High gain and multi band frequency selective antenna system design for 5G mobile terminals

The enormous developments in information and communication technologies and the increasing number of wireless users have also increased the demand for high data rates and channel capacity. Mobile communication systems, which started with the first generation (1G) in the 1980s, have been defeated as a new generation in every ten years. The fifth generation (5G) communication, which is also called as the next generation, has recently started. The 5G mobile spectrum covers a wide frequency range from 410 MHz to 86 GHz and is broadly classified as low band (below 1 GHz), mid band (1 to 6 GHz), and high band (above 6 GHz). The most prominent frequency bands determined by ITU for these 3 frequency regions are respectively; n78 (3.3-3.8 GHz), n258 (24.25-27.5 GHz or 26 GHz), n257 (26.5-29.5 GHz or 28 GHz), and n260 (37-40 GHz or 39 GHz). As in other mobile systems, one of the most important elements of 5G mobile systems is antennas. Microstrip antennas come to the forefront for mobile systems with their advantages such as having a small profile, low production cost, and ease of design and production. The most common disadvantage of conventional microstrip antennas is the narrowness of the bandwidth. Another disadvantage of microstrip antennas designed especially for the high frequency region, where atmospheric losses increase, is the low gain value. Techniques such as using a defected ground structures, adding shorting pins or slots, using frequency-independent architecture such as log-periodic and designing parasitic patches are used to increase the bandwidth of microstrip antennas. In addition, some of the gain enhancement techniques used by the researchers in microstrip antennas are; adding a shorting-pins or slots, adding electromagnetic band gap on antenna and using superstrates or frequency selective surfaces. In this thesis, 3 different architectures for next generation 5G mobile communication devices are used and a total of eight log-periodic patch antenna designs are presented. In the first of these studies, a log-periodic-like linear decreasing monopole patch antenna structure was designed for the 3.3-4.2 GHz frequency range for 5G applications below sub-6 GHz. The log-periodic structure of the proposed antenna is designed as a monopole with linear architecture. In the second and third antenna studies, 4 and 8 element MIMO architectures were applied to linear decreased log-periodic monopole antenna (LPMA) to increase the channel capacity. In these MIMO antennas, two more bands were added to the working bands with the differences in the ground planes. Three LPMAs designed with this first log-periodic architecture used FR4 substrates with a relative permeability of 4.3 and a thickness of 1.6 mm. In the second log-periodic geometry, a novel log-periodic dipole antenna (LPDA) consisting of non-crossed dipole elements instead of the crossed dipole in the traditional LPDA was designed and implemented as a two-element MIMO antenna system for n258 5G band. In the third log-periodic antenna architecture, a notched LPMA (NLPMA) structure is obtained by merging two-quarter wavelength strip lines with a 90° angle for each log-periodic element. In this NLPMA architecture, two-port MIMO designs are presented with 3 different frequency combinations in the 5G mm-wave band suc as; 28 GHz, 26/28 GHz and 26/39 GHz bands. Rogers RT 5880 with a relative permittivity of 2.2 and a thickness of 0.508 mm was used as the dielectric substrate in LPDA and ÇLPMA MIMO antenna structures designed for mm-wave bands. In addition, in order to reduce the effect of increasing atmospheric losses at mm-wave frequency region, gain increases of more than 50% have been achieved by adding superstrate and frequency selective surfaces obtained by using FR4 and Rogers RO3003 dielectric substrates to the antennas designed with LPDA and NLPMA architecture. In the last antenna system proposed in this thesis, a 12-element MIMO antenna structure is designed by combining the log-periodic architectures of the first and third designs in a multilayered manner. In the multilayer antenna structure designed as multi-band, there is a MIMO system consisting of 4 triangle-shaped LPMAs in the upper layer and operates in the frequency range of 3.3-3.8 GHz. The NLPMA structure, which is located in the lower layer of the antenna, is a MIMO system consisting of 8 elements with 26 GHz and 39 GHz resonances. In this last antenna system, the FR4 layer on the top acts as a superstrate for the mm-wave antennas and increase the antenna gain.