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On Methods to Determine Bounds on the Q-Factor for a Given Directivity
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.ORCID iD: 0000-0001-7269-5241
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
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2017 (English)In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 65, no 11, p. 5686-5696Article in journal (Refereed) Published
Abstract [en]

This paper revisit and extend the interesting case of bounds on the Q-factor for a given directivity for a small antenna of arbitrary shape. A higher directivity in a small antenna is closely connected with a narrow impedance bandwidth. The relation between bandwidth and a desired directivity is still not fully understood, not even for small antennas. Initial investigations in this direction have related the radius of a circumscribing sphere to the directivity, and bounds on the Q-factor have also been derived for a partial directivity in a given direction. In this paper, we derive lower bounds on the Q-factor for a total desired directivity for an arbitrarily shaped antenna in a given direction as a convex problem using semidefinite relaxation (SDR) techniques. We also show that the relaxed solution is also a solution of the original problem of determining the lower Q-factor bound for a total desired directivity. SDR can also be used to relax a class of other interesting nonconvex constraints in antenna optimization, such as tuning, losses, and front-to-back ratio. We compare two different new methods to determine the lowest Q-factor for arbitrary-shaped antennas for a given total directivity. We also compare our results with full electromagnetic simulations of a parasitic element antenna with high directivity.

Place, publisher, year, edition, pages
IEEE, 2017. Vol. 65, no 11, p. 5686-5696
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-218217DOI: 10.1109/TAP.2017.2748383ISI: 000414047400003OAI: oai:DiVA.org:kth-218217DiVA, id: diva2:1160879
Note

QC 20171128

Available from: 2017-11-28 Created: 2017-11-28 Last updated: 2018-11-15Bibliographically approved
In thesis
1. Antenna Limitations and Q-factor Trade-off between Parameters, Steps towards Optimal Antenna Design
Open this publication in new window or tab >>Antenna Limitations and Q-factor Trade-off between Parameters, Steps towards Optimal Antenna Design
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Sub-wavelength antennas have become ubiquitous in essential devices, such as mobile phones, sensors, Internet of things (IoT) and machine to machine (M2M) communication devices. Such antennas are often embedded as a small part of the device chassis or their circuit-boards. The size assigned both to the antennas as well as the device tends to shrink, while demands on antenna performance are increasing. In such a context knowledge of optimal performance is of increasing importance. The subject of this thesis is on the bounds of small antennas, in particular bounds on impedance bandwidth performance.

The main tool to obtain bounds is antenna current optimization. The bounds are mainly focused on determining limits on the Q-factor for small antennas, and hence implicitly on the available bandwidth at a given reflection coefficient. We investigate Q-factor bounds under a number of constraints including directivity, far-field radiation pattern, efficiency, and the embedded position of the antenna. In this process, we combine physical methods, mathematical tools, and antenna engineering. We use the Method of Moments (MoM) approach to solving the Electric Field Integral Equations (EFIE), in this context we formulate and solve antenna optimization problems where the surface current density is an unknown variable, and we solve convex and non-convex quadratically constrained quadratic programs (QCQPs). For non-convex problems, we investigate different methods to obtain the solution, but with the main focus on the semidefinite relaxation (SDR) technique. Different current optimization problems are solved for a range of shapes, where the Q-factor and the optimal surface current are determined; the results are compared with full-wave-simulation of antennas that approach the bounds.

To determine the Q-factor for an available space in the device is here proposed to be an initial step of an antenna design procedure. The current optimization helps us to determine the optimal trade-off between the different performance parameters of a small antenna, and it can inspire antenna design with better performance. We furthermore show that a multi-position feeding strategy to realize an optimal current successfully realize a non-standard far-field performance. As an example, we show that the desired radiation patterns are obtained with small costs of Q-factor. The thesis ends with a discussion of initial steps to a methodology with the goal of obtaining a Q-factor optimal antenna. Here the current optimization plays an important role in the antenna synthesis and analysis stages of the process. An application to the embedded antenna is discussed in detail.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. p. 76
Series
TRITA-EECS-AVL ; 2018:89
Keywords
Antenna limitation, Q-factor, directivity, radiation pattern, current optimization
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-239047 (URN)978-91-7873-019-3 (ISBN)
Public defence
2018-12-05, Kollegiesalen, Brinellvägen 8, Stockholm, 13:00 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research , AM130011
Note

QC 20181116

Available from: 2018-11-16 Created: 2018-11-15 Last updated: 2018-11-16Bibliographically approved

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