| 1. |
- Jonsson, B. Lars G., et al.
(författare)
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On bounds of the Q-factor as a function of array antenna directivity
- 2018
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Ingår i: Proceedings of the 2018 20th International Conference on Electromagnetics in Advanced Applications, ICEAA 2018. - : Institute of Electrical and Electronics Engineers Inc.. - 9781538667620 ; , s. 406-409
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Konferensbidrag (refereegranskat)abstract
- In this paper we determine the trade-off between the Q-factor and partial (super) directivity for linear arrays of dipoles. The dipoles are densely spaced in a one or two wavelength long array. We compare the bound for port-feed dipoles with the current optimization approach, with optimal currents on the whole dipole area, and also with an enclosing plate. It is interesting to note that port-feed dipoles are close to the optimal current dipoles in the endfire case, for sufficiently many dipoles in the array, for a wide range of directivities. We also show that the optimization problem for the 'optimal current' and 'the port-feed' cases are very similar and that the Pareto-front (trade-off curve) can be obtained and compared for the different cases.
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| 2. |
- Ludvig-Osipov, Andrei, et al.
(författare)
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Fundamental bounds on extraordinary transmission with experimental validation
- 2018
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- This paper presents a study of extraordinary transmission (EoT) through arrays of sub-wavelength apertures. Fundamental limitations for this phenomenon are formulated as a sum rule, relating the transmission coefficient over a bandwidth to the static polarizability. The sum rule is rigorously derived for arbitrary periodic apertures in thin screens. By this sum rule we establish a physical bound on the bandwidth of EoT which is verified numerically for a number of aperture array designs. We utilize the sum rule to design and optimize sub-wavelength frequency selective surfaces with a bandwidth close to the physically attainable. Finally, we verify the sum rule and simulations by measurements of an array of horseshoe-shaped slots milled in aluminum foil.
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| 3. |
- Ludvig-Osipov, Andrei
(författare)
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Fundamental Bounds on Performance of Periodic Electromagnetic Radiators and Scatterers
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, the optimal bandwidth performance of periodic electromagnetic radiators and scatterers is studied. The main focus is on the development and application of methods to obtain fundamental physical bounds, relating geometrical parameters, frequency bandwidth, efficiency and radiation characteristics of periodic electromagnetic structures.Increasing demand on the performance of wireless electromagnetic systems in the modern world requires miniaturization, high data rates, high efficiency, and reliability in harsh electromagnetic environments. Attempts to improve all these design metrics at once confront the inevitable physical limitations. For example, an antenna’s size is fundamentally bounded with bandwidth performance, and attempts to decrease size result in reduced performance capabilities. Knowledge of such physical bounds is vital to achieve high performance: to gain an understanding of the trade-off between parameters and requirements, or to evaluate how optimal the realized design is.Periodic structures are indispensable components in many wireless systems. As antenna arrays, they are in base stations of mobile phone networks, in radio astronomy, in navigation systems. As functional structures, they are used as frequency-selective filters, polarizers and metamaterials.In this thesis, methods to construct fundamental bounds on Q-factor – a quantity inversely proportional to bandwidth – are presented for periodic structures. First, the Q-factor representation is derived in terms of the electric current density in a unit cell. Then, the bounds are obtained by minimizing the Q-factor over all current densities, that are supported in a specified spatial subset of a unit cell, with possibly additional constraints (e.g. on conductive losses, or on polarization) imposed.Moreover, an alternative approach for obtaining fundamental bandwidth bounds is investigated – the sum rules, that are based on representing a physical phenomenon as a passive input-output system. Transmission of a plane wave through a periodically perforated metal screen is described by a passive system, and the sum rule bounds the transmission bandwidth with the static polarizability of the unit cell. Such a bound is shown to be tight for simulated and measured perforated screens.
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| 4. |
- Ludvig-Osipov, Andrei, et al.
(författare)
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Fundamental Bounds on Transmission Through Periodically Perforated Metal Screens With Experimental Validation
- 2020
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Ingår i: IEEE Transactions on Antennas and Propagation. - : IEEE. - 0018-926X .- 1558-2221. ; 68:2, s. 773-782
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Tidskriftsartikel (refereegranskat)abstract
- This article presents a study of transmission through arrays of periodic sub-wavelength apertures. Fundamental limitations for this phenomenon are formulated as a sum rule, relating the transmission coefficient over a bandwidth to the static polarizability. The sum rule is rigorously derived for arbitrary periodic apertures in thin screens. By this sum rule we establish a physical bound on the transmission bandwidth which is verified numerically for a number of aperture array designs. We utilize the sum rule to design and optimize sub-wavelength frequency selective surfaces with a bandwidth close to the physically attainable. Finally, we verify the sum rule and simulations by measurements of an array of horseshoe-shaped slots milled in aluminum foil.
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| 5. |
- Ludvig-Osipov, Andrei, et al.
(författare)
-
Physical limitations of phased antenna arrays
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In this paper, the bounds on the Q-factor, a quantity inversely proportional to bandwidth, for phased antenna arrays are derived and investigated. Arrays in free space and above a ground plane are considered. The bounds are determined by optimizing the Q-factor represented in terms of electric current density on a unit-cell element. The optimization problem is formulated as a QCQP minimization of the Q-factor over all possible current densities on an enclosing element shape in a unit cell environment. The constraints can include conductive losses and polarization purity. We demonstrate how such bounds can be used to build Pareto-type trade-off relations between the Q-factor and other design specifications: element form-factor and size, efficiency and polarization purity.
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| 6. |
- Ludvig-Osipov, Andrei, et al.
(författare)
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Stored energies and Q-factor of two-dimensionally periodic antenna arrays
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The Q-factor for lossless three-dimensional structures with two-dimensional periodicity is here derived in terms of the electric current density. The derivation in itself is shape-independent and based on the periodic free-space Green’s function. The expression for Q-factor takes into account the exact shape of a periodic element, and permits beam steering. The stored energies and the radiated power, both required to evaluate Q-factor, are coordinate independent and expressed in a similar manner to the periodic Electric Field Integral equation, and can thus be rapidly calculated. Numerical investigations, performed for several antenna arrays, indicate fine agreement, accurate enough to be predictive, between the proposed Q-factor and the tuned fractional bandwidth, when the arrays are not too wideband (i.e. when Q≥5). For completeness, the input impedance Q-factor, proposed by Yaghjian and Best in 2005, is included and agrees well numerically with the derived Q-factor expression. The main advantage of the proposed representation is its explicit connection to the current density, which allows the Q-factor to give bandwidth estimates based on the shape and current of the array element.
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| 7. |
- Ludvig-Osipov, Andrei, et al.
(författare)
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Stored Energies and Q-Factor of Two-Dimensionally Periodic Antenna Arrays
- 2020
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Ingår i: IEEE Transactions on Antennas and Propagation. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-926X .- 1558-2221. ; 68:8, s. 5989-6002
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Tidskriftsartikel (refereegranskat)abstract
- The Q-factor for lossless 3-D structures with 2-D periodicity is here derived in terms of the electric current density. The derivation in itself is shape-independent and based on the periodic free-space Green's function. The expression for Q-factor takes into account the exact shape of a periodic element and permits beam steering. Both the stored energies and the radiated power, required to evaluate Q-factor, are coordinate-independent and expressed in a similar manner to the periodic electric field integral equation and can thus be rapidly calculated. Numerical investigations, performed for several antenna arrays, indicate fine agreement, accurate enough to he predictive, between the proposed Q-factor and the tuned fractional bandwidth, when the arrays are not too wideband (i.e., when Q >= 5) . For completeness, the input-impedance Q-factor, proposed by Yaghjian and Best in 2005, is included and agrees well numerically with the derived Q-factor expression. The main advantage of the proposed representation is its explicit connection to the current density, which allows the Q-factor to give bandwidth estimates based on the shape and current of the array element.
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| 8. |
- Nedic, Mitja, 1990-, et al.
(författare)
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Herglotz functions and applications in electromagnetics
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Herglotz functions inevitably appear in pure mathematics, mathematical physics, and engineering with a wide range of applications. In particular, they are the pertinent functions to model passive systems, and thus appear in modeling of electromagnetic phenomena in circuits, antennas, materials, and scattering. In this chapter, we review the basic theory of Herglotz functions and its applications to determine sum rules and physical bounds for passive systems.
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