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Spiral Elements for Broad-Band Phased Arrays
Air Force Research Laboratory, Hanscom AFB.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Saab Bofors Dynamics AB.
2005 (English)In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 53, no 8, 2558-2562 p.Article in journal (Refereed) Published
Abstract [en]

In this paper, we present a numerical analysis of an infinite periodic array of planar spiral elements with octave bandwidth. For off-broadside scan the array is found to exhibit very narrow resonances, which are independent of scan angle. They occur when the spiral arms are multiples of half a wavelength, in which case the current forms a high amplitude standing wave along the spiral arms. The resonances are conveniently suppressed by making the arms unequally long. We also discuss the equivalent 3-port for this nonsymmetrical array element and evaluate the element polarization performance.

Place, publisher, year, edition, pages
2005. Vol. 53, no 8, 2558-2562 p.
Keyword [en]
broad-band arrays, phased arrays, spiral antennas
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-8537DOI: 10.1109/TAP.2005.852309ISI: 000231156100024Scopus ID: 2-s2.0-27644455861OAI: oai:DiVA.org:kth-8537DiVA: diva2:13888
Note
QC 20100906Available from: 2008-05-28 Created: 2008-05-28 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Electromagnetic Waves in Media with Ferromagnetic Losses
Open this publication in new window or tab >>Electromagnetic Waves in Media with Ferromagnetic Losses
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The operation of a wide variety of applications in today's modern society are heavily dependent on the magnetic properties of ferromagnetic materials and their interaction with electromagnetic fields.

The understanding of these interactions and the associated loss mechanisms is therefore crucial for the improvement and future development of such applications.

This thesis is concerned with electromagnetic waves in media with ferromagnetic losses. We model the dynamics of the magnetization of a ferromagnetic material with the nonlinear Landau-Lifshitz-Gilbert (LLG) equation and study stability conditions on static solutions. Furthermore, with the aid of a small signal analysis this equation is linearized around a stable static solution. From this analysis we obtain a small signal permeability, which shows that ferromagnetic material in general are gyrotropic with a resonant frequency behavior similar to that of a Lorentz material. In difference to dielectric Lorentz material, this resonance frequency can be shifted with the aid of a bias field. For a specific bias field we obtain a frequency behavior that mimics that of a material with electric conductivity losses. In terms of losses per unit volume it is then possible to define a magnetic conductivity which is independent of frequency.

We treat composite materials built from ferromagnetic inclusions in a nonmagnetic and nonconductinig background material. The composite material inherits the gyrotropic structure and resonant behavior of the single particle. The resonance frequency of the composite material is found to be independent of the volume fraction, unlike dielectric composite materials. For small enough particles, typically around 100 nm, it becomes energetically favorable to form a single domain in the particle, where disturbances in the magnetization can propagate in the form of spin waves. We study the possibility of exciting spin waves and derive a susceptibility that takes spin waves into account. It is found that spin wave resonances are excited in the gigahertz range and this could offer a way to increase the losses in a composite material. We also discuss some concerns regarding stability and causality of effective material parameters for biased ferromagnetic materials.

Finally, we discuss the possibility of using magnetic materials in absorbing applications. We analyze the scattering of electromagnetic waves from a metal surface covered with a thin magnetic lossy sheet. It is found that very thin magnetic layers can provide substantial specular absorption over a wide frequency band. However, magnetic specular absorbers, where the waves propagates just a fraction of the wavelength in the material, seem to require a certain amount of ferromagnetic material which make them quite heavy and thereby limit its practical use. On the other hand, for nonspecular absorbers where the waves propagates several wavelengths in the material, the amount of magnetic material required for efficient absorption seems to be substantially less than for specular absorbers. Thus, as nonspecular absorbers, magnetic lossy materials could offer very thin and light designs.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. xii, 30 p.
Series
Trita-EE, ISSN 1653-5146 ; 2008:029
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-4776 (URN)978-91-7415-011-7 (ISBN)
Public defence
2008-06-05, D3, D, Lindstedtsv 5, Stockholm, 13:15 (English)
Opponent
Supervisors
Note
QC 20100906Available from: 2008-05-28 Created: 2008-05-28 Last updated: 2010-09-06Bibliographically approved

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