| 11. |
- Bernekorn, Peter, et al.
(författare)
-
Propagation of transient electromagnetic waves in inhomogeneous and dispersive waveguides
- 1995
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Transient wave propagation in a waveguide filled with an inhomogeneous dispersive medium is analyzed. The waveguide is inhomogeneous in the longitudinal direction, but is homogeneously filled in the transverse directions. The analysis is performed in the time domain and is based upon a wave splitting technique and the method of propagators. The propagator maps the exciting field from one position in the waveguide to the field at another position. This mapping is represented as time convolution integrals. The theory is exemplified by numerical examples where it is shown how wave trains of different shapes are propagating in a waveguide filled with a homogeneous dispersive medium.
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| 12. |
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| 13. |
- Björkberg, Jonas, et al.
(författare)
-
Three-dimensional subterranean target identification by use of optimization techniques
- 1997
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Ingår i: Progress in Electromagnetics Research PIER. - 1070-4698. ; 15, s. 141-164
-
Tidskriftsartikel (refereegranskat)abstract
- The identification of a subterranean metallic ore from scattering experiments, conducted on the surface of the ground or in a bore hole, is a classic geophysical problem. In general this problem is not well-posed. However, a priori information about the shape of the target provides enough regularization to make the problem numerically stable. The problem is solved by minimizing the mean-square error between an eleven parameter model, based on the null field approach, and the data. The optimization is done with a Newton technique in which a singular value decomposition of the model Jacobian is employed. The algorithm is very stable to noise and makes good reconstructions from feasible starting guesses, for realistically noise contaminated data.
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| 14. |
- Björkberg, Jonas, et al.
(författare)
-
Three-dimensional subterranean target identification by use of optimization techniques
- 1991
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The identification of a subterranean metallic ore from scattering experiments, conducted on the surface of the ground or in a bore hole, is a classic geophysical problem. In general this problem is not well-posed. However, a priori information about the shape of the target provides enough regularization to make the problem numerically stable. The problem is solved by minimizing the mean-square error between an eleven parameter model, based on the null field approach, and the data. The optimization is done with a Newton technique in which a singular value decomposition of the model Jacobian is employed. The algorithm is very stable to noise and makes good reconstructions from feasible starting guesses, for realistically noise contaminated data.
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| 15. |
- Bolin, Thomas, et al.
(författare)
-
Two-antenna receive diversity performance in indoor environment
- 2005
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Ingår i: Electronics Letters. - : Institution of Engineering and Technology (IET). - 1350-911X .- 0013-5194. ; 41:22, s. 1205-1206
-
Tidskriftsartikel (refereegranskat)abstract
- Receive diversity gain performance of a two-antenna setup in an office environment is reported. The terminal is handheld in front of the user simulating a data mode scenario while walking around. The dual polarised base station sector antenna is placed at the end of a corridor. The results indicate that experimentally achieved diversity performance is similar to previously predicted theoretical data.
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| 16. |
- Cheney, Margaret, et al.
(författare)
-
Optimal electromagnetic measurements
- 2001
-
Ingår i: Journal Electromagnetic Waves and Applications. - 1569-3937. ; 15:10, s. 1323-1336
-
Tidskriftsartikel (refereegranskat)abstract
- We consider the problem of obtaining information about an inaccessible half-space from electromagnetic measurements made in the accessible half-space. If the measurements are of limited precision, some scatterers will be undetectable because their scattered fields are below the precision of the measuring instrument. How can we make optimal measurements? In other words, what incident fields should we apply that will result in the biggest measurements? There are many ways to formulate this question, depending on the measuring instruments. In this paper we consider a formulation involving wave-splitting in the accessible half-space: what downgoing wave will result in an upgoing wave of greatest energy? This formulation is most natural for far-field problems. A closely related question arises in the case when we have a guess about the configuration of the inaccessible half-space. What measurements should we make to determine whether our guess is accurate? In this case we compare the scattered field to the field computed from the guessed configuration. Again we look for the incident field that results in the greatest energy difference. We show that the optimal incident field can be found by an iterative process involving time reversal "mirrors". For band-limited incident fields and compactly supported scatterers, this iterative process converges to a sum of time-harmonic fields.
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| 17. |
- Cheney, Margaret, et al.
(författare)
-
Optimal Electromagnetic Measurements
- 2000
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We consider the problem of obtaining information about an inaccessible halfspace from electromagnetic measurements made in the accessible half-space. If the measurements are of limited precision, some scatterers will be undetectable because their scattered fields are below the precision of the measuring instrument. How can we make optimal measurements? In other words, what incident fields should we apply that will result in the biggest measurements? There are many ways to formulate this question, depending on the measuring instruments. In this paper we consider a formulation involving wavesplitting in the accessible half-space: what downgoing wave will result in an upgoing wave of greatest energy? This formulation is most natural for far-field problems. A closely related question arises in the case when we have a guess about the configuration of the inaccessible half-space. What measurements should we make to determine whether our guess is accurate? In this case we compare the scattered field to the field computed from the guessed configuration. Again we look for the incident field that results in the greatest energy difference. We show that the optimal incident field can be found by an iterative process involving time reversal “mirrors”. For band-limited incident fields and compactly supported scatterers, this iterative process converges to a sum of time-harmonic fields.
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| 18. |
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| 19. |
- Cheney, Margaret, et al.
(författare)
-
Resonance Enhancement in Noise
- 2023
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- This document compares two approaches to finding resonance peaks from asingle sensor: 1) averaging multiple measurements of the transfer function orfrequency-domain scattering matrix, versus 2) using the iterative time-reversalprocess, which involves iteratively re-transmitting a time-reversed version ofthe scattered field at the previous iterations. The averaging method has theadvantage of handling arbitrarily much noise if sufficiently many averages areused. On the other hand, up to a certain level of noise, the time-reversalmethod has dramatic advantages over the averaging method; but it also requires more complex equipment. This document discusses the tradeoffs involved, with the goal of providing information that may be useful in the system design process.
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| 20. |
- Cheney, Margaret, et al.
(författare)
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Three-dimensional inverse scattering: layer-stripping formulas and ill-posedness results
- 1988
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Ingår i: Inverse Problems. - : IOP Publishing. - 0266-5611 .- 1361-6420. ; 4:3, s. 625-642
-
Tidskriftsartikel (refereegranskat)abstract
- The authors consider the three-dimensional direct and inverse scattering problems for the Schrodinger equation and for the reduced wave equation with variable velocity. The scatterer is probed with either point sources or plane waves of fixed frequency. They ask the question, 'How does the wave field change when the scatterer is truncated?' Simple formulae for the derivative of the wave field with respect to the truncation parameter are obtained. Similar formulae are obtained for the scattering amplitudes. These formulae are used to derive ill-posedness results for various inverse scattering problems. The ill-posedness results apply when data are collected over a range of frequencies.
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