Design of Millimeter-wave Substrate Integrated Waveguide Antennas and Arrays for 5G Applications

應用於第五代移動通信的毫米波基片集成波導天線與陣列設計

Student thesis: Doctoral Thesis

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Award date11 Sept 2018

Abstract

This thesis presents the investigations of the millimeter-wave substrate integrated waveguide (SIW) antenna and array designs for 5G applications. The separate releases of the unlicensed 57-64 GHz and 64-71 GHz in the US make a 14 GHz contiguous unlicensed band available for the next generation communications to enable fiber-like wireless connectivity. Due to the high attenuation of waves in this band, antennas with high gain are preferred and can be realized by composing arrays. Moreover, SIW not only inherits the merits of a conventional waveguide by having an enclosed structure with low loss but is also quite compatible with the conventional printed-circuit-board (PCB) and plated-through-hole technologies, thus making SIW suitable for mass productions. To realize high gain and low loss planar array antennas, corporate feed networks by SIW are required.

First, several SIW circularly polarized (CP) array antennas are studied. One element design has two open loops with small gaps to excite a traveling wave for CP radiation. Both open loops are fed in balance by two vias standing on opposite sides of the aperture, which is etched on the top surface of the SIW. The element covers 7.9% of the 3-dB axial ratio (AR) bandwidth with a standing wave ratio (SWR) of better than 1.3 over the entire AR band and a peak gain of 8.3 dBic. Three sequentially rotated arrays are designed with sizes of 2 × 2, 8 × 8, and 16 × 16 elements. Another element design is a two-arm spiral with different arm lengths to achieve better circular polarization. Two 4 × 4 arrays with different feed networks are designed with or without sequential rotation. On the one hand, the array without sequential rotation has 15.3% and 14.2% for impedance bandwidth and 3.2-dB AR bandwidth, respectively, with a gain of 18.7 dBic at 60 GHz. On the other hand, a 2 × 2 subarray is designed using a sequentially rotated feed network for wider AR bandwidth and lower AR values. The array covers an impedance bandwidth of 14.1%, and a 3-dB AR bandwidth of 21.1% with a gain up to 19.5 dBic. All sequentially rotated arrays fully cover the entire 57-64 GHz unlicensed band, and they can be applied to high-speed Wireless Gigabit Alliance (WiGig) and 5G applications that require CP radiation.

Second, an LP antenna with conical beam is designed to operate in the unlicensed band of 60 GHz. To generate a conical beam, a cavity excited by two apertures, which supports the TM01 mode, is used. The two apertures are located on the top surface and on the opposite longitudinal walls of an SIW feed. The proposed antenna achieves an impedance bandwidth of 14.0%, covers the frequency range of 55.7 to 64.1 GHz, and realizes a gain of approximately 6 dBi. The proposed antenna can be used for short-range, point-to-multipoint wireless communications.

Third, wideband LP array antennas with complementary sources are investigated. The single element is composed of a slot, two dipoles with standing vias, and a cavity. In this design, the slot is used not only to couple energy from the SIW feed to the radiating structure but also to function as a magnetic dipole together with the cavity. Furthermore, the two dipoles function as a composite electric dipole. When they are excited with the proper amplitude and phase, stable unidirectional cardiac-shaped radiation patterns with low backlobes and low cross-polarization in the E- and H-planes are obtained. The element covers a bandwidth of 38.7% with a peak gain of 9.4 dBi. To generate arrays, a 2 × 2 subarray is initially formed, using a four-way broad wall coupler, which further suppresses cross-polarization of the single element. Subsequently, two larger arrays with double-layer full-corporate feed networks have been designed and measured. The 4 × 4 array presents a bandwidth of 26.7% with a peak gain of 21.5 dBi, fully covering the 57-71 GHz unlicensed band. The 8 × 8 array antenna exhibits a 14.5 GHz (22.9%) bandwidth with a peak gain of 26.7 dBi, which covers the 57-71 GHz band with a slight frequency shift. The proposed antenna element and arrays provide wideband millimeter-wave antenna solutions for future 5G applications.

Fourth, a wideband 45° polarized array antenna with a triple-layer corporate feed network is proposed. The radiating element consists of a slot and a cavity. To generate 45° polarization, both the slot and the cavity are rotated −45°. The element covers a bandwidth of 23.4%, from 56.6 to 71.6 GHz with an average gain of 7.9 dBi. To form a high gain array, the conventional double-layer full-corporate feed network is used first. However, the adjacent slots are excited with different amplitudes and phases, leading to grating lobes. Under this circumstance, a triple-layer feed network is proposed. The original 2 × 2 subarray in the double-layer configuration is separated as two 2 × 1 subarrays with equal excitation in the triple-layer. Two arrays with 4 × 4 and 8 × 8 elements are demonstrated. Both arrays cover bandwidths of 27.8% and 25.2%, respectively, in measurement, fully covering the entire 57-71 GHz unlicensed band. In addition, the measured gains for both arrays are 20.0 and 25.5 dBi, respectively. The sidelobes in the E- and H-planes are below −23 dB because the theoretical first sidelobe level of a uniformly excited square array is −26.4 dB in the diagonal plane. This kind of array can be used for vehicle radars and point-to-point communications.

Fifth, LP arrays with three-dimensional (3D) printed cuboids are designed to cover the 57-71 GHz band. In contrast to previous designs with PCBs, the cuboid is fabricated by 3D printing technology, which gives more flexibility to the design of the radiating part. The bandwidth of a single element is broadened by adding the dielectric cuboid on top of the aperture. Then, conventional double-layer corporate feed networks are applied to the 8 × 8 and 16 × 16 arrays. Links and fixtures are added between or around cuboids for the easy assembly of the prototype. The 8 × 8 array covers a bandwidth of 22.2% with a peak gain of 23.8 dBi. In addition, the 16 × 16 array covers a bandwidth of 23.4% with a peak gain of 29.2 dBi. Both arrays fully cover the 57-71 GHz unlicensed band. Therefore, this design demonstrates a novel way for the fabrication of an antenna using both PCB and 3D printing technologies.