antenna radiation pattern.
As a result, it was decided to use the Taylor function, which has the minimum effect on the antenna radiation pattern, in the calculation of the amplitude coefficients. The impedance values and thicknesses of the transmission lines were recalculated using these coefficients and the power distribution was examined through the simulation model. Results are consistent with the theoretical calculations.
After the serial feeding circuit was created, the dimensions of the antenna element were calculated for the 77 GHz frequency and the dimensions were optimized to obtain a symmetrical geometry. Antenna elements are directly connected to the serial feed line designed by applying amplitude distribution and simulated. The reflection coefficient and radiation pattern graphs of the array antenna are given. It can be seen that the electric field direction of the array antenna moves along the y-axis, so the antenna is in vertical polarization.
A new model was created with the design rotated 45° in order to obtain slant polarization in which the electric field direction moves along the x- and y- axes in order to receive signals in both horizontal and vertical polarization. A second feedline has been added to the antenna array to provide polarization diversity to increase the accuracy of the data used in the vehicle's interaction with its environment. Thus, a series of antenna designs with dual slant polarization and low side-lobe level have been obtained. At the 77 GHz center operating frequency, the gain of the antenna array is 16.3 dBi, and the half-power beamwidths in the azimuth and elevation axes are 10° and 65.9°, respectively. The side lobe level was obtained as -20 dB. When the electric field graphs are examined, it is seen that the direction of the electric field moves on a line of +45° and -45° between the x- and y- axes, respectively, when port 1 and port 2 are used.