5G uygulamaları için dairesel polarizasyonlu ve metayüzeyli mikroşerit MIMO anten tasarımı
Design of circularly polarized microstrip MIMO antenna with metasurface for 5G applications
Yazar: MUSTAFA KOÇER
Danışman: PROF. DR. MURAT TAYFUN GÜNEL
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: 5G = Five-Generation Wireless Telephone Technology ; Anten = Antenna ; Metayüzeyler = Metasurfaces ; Mikroşerit antenler = Microstrip antennas ; Polarizasyon = Polarization

Özet:
Bu tez çalışmasında, 6 GHz altı 5G frekans spektrumuna yönelik n78 bandı olarak adlandırılan 3.3-3.8 GHz frekans aralığında 4x4 çok girişli çok çıkışlı bir anten tasarımı hedeflenmiştir. Öncelikle çok girişli, çok çıkışlı anten tasarımında kullanılacak olan mikroşerit anten tasarımı gerçekleştirilmiştir.
Mikroşerit antenin dairesel polarizasyonda çalışması için kare yama tercih edilmiştir. Tercih edilen kare yamanın köşelerinde sol el dairesel polarizasyona yönelik kesik daire ve kesik üçgen kullanılmıştır. Yapılan tasarımlara göre köşelerinden kesik daire kullanılarak yapılan mikroşerit anten tasarımı kesik üçgen kullanılarak yapılan tasarıma göre daha iyi sonuç vermiştir.
Sol el dairesel polarizasyona yönelik köşelerinde kesik daire oluşturularak tasarlanan mikroşerit antenin performansını artırmak için üzerine hava boşluğu olmadan 4x4 metayüzey yerleştirilmiştir. Böylece metayüzeyin oluşturduğu yüzey dalgalarının kesim frekanslarında mikroşerit anten rezonansa girmiştir. Mikroşerit antenin istenilen yansıma katsayısı ve eksenel oranı aşağı frekansta (3.4 GHz) iken metayüzeyin yüzey dalgaları yukarı frekanslardadır (3.9-4 GHz). Bu sayede anten geniş bantta çalışmaktadır. Kullanılan metayüzey sayesinde FR-4 alttaş malzemesi ile tasarlanan bu antenin verimliliği, kazancı artmıştır ve geniş bantta dairesel polarizasyonda çalışması sağlanmıştır. Ayrıca mikroşerit yama antenin ortasında çapraz yarık oluşturularak metayüzeyli mikroşerit anten daha düşük yansıma katsayısına sahip olmuştur.
Metayüzeyli mikroşerit antenin tasarımında kullanılan alttaş malzemesinin kalınlıkları ve kullanılan alttaş malzemesinin performansa etkileri incelenmiştir. Ayrıca tasarımda kullanılan 4x4 metayüzeyin köşelerinden kesik daire şekli oluşturularak antenin dairesel polarizasyon bant genişliği artırılmış ve daha geniş bantta yüksek kazanç elde edilmesi sağlanmıştır.
TLC-32 alttaş malzemesi kullanılarak tasarlanan metayüzeyli mikroşerit anten ile 6 GHz altı 5G uygulamalarına yönelik 3.3-3.8 GHz' te dört kapılı (iki kapı sağ el dairesel, iki kapı sol el dairesel polarizasyon) olacak şekilde MIMO anten tasarımı gerçekleştirilmiştir. Tasarlanan MIMO antenin 1. ve 3. kapıları sol el dairesel polarizasyona yönelik iken 2. ve 4. kapıları sağ el dairesel polarizasyona yöneliktir ve her kapı kendi aralarında 90° döndürülerek tasarlanmıştır. MIMO antenin izolasyonunu artırmak için MIMO antenin ortasında alttaşa entegreli dalga kılavuzu yapısı kullanılmıştır. Buna ek olarak mikroşerit anten katmanına ve metayüzey katmanına parazitik elemanlar eklenmiştir. Böylece tasarımı gerçekleştirilen MIMO anten yüksek izolasyonlu ve kapılarının hepsi yüksek kazançlı olacak şekilde dairesel polarizasyonda çalışmaktadır.

Abstract:
In this thesis, a 4x4 multi-input, multi-output antenna design is presented for the 5G applications in the frequency ranges of 3.3–3.8 GHz, which is called the n78 band. First of all, a microstrip antenna was designed as a part of multi-input, multi-output antenna.
A square patch was preferred for the microstrip antenna to operate in circular polarization. At the corners of the square patch, a truncated circle and truncated triangle for left-hand circular polarization were used. According to the results a truncated circle gave better results than truncated triangle.
In order to improve the performance of the left-hand circularly polarized microstrip antenna with a truncated circle at the corners, a 4x4 metasurface without an air gap was placed on it. Thus, the microstrip antenna resonated at the cutoff frequencies of the surface waves formed by the metasurface.
The desired reflection coefficient and the axial ratio of the microstrip antenna were at low frequencies (3.4 GHz), while the surface waves of the metasurface were at high frequencies (3.9–4 GHz). In this way, the bandwidth of the microstrip antenna increased from 170 MHz to 1.57 GHz, its axial ratio bandwidth increased from 50 MHz to 750 MHz, its efficiency increased from 58.7% to 90% and its gain increased by 2.57 dBic.
In addition, the microstrip patch with diagonal slot improved the metasurface microstrip reflection coefficient value from -30,26 dB to -34,4 dB at 3,9 GHz. Thus, it exhibited high gain and high efficiency in the desired frequency ranges, with a gain over 6.13 dBic and an efficiency over 87.17% in the n78 band. The size of the antenna with the metasurface was 48x48x3.2 (mm).
The thickness of the substrate material used for the design of the microstrip antenna with metasurface was changed and its effects on performance were examined. FR-4 substrate material with 1.2 mm, 1.6 mm and 2 mm thickness were used for the design. When FR-4 material with a 1.2 mm substrate thickness was used; bandwidth, circular polarization bandwidth and gain of the antenna were obtained as 1.23 GHz, 570 MHz and 7.1 dBic respectively. When FR-4 material with a 1.6 mm substrate thickness was used; bandwidth, circular polarization bandwidth and gain of the antenna were obtained as 1.57 GHz, 750 MHz and 6.66 dBic respectively. When FR-4 material with a 2 mm substrate thickness was used; bandwidth, circular polarization bandwidth and gain of the antenna were obtained as 1.77 GHz, 870 MHz and 6.25 dBic respectively. The dimensions of the antenna were 56X56X2.4 (mm), 48X48X3.2 (mm) and 42X42X4 (mm), respectively, for 1.2 mm, 1.6 mm and 2 mm thickness of substrate material.
As a result high gain was obtained with the low-thickness substrate material, while the gain decreased with the high-thickness substrate material. However, with a high-thickness substrate material, high gain and high efficiency were obtained in a wider bandwidth, while the size reduced.
The performances of metasurface microstrip antennas designed using FR-4 substrate material with a thickness of 1.6 mm and TLC-32 substrate material with a thickness of 1.575 mm were investigated. When FR-4 material with a high dielectric constant (4.4) was used; bandwidth, circular polarization bandwidth, gain and efficiency of the antenna were obtained as 1.57 GHz, 750 MHz, 6.66 dBic and 89.7% respectively. When TLC-32 material with a low dielectric constant (3.2) was used; bandwidth, circular polarization bandwidth, gain and efficiency of the antenna were obtained as 1.21 GHz, 550 MHz, 8.55 dBic and 99.55% respectively. The size of the antenna with FR-4 material was 48x48x3.2 (mm), while the size of the antenna with TLC-32 material was 62.9x62.9x3.15 (mm).
As a result, high gain was obtained by using the low dielectric constant (3.2) substrate material (TLC-32), while low gain was obtained by using the high dielectric constant (4.4) substrate material (FR-4). However, with a high dielectric constant substrate material, wide bandwidth and wide circular polarization bandwidth were obtained in a wider bandwidth, while the size of the antenna was lowered.
In addition, the circular polarization bandwidth of the antenna increased by 130 MHz by creating a cut circle shape at the corners of the 4x4 metasurface with the TLC-32 substrate material, and a high gain was achieved in the 200 MHz wider bandwidth.
A metasurface microstrip antenna with TLC-32 substrate material was used to design a four-port (two ports right-hand circular, two ports left-hand circular polarization) MIMO antenna between 3.3 and 3.8 GHz for 5G applications. While the 1st and 3rd ports of the MIMO antenna have left-hand circular polarization, the 2nd and 4th ports have right-hand circular polarization, and each port was designed by rotating 90° among themselves. As a result of this, the isolation between the antenna elements of the MIMO antenna in the n78 frequency band became greater than 23.34 dB.
In order to increase the isolation of the MIMO antenna, parasitic elements were added to the microstrip antenna plane and the metasurface plane. Thus the isolation between the antenna elements of the MIMO antenna in the n78 frequency band became greater than 28.29 dB.
In order to increase the isolation of the MIMO antenna, as a different approach substrate-integrated waveguide structure were used in the middle of the MIMO structure. The substrate-integrated waveguide have 8 metallic vias with a diameter 1 mm and a length of 3.15 mm and they were connected from the ground layer to the metasurface layer of the antenna. As a result of this, the isolation between the antenna elements of the MIMO antenna in the n78 frequency band became greater than 28.07 dB.
In addition, a combination of parasitic elements and substrate-integrated waveguide were also used. Thus, isolation value of the MIMO antenna in the n78 frequency band became greater than 32.02 dB and its ports operated in circular polarization and high gain was obtained. The isolation between the antenna elements increased from 23.34 dB to 32.02 dB when substrate-integrated waveguide and parasitic elements were used together.
Finally the MIMO antenna was produced and also measured in the laboratory. According to the measurement results of the MIMO antenna for n78 frequency band the minimum isolation value is equal to -30.81 dB, the highest ECC value is equal to 0.012, the lowest DG value is equal to 9.891 dB, the lowest TARC value is equal to -8.31 dB, and the highest circular polarization gain is equal to 7.63 dBic at the 3rd port. In addition, the total size of the MIMO antenna becomes 132x132x3.15 (mm).