Array Antennas and Radomes for Millimetre-Wave 5G A[[;ocatopms
Warren Wai Yan Yong is a PhD student in the department Radio Systems. (Co)promotors are prof.dr.ir. A.B.J. Kokkeler and dr. A. Alayón Glazunov from the faculty of Electrical Engineering, Mathematics and Computer Science.
The deployment of the millimetre-wave (mmWave) band for 5G wireless communication has attracted significant attention and support from government, industry, and academic institutions over the past years due to the large chunks of available bandwidth. Nonetheless, for 5G mmWave communication systems to be successful for commercial applications, several fundamental issues in hardware development must be addressed. Here of particular relevance, is the development of mmWave antenna systems. Fundamental challenges in mmWave antenna system design include substantial losses such as material and propagation path losses and hardware implementation costs. To mitigate propagation path loss, a directive antenna is required. An array antenna is among the techniques used to realise directive antennas. Nonetheless, with the directive beam, the array antenna's coverage area will be reduced. However, a wide coverage area feature is one of the essential characteristics required by the base station to provide high throughput to individual users. Therefore, the antenna beam of an array antenna must be able to be steered in various directions covering up to 120°. In practical deployments of antenna systems, antennas are typically covered with a dielectric radome that serves to protect the antenna system or to provide additional spatial and/or frequency filtering to the antenna system. Conventional dielectric radomes have demonstrated satisfactory performance at low microwave frequencies. Nonetheless, at the mmWave band, the dielectric radome deployment has become an additional source of losses for the mmWave antenna system.
Considering the issues above of mmWave antenna systems for 5G wireless communications, this doctoral dissertation objective is twofold: (I) to design and characterise wideband mmWave array antennas and (II) to design and characterise antenna radomes for 5G applications. This dissertation therefore consists of two Parts. Part I of the thesis focuses on the design of mmWave array antennas for 5G applications. In this part, one of the primary objectives is to develop a cost-effective wideband array antenna that covers both the N257 (26.5 - 29.5 GHz) and N258 (24.25 - 27.5 GHz) frequency bands. The gap waveguide (GW) technology is a newly introduced transmission line technology that has shown potential as a contender for mmWave transmission lines. The GW technology possesses the same advantages as conventional waveguides, but it can be manufactured at a much lower cost. Like conventional waveguide-based array antennas, GW-based array antennas exhibit promising loss performance at mmWave frequencies, but their operating bandwidth is confined to approximately 15-20% Thus, a question arises, how can the bandwidth performance of these gap waveguide-based array antennas be enhanced? To address this, two different techniques have been proposed in this thesis, (i) modification of the cavity slot of the array antenna and (ii) designing the gap waveguide-based array antenna using a wideband radiating element such as the magneto-electric dipole (MED). Both of these methods provided in this dissertation proved a promising bandwidth enhancement technique for GW-based array antennas. However, the resulting GW-based array antennas cannot be deployed with beam steering capabilities due to their bulky unit cell characteristics that may result in unwanted grating lobes.
Therefore, to realise a compact unit cell antenna element suitable for the realisation of an array antenna with beam steering capability, substrate-based antennas remain the most popular candidates, as the dielectric allows for miniaturisation. To design the substrate-based mmWave array antenna, the antenna's transmission line losses must be minimised. Consequently, the antenna-in-package (AiP) concept is utilised because it permits the transmission lines between the antenna and RF components to be drastically abridged, thereby reducing the antenna and RF components' overall losses. The earlier mentioned MED antenna with wideband performance used in the design of the GW-array antenna is selected as the radiating element. However, instead of the fully metallic structure as in the GW-array antenna, the proposed MED is realised over the substrates material. The MED AiP is designed on a Panasonic Megtron-6 substrate, which covers both the N257 and N258 5G frequencies. The proposed MED subsequently incorporates two active beamformers encompassing the N257 and N258 bands. This thesis comprehensively discusses the procedure for integrating the active beamformer into the proposed MED array antenna. In addition, four distinct commercially available dielectric substrates are used to assess the effect of dielectric properties on the scanning performance of the MED array antenna resulting in design guidelines.
The focus of Part II of the dissertation is the design and implementation of low-loss bandpass frequency selective surface (FSS) radomes. As previously indicated, the conventional dielectric FSS radome is usually a source of antenna system loss. To realise minimal loss of an FSS radome, this dissertation proposes a fully metallic FSS radome. To enhance the overall filtering performance of the FSS, the GW technology is utilised for the first time in the design of the FSS radome. The proposed GW-FSS radome demonstrates promising filtering characteristics and minimal insertion loss. Since the proposed GW-FSS radome is entirely metallic, the unit cell dimensions are relatively large because the dielectric substrate is absent from the design. Therefore, it is only effective in the broadside radiation direction of the array antenna. As a result of this research, a new research topic has emerged addressing the miniaturisation of all-metallic FSS. We ask a questions as the following: Can the miniaturisation technique utilised in dielectric-based FSS also be utilised in all-metallic-based FSS? As a result, in the second half of Part II we provide a solution to the miniaturisation problem of an all-metallic FSS. The meandering technique commonly employed in the miniaturisation of substrate-based FSS has been proposed to be applied to achieve the miniaturisation of an all-metallic FSS straightforwardly. The results presented in this dissertation indicate that due to the meandering of the slot in the all-metallic FSS, the dimensions of the unit cell FSS can be significantly reduced, allowing it to function adequately for both broadside and oblique angles of incidence.
As a result of the performed investigation, numerous breakthroughs can be revealed. First, it is shown that by modifying the cavity slot, the overall impedance bandwidth of the GW-based antenna can be considerably increased from 18% to 28%. In addition, by employing the MED antenna as the radiating element and feeding it through the single-layer corporate feeding network, the proposed antenna exhibits a bandwidth of 18.9%, which increases the overall bandwidth of the single-layer corporate feed array antenna by more than 10%. Furthermore, its bandwidth performance is comparable to a cavity-backed slot antenna without requiring an additional cavity layer. Next, it is demonstrated for the first time that with a single wideband MED antenna, the AiP can be customised to operate in two distinct bands (either N257 or N258) by integrating it with two separate active beamformers that each covers a different band and shows excellent performance. The proposed MED AiP operates over 24.25 - 29.5 GHz and is integrated with NXP MMW 9004 KC and MMW 9002 KC analogue beamformer chips that operates at 26.5 - 29.5GHz and 24.25 - 27.5GHz, respectively. Besides that, from the numerical evaluation, the proposed MED AiP can also be further designed as a phased array with scanning capability up to and for E- and H-planes, respectively. In addition, the GW technology has been utilised for the first time to realise a bandpass FSS radome with minimal loss and sharp filtering. The proposed GW-FSS operates at 26 – 30 GHz at broadside with the insertion loss of dB. In addition, the miniaturisation technique based on the convoluted slot is applied to the all-metallic FSS slot to accomplish an FSS with a compact cell size that functions adequately at oblique angles of incidence. The proposed miniaturised FSS demonstrates a stable bandpass filtering performance at 24.9 - 31.4GHz for both transverse electric (TE) and transverse magnetic (TM) polarisations at broadside performance. Moreover, the proposed miniaturised all-metallic FSS performs adequately at oblique incidence angles up to
