Research On High Gain Antennas And Dielectric Surfaces For RCS Reduction

The next generation of cellular networks (5G) is expected to meet the dramatically growing demands for data (high speed data transmission of several gigabits per second) through the using of huge number of small cells (base stations). The availability of huge amount of many light licensed/unlicensed gigahertz of spectrum at millimeter waves, such as E-band (71-76GHz for uplink,81-86GHz for downlink) with relatively low atmospheric absorption, makes these frequencies a promising candidate with clear technological and economic advantages to be used for 5G cellular networks as a robust solution to the conventional microwave band below 10 GHz, which is presently used by almost all cellular communication systems and going to reach its saturation level in the coming few years. High gain antennas with high aperture efficiency, low side lobes, low cross polarization, wide gain bandwidth (flat gain),return loss less than-10dB and low fabrication cost are highly required for the backhaul part of the 5G cellular networks to realize a reliable Gbps wireless point-to-point communication links. The challenge of these antennas to be suitable for E-band communication is they must have flat gain bandwidth over the frequency band 71-86GHz and in the same time high aperture efficiency. The first part of this Ph.D. dissertation is contributes to the analysis and design of high gain antennas for the next generation mobile networks and the second part is focuses on the design of dielectric surfaces for Radar Cross Section (RCS) reduction of solid metallic objects. The major contributions of this dissertation are organized in five chapters:Chapter one presents the introduction, a brief history of E-band, E-band frequency allocation, benefits of E-band over other wireless technologies, light licensing for E-band, free space propagation and other background information about E-band.Chapter two presents the design, manufacturing and measurements of light weight, compact size and high directivity conical horn lens antenna and pyramidal horn lens antenna that covers both 71-76GHz (uplink) and 81-86GHz (downlink) bands for 5G networks applications with almost flat gain bandwidth. The aperture of conventional conical moderate gain horn antenna is covered by planar inhomogeneous dielectric flat circular lens. A directivity of 26.4dBi,9dBi improvement at 74GHz and directivity of 27dBi,9.9dBi improvement at 86GHz were achieved. The proposed conical horn lens antenna is 44.46% shorter than the conventional conical horn antenna producing the same directivity. The second part of chapter two deals with the theory, design, fabrication and measurements of high gain compact size wide flare angle pyramidal horn antenna integrated with fully flat inhomogeneous low loss (≤2dB) dielectric lens that is covering the horn antenna aperture without additional impedance matching layers. The design formulas of the lens are presented and investigated in details. A measured peak gain of 35dB at 72.5GHz is achieved with aperture efficiency of 47.6%, SLL<-21dB, F/B ratio≈35.1dB and |S11|<-10dB. The designs in this chapter have been published in IEEE Antennas and Propagation Letters (IEEE-AWPL) and 2014 Asia-Pacific Conference on Antennas and Propagation (APCAP2014) and achieved the best student paper award of APCAP2014 and conference paper in IEEE International Symposium (IWS2015) and have been selected as finalist for the best student paper contest.Chapter Three presents the possibility of beam scanning at E-band for next generation high data rate point-to-point communication systems. First the design and results of low loss air fed discrete dielectric flat lens for E-band is discussed and then optimized to cover the frequencies 71-86GHz. Next the beam scanning capabilities of this single layer discrete dielectric lens at the central frequencies of E-band (73.5GHz and 83.5GHz) are investigated in details. Radiation characteristics such as scanning range, beam scanning gain loss, side lobe level, beam width and |S11| matching are investigated in details for the scanned beams both numerically and experimentally. It is found that using this kind of lenses its possible to steer the main beam by 40° (±20° or ±7 beam widths) away from the lens main axis in both horizontal and vertical planes with scan gain loss less than 1.3dB at 73.5GHz and less than 1dB at 83.5GHz with low SLL and stable beam width and measured |S11|<-10dB for all scanned beams. The lens has almost stable beam scanning characteristics over the entire E-band frequencies. The designs in this chapter have been published in IEEE Antennas and Propagation Letters (IEEE-AWPL) and in IEEE International Wireless Symposium (IWS2015).Chapter Four presents the phase error analysis of a single layer discrete dielectric lens that uses non-resonant unit cell. An extensive detailed study of the non-resonant unit cell is performed to understand the phase error loss mechanism and to find the best periodicity (unit cell size or inter-element spacing) that produces almost the same phase response not only at different frequencies but also for both normal and oblique incidence which leads to lower oblique phase error loss across the lens aperture and higher aperture efficiency. Based on this study three lenses of identical aperture area (100×100×6.35mm3) but different unit cell sizes (0.5λ94GHz,0.53λ94GHz and 0.62Xλ94GHz) are designed, fabricated and tested. Its found that the maximum oblique phase error is reduced from about 65°to less than 25°, aperture efficiency is improved by about 13.4% at 94GHz and the total number of unit cells is reduced by about 42.6% when periodicity changed from 0.47λ94GHz to 0.62λ94GHz, respectively. The designs in this chapter have been published in IEEE Transactions on Antennas and Propagation (IEEE-TAP).Chapter five proposes dielectric non-absorptive surfaces for RCS reduction and creating diffuse reflections at mmWaves. Various dielectric surfaces are designed, fabricated and tested. The proposed dielectric surfaces are composed of dielectric unit cells that designed in such away to make the incident EM-wave backscattered in angles other than specular direction and the monostatic and bistatic RCS will significantly be reduced.In the first part of chapter five the design of dielectric surfaces based on antenna array theory is presented and the incident EM waves will be reflected into predefined angles far away from the incident angle and thus a RCS reduction can be achieved. The number of unit cells in the building block of proposed RCS reducer dielectric surfaces is studied carefully to more understand the behavior of the RCS reduction level, RCS reduction bandwidth, grating lobe direction and level over 70-90GHz band and the optimum building block is addressed. Then the effect of the unit cells distribution across the aperture of the random dielectric surface on the RCS reduction properties is investigated as well.In the second part of chapter five the design of 1-bit and 2-bit dielectric surfaces of random distribution of effective permittivity across their aperture and creates low level backward diffuse reflections are presented. The proposed designs showed significant RCS reduction properties. The proposed surfaces are fabricated and then characterized experimentally. The results of this chapter are published at 2015 Asia-Pacific Microwave Conference and IEEE Transactions on Antennas and Propagation (submitted).