Postdoc, North Carolina State University
The smaller an antenna gets, the worse it performs; can we get around this by playing clever tricks?
An antenna is the critical part of any wireless communication device which transmits and receives electromagnetic (e.g., radio) waves. For over half a century, engineers and physicists have asked the question: what is the “best” antenna possible for a given wireless system? Mathematically this question is a tough one to solve, but over the years various researchers have arrived at key results known as fundamental antenna bounds. For wireless systems with small antennas (mobile phones, small Internet of Things devices), the results from these bounds aren't good. As an antenna becomes smaller, several performance parameters important for maintaining a good wireless data link become irretrievably poor. This degradation with decreasing size suggests there exists a hard limit of how fast and efficiently small devices can send and receive data. So are small devices doomed to run up against their fundamental limit as expectations for portable high-data-rate wireless applications (streaming high-definition video, wireless virtual reality, video calling) continue to increase year after year?
My research approaches this problem by studying the assumptions under which the fundamental bounds on antenna performance were derived. In most cases, these assumptions are based on how radios and antennas have been built since the earliest days of wireless communications. By critically examining these assumptions, we can find conditions under which the fundamental bounds no longer hold. Then by identifying and exploiting these conditions, we can design new kinds of antenna and radio architectures which are totally unlike the status quo and which are no longer hindered by the fundamental bounds. This work covers fundamental theoretical problems associated with the physics of small antennas, it explores performance implications of different design strategies used to break the fundamental bounds, and it includes construction and field assessment of prototypes to demonstrate the validity of very unconventional yet potentially groundbreaking antenna systems.
Abstract: Ambiguities in the definition of stored energy within distributed or radiating electromagnetic systems motivate the discussion of the well-defined concept of recoverable energy. This concept is commonly overlooked by the community and the purpose of this communication is to recall its existence and to discuss its relationship to fractional bandwidth. Using a rational function approximation of a system's input impedance, the recoverable energy of lumped and radiating systems is calculated in closed form and is related to stored energy and fractional bandwidth. Lumped circuits are also used to demonstrate the relationship between recoverable energy and the energy stored within equivalent circuits produced by the minimum phase-shift Darlington's synthesis procedure.
Pub.: 23 Jan '17, Pinned: 01 Jul '17
Abstract: Though commonly used to calculate Q-factor and fractional bandwidth, the energy stored by radiating systems (antennas) is a subtle and challenging concept that has perplexed researchers for over half a century. Here, the obstacles in defining and calculating stored energy in general electromagnetic systems are presented from first principles as well as using demonstrative examples from electrostatics, circuits, and radiating systems. Along the way, the concept of unobservable energy is introduced to formalize such challenges. Existing methods of defining stored energy in radiating systems are then reviewed in a framework based on technical commonalities rather than chronological order. Equivalences between some methods under common assumptions are highlighted, along with the strengths, weaknesses, and unique applications of certain techniques. Numerical examples are provided to compare the relative margin between methods on several radiating structures.
Pub.: 22 May '17, Pinned: 01 Jul '17
Abstract: The optimal currents on arbitrarily shaped radiators with respect to the minimum quality factor Q are found using a simple and efficient procedure. The solution starts with a reformulation of the problem of minimizing quality factor Q as an alternative, so-called dual, problem. Taking advantage of modal decomposition and group theory, it is shown that the dual problem can easily be solved and always results in minimal quality factor Q. Moreover, the optimization procedure is generalized to minimize quality factor Q for embedded antennas, with respect to the arbitrarily weighted radiation patterns, or with prescribed magnitude of the electric and magnetic near-fields. The obtained numerical results are compatible with previous results based on composition of modal currents, convex optimization, and quasi-static approximations; however, using the methodology in this paper, the class of solvable problems is significantly extended.
Pub.: 22 Dec '16, Pinned: 01 Jul '17
Join Sparrho today to stay on top of science
Discover, organise and share research that matters to you