Planar Beam-forming Antenna Array for 60-GHz Broadband Communication

The 60-GHz frequency band can be employed to realise the next-generation wireless high-speed communication that is capable of handling data rates of multiple gigabits per second. Advances in silicon technology allow the realisation of low-cost radio frequency (RF) front-end solutions. Still, to obtain the link-budget that is required for wireless gigabit-per-second communication, antenna arrays are needed that have sufficient gain and that support beam-forming. This requires the realisation of antenna arrays that maintain a high radiation efficiency while operating at millimeter-wave frequencies. Moreover, the antenna array and the RF front-end should be integrated into a single low-cost package that can be realised in a standard production process. In this thesis, antenna solutions have been presented that meet these requirements. This work covers the complete development cycle, viz modelling, design, optimisation, manufacturing, measurement and verification for three antenna prototype generations. An in-depth view of each development step is provided, while the combined work provides an overview of millimeter-wave antenna development. Modelling is a crucial step in the development cycle and has been discussed in Chapter 2. The production processes that are used for antenna design and packaging realise planar multi-layered structures. Therefore, the modelling of electromagnetic (EM) structures in stratified media has been considered. First, the Green’s function for stratified media has been derived. Second, a MoM-based approach has been proposed that provides an accurate analysis of the physical behaviour of these structures. Special attention has been given to the analysis of surface waves that propagate in the planar geometry, because they can significantly affect the radiation efficiency of planar antennas. The resulting model provides a computationally efficient tool (Spark) for the analysis and design of a wide range of planar antenna topologies. The first prototype is the balanced-fed aperture-coupled patch (BFACP) antenna element, that employs a unique topology and therefore exhibits excellent performance regarding bandwidth and radiation efficiency. The modelling and design of this antenna has been discussed in Chapter 3. It has been shown that the use of two coupling slots improves the bandwidth of the antenna as well as the radiation efficiency. Simultaneously, the back radiation is significantly reduced by employing a reflector element. The resulting antenna design has a measured bandwidth of 15% in combination with a radiation efficiency that is larger than 80% and an accompanying measured gain of 5.6 dBi. In Chapter 3, an extension of the BFACP antenna element has been presented that supports dual polarisation and/or circular polarisation as well. The proposed BFACP antenna designs can be employed both as single-element antenna and as a building block for antenna arrays. Obviously, the accurate measurement of the manufactured antenna prototypes is of importance for verification of both the modelling methods and the antenna designs. For this purpose, specific measurement setups have been designed. In Chapter 4 these setups have been introduced, motivated and explained. To obtain a reliable interconnection between the measurement equipment and the antenna under test, RF probes have been employed. Additional transitions (coplanar waveguide to microstrip transition, balun) have been designed to convert the single-ended signal of the measurement equipment to the balanced signal that is required by the antenna under test. Moreover, a far-field radiation pattern measurement setup has been developed from scratch which is completely tailored for the measurement of millimeter-wave antennas and beam-forming antenna arrays. It has been shown that these setups provide reliable measurement data that is in good agreement with the results obtained from the derived models. To maximise the performance of the antenna, an optimisation algorithm has been presented in Chapter 5 that gives the designer the flexibility to obtain the best antenna design for the considered application. This algorithm extends the derived EM model of the BFACP antenna (Chapter 3) to include sensitivity information about design parameters. The sensitivity has been employed to jointly optimise the bandwidth and the radiation efficiency of the antenna element. In Chapter 6, the optimised antenna element is used in the design of antenna arrays. Here, the modelling of beam-forming antenna arrays is discussed and the performance of several array configurations is compared. It has been concluded that a 6-element circular array shows best performance in terms of gain and radiation efficiency. Moreover, the mutual coupling between the elements of this array is low such that the active reflection coefficient remains well below -10 dB throughout the entire scan range. A second prototype has been designed that demonstrates beam-forming. For this prototype, 6-element circular arrays have been designed in combination with fixed feed networks that provide each antenna element with an RF signal that has the appropriate phase for beam-forming to a specific angle. The performance of these antenna arrays has been investigated in terms of radiation efficiency, bandwidth and gain. The prototype has a maximum measured gain of 11.8 dBi for broadside scan and it has been shown that these antenna arrays can be readily employed for the realisation of adaptive beam-forming at millimeter-wave frequencies. Chapter 7 discusses the packaging of the transceiver. First, the package requirements are listed and several package topologies are discussed. For example, the performance of superstrate topologies is analysed. Additionally, a package is proposed that embeds the BFACP antenna. This package combines ceramic-based layers and teflon-based layers. The ceramic-based layers provide the package with stiffness and are used to realise the RF feed network, whereas the teflon-based layers are employed to allow an antenna design that has a high radiation efficiency. For a high-performance package design, it is important that the electrical properties of the materials used is welldefined. Therefore, special efforts have been undertaken to characterise the electrical material properties of the materials used at millimeter-wave frequencies. For this purpose, ring resonators have been designed. Measurement results indicate that the electrical properties at higher frequencies can differ significantly from the values that are specified by the manufacturer for an operating frequency of 10 GHz. To demonstrate the performance of the BFACP antenna in a package configuration, a third prototype has been developed, in which the BFACP antenna is packaged in combination with active electronics. This prototype demonstrates that the antenna can be embedded in a package that contains not only the antenna, but also the RF electronics, RF feed network and control circuitry. In the prototype, the BFACP antenna has been connected to a power amplifier that has been realised in CMOS technology. The PA has been connected to the RF feed through a flip-chip interconnection process. It has been demonstrated that the proposed packaging topology results in an efficient transmitter. In conclusion, three antenna prototype generations have been presented and it is demonstrated that the presented concepts can be readily used for the design of a transceiver package that embeds a beam-forming antenna array and that supports gigabit-per-second communication.