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Karu Esselle

Karu Esselle

Distinguished Professor Karu Esselle
University of Technology Sydney
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Prof. Karu Esselle received the B.Sc. degree in electronic and telecommunication engineering (first-class honors) from the University of Moratuwa, Sri Lanka, and the M.Sc. and Ph.D. degrees in electrical engineering from the University of Ottawa, Canada. He is a professor of electronic engineering at Macquarie University, Sydney, and the past-associate dean of Higher Degree Research of the Division of Information and Communication Sciences. He has also served as a member of the Dean’s Advisory Council and the division executive from 2003 to 2008 and several times as the head of the Department of Engineering. He is the chair of the board of management of the Australian Antenna Measurement Facility, deputy director (engineering) of the WiMed Research Center, and elected 2016 chair of both the IEEE New South Wales (NSW) Section and the IEEE NSW Antennas Propagation/Microwave Theory and Techniques Chapter. He directs the Center for Collaboration in Electromagnetic and Antenna Engineering.

When Prof. Esselle was elected to the AP-S Administrative Committee for a three-year term in 2014, he became the only person residing in the Asia-Pacific Region (IEEE Region 10) to be elected to this highly competitive position for over a period of at least six years (2010–2015). He is a Fellow of the IEEE and Engineers Australia. Prof. Esselle has authored more than 450 research publications, and his Google Scholar h-index is the highest among Australian antenna researchers. Since 2002, his research team has been involved with research grants, contracts and Ph.D. scholarships worth over US$15 million, awarded from national and international organizations, including industries and governments.

Many names, many advantages – Are resonant cavity antennas the killer planar space-saving approach to get 15-25 dBi gain?

No other antenna concept has more names. At present these antennas are known as Fabry-Perot cavity resonator antennas, Partial Reflector Surface (PRS) based antennas, Electromagnetic Band Gap (EBG) Resonator antennas (ERAs) and Two-Dimensional Leaky-Wave Antennas, and more names are forthcoming. Yet they all have more or less the same configuration consisting of a resonant cavity, formed between a partially reflecting superstructure and a fully reflecting (ground) plane. The resonant cavity is excited by a small feed antenna. Hence, they are referred to as resonant cavity antennas (RCAs) in this presentation. Since the concept of using a “partially reflecting sheet array” superstructure to significantly enhance the directivity was disclosed by Trentini in 1956, it has been an attractive concept to several antenna researchers for several reasons, including its theoretical elegance, relationships to other well-researched area such as leaky-waves, EBG, frequency selective surfaces and metasurfaces, and practical advantages as a low-cost simple way to achieve high-gain (15-25 dBi) from an efficient planar antenna without an array, which requires a feed network. The RCA concept is one of the main beneficiaries of the surge of research on electromagnetic periodic structures in the last decade, first inspired by EBG and then to some extent by metamaterials. As a result, RCAs gained a tremendous improvement in performance in the last 10 years, in addition to other advantages such as size reduction. As an example, achieving 10% gain bandwidth from such an antenna with a PSS was a major breakthrough in 2006 but now there are prototypes with gain bandwidths greater than 50%. Until recently most RCAs required an area in the range of 25-100 square wavelengths but the latest extremely wideband RCAs are very compact, requiring only 1.5-2 square wavelengths at the lowest operating frequency. Once limited to a select group of researchers, these advantages have attracted many new researchers to RCA research domain, and the list is growing fast, as demonstrated by the diversity of authors in recent RCA publications. RCAs have already replaced other types of antennas, for example as feeds for reflectors. Have they become the killer planar alternative to 3D antennas such as horns and small reflectors? If not, what needs to be done to reach that stage?

This presentation will take the audience through historical achievements of RCA technology, giving emphasis to breakthroughs in the last 10 years. Special attention is given to methods that led to aforementioned bandwidth enhancement and area reduction, dramatic improvement of gainbandwidth product and unprecedented gain-bandwidth product per unit area demonstrated by RCAs, both theoretically and experimentally. Several choices of superstructures are discussed and performance achieved from them are compared. These superstructures include all dielectric superstrates with axial permittivity gradients and transverse permittivity gradients developed by the speaker’s team and printed superstructures also known as PSSs or metasurfaces developed by several research teams. Due to ultra-compactness of modern designs and edge radiation becoming a significant player in the principle of operation, different optimisation methods and strategies have been developed to replace previous unit-cell based methods, which were only suitable for previous larger RCAs. In particular, optimisation of RCAs using automated optimisation methods, including evolutionary algorithms such as Genetic algorithms and Particle Swarm algorithms as well as statistical optimisation algorithms, is described, illustrating the improvements that have been achieved from such optimisations by the speaker’s team and others. Taking one step back, methods of designing phase correction structures (PCS) to enhance near-field phase uniformity, and hence far-field directivity, of conventional larger RCAs are presented, highlighting physical reasons for the phase non-uniformity and pros and cons of each solution approach. Both printed (metasurface-type) PCSs and all-dielectric PCSs are included in this discussion. Sparse arrays of RCAs are considered as a way to achieve higher gain from RCAs. The presentation will conclude with yet unresolved issues, which could be addressed in future research.

Leaky-wave antennas: from niche applications to mass market

Since the discovery of efficient leaky-wave radiation from a slot in a wave guide by Oliner, leakywave antennas have attracted a lot of interest in applications that require beam scanning. Printed planar configurations of LWAs have become very popular, due to low cost. Half-width LWAs based on microstrip lines and substrate-integrated-wave guides have provided an additional advantage of narrow footprint. Professor Christophe Caloz (AP-S Distinguished Lecturer 2014-16) has brilliantly summarised in his distinguished lectures and illustrated how the problem of massive drop of antenna efficiency, when the beam is attempted to steer in the broadside direction, has been solved using Composite Right/Left-Handed (CRLH) structures and other methods. After briefly reviewing such crucial historical milestones in LWAs, this presentation will focus on recent developments in LWA antenna research and practical outcomes, including some that have the potential to further extend applications of LWAs from current niche scanning applications to mass communication applications such as wireless local area networks and emerging 5G mobile communications

One of them is fixed-frequency beam steering using only two values of bias voltages, for applications where sweeping the operating frequency is not possible. Several methods of LWA fixed-frequency beam steering has been demonstrated, including one recently developed by the speaker’s team that requires only two bias voltage values to steer the beam. This is very promising for millimetre-wave communication systems such as Wi-Gig and potential millimetre-wave modes of 5G. The principle underlying these LWAs is formation of a multi-state radiating structure by cascading several binary reconfigurable unit cells. Thus the basic building block of the antenna is a reconfigurable binary unit cell, switchable between two states. A macro cell is created by combining several reconfigurable unit cells and the periodic LWA is formed by cascading identical macro cells. Antenna beam is digitally steered in small steps by switching to different macro-cell states. Microwave prototypes based on this concept have demonstrated excellent beam steering over 30 degrees with negligible gain variation (of about 1 dB) and good input matching. As all switches in the antenna are binary, only two bias voltage values are required for beam steering, and the antenna sub-system can be controlled easily using digital electronics. Other recent developments presented in the lecture include (i) steering two side beams simultaneously by sweeping the operating frequency, using the second higher order mode of a microstrip, and (ii) dual-band beam scanning by frequency sweeping, with one beam scanning forward directions and the other one scanning backward directions. At the end, selected topics suitable to future research in this area will be discussed.

Through Body, On Body and Off Body: Antennas for Better Health

Wireless health monitoring requires antennas with different characteristics to transfer information including vital signs through a living body (from an implantable device to outside), over the body (between body-worn wireless devices) and off-body (to wireless hubs in the vicinity). The requirements of these antennas are significantly different, depending on the application and frequencies suitable or allowed for such communication systems. This lectures reviews antenna solutions developed by the speaker’s team and others for such narrowband, multi-band and ultrawideband systems, including medical telemetry. For wearable devices, antenna flexibility is often an additional requirement. Hence, several modern fabrication technologies, including embroidery, have been developed and tested. Another is the need to reduce radio-frequency energy deposition in the body, not only to keep the specific absorption rate within limits but also to get maximum mileage out of a given battery in a battery-powered device. This lecture will outline methods developed by us and others to address some of the design challenges and conflicting requirements. They include the use of a full-size conducting ground plane to shield the body from a wearable antenna. Another is to make a wearable antenna beam-switchable, to switch between a few difference radiation patterns to maximise the signal level in a given scenario. The lecture will also cover antennas with switchable beams, which are suitable for wireless hubs, to increase signal quality beyond what can be achieved from a wide-and-fixed-beam antenna. A specific example of converting an on-body health monitoring device to an implantable device is described, outlining how implanted antenna detuning problems and device bio-compatibility requirements were addressed and difficulties encountered in the process. System–level tests of the bio-telemetry device and range of the wireless link in different directions are provided as examples.