| 1. |
- Shirvany, Yazdan, 1980, et al.
(författare)
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Evaluation of a finite-element reciprocity method for epileptic EEG source localization: Accuracy, computational complexity and noise robustness
- 2013
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Ingår i: Biomedical Engineering Letters. - : Springer Science and Business Media LLC. - 2093-985X .- 2093-9868. ; 3:1, s. 8-16
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Tidskriftsartikel (refereegranskat)abstract
- PURPOSEThe aim of this paper is to evaluate the performance of an EEG source localization method that combines a finite element method (FEM) and the reciprocity theorem.METHODSThe reciprocity method is applied to solve the forward problem in a four-layer spherical head model for a large number of test dipoles. To benchmark the proposed method, the results are compared with an analytical solution and two state-of-the-art methods from the literature. Moreover, the dipole localization error resulting from utilizing the method in the inverse procedure for a realistic head model is investigated with respect to EEG signal noise and electrode misplacement.RESULTSThe results show approximately 3% relative error between numerically calculated potentials done by the reciprocity theorem and the analytical solutions. When adding EEG noise with SNR between 5 and 10, the mean localization error is approximately 4.3 mm. For the case with 10 mm electrode misplacement the localization error is 4.8 mm. The reciprocity EEG source localization speeds up the solution of the inverse problem with more than three orders of magnitude compared to the state-of-the-art methods.CONCLUSIONSThe reciprocity method has high accuracy for modeling the dipole in EEG source localization, is robust with respect to noise, and faster than alternative methods.
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| 2. |
- Talcoth, Oskar, 1980, et al.
(författare)
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Monolithic Multi-Scale Modeling of MR-Induced Pacemaker Lead Heating
- 2011
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Ingår i: 13th International Conference on Electromagnetics in Advanced Applications (ICEAA), 2011, Torino, 12-16 September 2011. - 9781612849768 ; , s. 599 - 602
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Konferensbidrag (refereegranskat)abstract
- Modeling of MR-induced pacemaker lead heating is complicated by, among other things, the multi-scale characteristics of the problem. In this paper, the method of moments is used to model a wide range of length scales of the problem simultaneously including the helix-shaped conducting wires present in the pacemaker lead. A cross-section area conserving meshing scheme for helices is proposed and evaluated. For a relative error of 1%, the meshing scheme reduces the number of thin wire segments needed to model a helix with a factor of roughly 3 to 6, as compared to the conventional approach. In addition, we study the relationship between the maximum induced current on the lead and the number of turns of the helices.
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| 3. |
- Talcoth, Oskar, 1980, et al.
(författare)
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MR-induced heating of pacemaker leads: A parameter study of contributing factors based on multi-scale modeling
- 2011
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Ingår i: Proceedings of Medicinteknikdagarna 2011. ; , s. 52-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Magnetic resonance imaging (MRI) is a valuable tool for diagnostic imaging inhealthcare and the number of examinations using this imaging modality increases each year.At the same time, pacemakers and other types of active implantable medical devices (AIMD)are getting increasingly common. Unfortunately, patients with an AIMD are currentlydeprived of the benefits of MRI due to the potentially harmful interactions of the implant withthe electromagnetic fields present during MRI. In particular, currents induced by the radiofrequency (RF) field can give rise to excessive heating near sharp corners and edges of theimplant. The occurrence of such heating at the electrodes of a pacemaker lead can result inreduction or loss of the pacemaker’s pacing ability.As a consequence, there is a need for MR-safe pacemaker systems. Experiments andnumerical modeling have been frequently used for investigations of the heating phenomenon.Such modeling is difficult due to the heterogeneity of the human body, the multi-scale natureof the problem, and inter-patient variations in factors like implant configuration.In this work, we model a wide range of length scales of the problem simultaneously. Themodeled MRI birdcage antenna and the human-body phantom have sizes of approximatelyone fifth of a wavelength whereas the helix-shaped conducting wires of the bi-polarpacemaker lead vary on a length scale of roughly one thousandth of a wavelength. The modelbeing entirely parameterized, we perform parameter studies to investigate how the heating isinfluenced by different factors, including the dielectric constants of the phantom material, thenumber of wire turns of the conductors, and the radii of the helices. Furthermore, implicationsof these results for the design of MR-safe pacemaker leads are discussed.
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| 4. |
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| 5. |
- Talcoth, Oskar, 1980, et al.
(författare)
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Optimal measurements in magnetic tracking for organ-positioning during radiotherapy
- 2011
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Ingår i: Proceedings of Medicinteknikdagarna 2011. ; , s. 79-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In a magnetic tracking system, the position and orientation of a coil is determined by its low frequency (LF)magnetic interactions with a coil array of known position and orientation. There are two main types of systems:(i) the coil with unknown position works as a transmitter with a receiving array of sensors, or (ii) the coil array isthe transmitter and the coil with unknown position is used as sensor. As LF magnetic fields do not interact withthe human body, magnetic tracking is particularly well-suited for tracking of objects in and around the humanbody. For example, this tracking technique has in recent years been proposed for eye tracking, tracking of tonguemovements, and organ-positioning during radiotherapy.We focus on an organ-positioning system for use during radiotherapy. The system, which is of type (i) above,consists of a transmitter that is implanted near the organ of interest and an external array of sensors. Of particularinterest is the spatial layout of the sensor array, since this strongly influences the performance of the trackingsystem. A well-designed sensor array will give good positioning accuracy whereas an ill-designed array canresult in an unsolvable positioning problem.In this work, we use Fisher-information theory from the field of optimal measurements to formulate anoptimization problem for the layout of the sensor array. Furthermore, we solve the optimization problem anddiscuss the implications of the results on the design of magnetic tracking systems.
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| 6. |
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| 7. |
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| 8. |
- Jensen, P. D., et al.
(författare)
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Parameter Scaling in Non-Linear Microwave Tomography
- 2012
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Ingår i: 2012 Loughborough Antennas & Propagation Conference, Loughborough, United Kingdom, 12-13 November 2012. - 9781467322201
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Konferensbidrag (refereegranskat)abstract
- Non-linear microwave tomographic imaging of the breast is a challenging computational problem. The breast is heterogeneous and contains several high-contrast and lossy regions, resulting in large differences in the measured signal levels. This implies that special care must be taken when the imaging problem is formulated. Under such conditions, microwave imaging systems will most often be considerably more sensitive to changes in the electromagnetic properties in certain regions of the breast. The result is that the parameters might not be reconstructed correctly in the less sensitive regions. In this paper, a method for obtaining a more uniform sensitivity throughout the breast is investigated. The method for obtaining uniform sensitivity throughout the imaging domain is a scaling of the parameters to be reconstructed. This scaling, based on the norms of the columns of the Jacobian, is also introduced as a measure of the sensitivity. The scaling of the parameters is shown to improve performance of the microwave imaging system when applied to reconstruction of images from 2-D simulated data and measurement data.
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| 9. |
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| 10. |
- Talcoth, Oskar, 1980, et al.
(författare)
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Convex optimization of measurement allocation for magnetic tracking systems
- 2017
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Ingår i: Optimization and Engineering. - : Springer Science and Business Media LLC. - 1573-2924 .- 1389-4420. ; 18:4, s. 849-871
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic tracking is a popular technique that exploits static and low-frequency magnetic fields for positioning of quasi-stationary objects. One important system design aspect, which substantially influences the performance of the tracking system, is how to collect as much information as possible with a given number of measurements. In this work, we optimize the allocation of measurements given a large number of possible measurements of a generic magnetic tracking system that exploits time-division multiplexing. We exploit performance metrics based on the Fisher information matrix. In particular, the performance metrics measure worst-case or average performance in a measurement domain, i.e. the domain where the tracking is to be performed. An optimization problem with integer variables is formulated. By relaxing the constraint that the variables should be integer, a convex optimization problem is obtained. The two performance metrics are compared for several realistic measurement scenarios with planar transmitter constellations. The results show that the worst performance is obtained in the most distant parts of the measurement domain. Furthermore, measurement allocations optimized for worst-case performance require measurements in a larger area than measurement allocations optimized for average performance.
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| 11. |
- Talcoth, Oskar, 1980, et al.
(författare)
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Convex optimization of sensor positions for organ-positioning during radiotherapy
- 2012
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Ingår i: Proceedings of Medicinteknikdagarna 2012. ; , s. 87-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Real-time organ positioning is a promising technique for enhancing radiotherapy of cancer tumors in situations where the tumor position can be believed to change between imaging and treatment and/or during treatment. In this work, we exploit theory on optimal measurements to formulate an optimization problem for the sensor positions of a magnetic tracking system. The optimization problem is relaxed into a convex problem, which is subsequently solved.
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| 12. |
- Talcoth, Oskar, 1980
(författare)
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Electromagnetic Modeling and Design of Medical Implants and Devices
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis covers two topics in biomedical electromagnetics: pacemaker lead heating in magnetic resonance imaging (MRI) and optimization of sensorpositions in magnetic tracking.The electromagnetic part of pacemaker lead heating during MRI is a resonant phenomenon which is complicated by, among other factors, the wide range of length scales involved in the problem. In this work, the multi-scalepart of the problem is taken into special consideration during the modeling process. The model incorporates a radio frequency coil, a human body phantom,and a highly detailed model of a pacemaker system with a bipolar lead that features helix-shaped conductors.Several configurations of pacemaker systems exposed to MRI are modeled and the results clearly show the importance of detailed lead modeling. Furthermore, modeling of resonant structures is investigated by a comparison between different modeling techniques. In addition, a meshing schemefor thin-wire approximations of helices is proposed, evaluated, and found to have improved convergence properties as compared to the conventional meshing approach.In recent years, magnetic tracking has been applied in many biomedical settings due to the transparency of the human body to low-frequency magnetic fields. In this work, the sensor positions of a magnetic tracking system are optimized by exploiting an analytical model where the transmitting and sensing coils of the system are approximated by magnetic dipoles.In order to compare different sensor array layouts, two performance measures based on the Fisher information matrix are discussed and compared for the optimization of the sensor positions of a circular sensor array. Furthermore,the sensor positioning problem is formulated as an optimization problem which is cast as a sensor selection problem. The sensor selection problem is solved for a planar sensor array by the application of a convex relaxation. Several transmitter positions are considered and general results are established for the dependence of the optimal sensor positions on the transmitter’s position and orientation.
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| 13. |
- Talcoth, Oskar, 1980
(författare)
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Electromagnetic Modeling and Sensitivity-Based Optimization of Medical Devices
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Electromagnetics is a fundamental part of biomedical engineering and modern healthcare due to the electromagnetic nature of several important processes in the human body and the interactions of electromagnetic fields with the human body. As a consequence, electromagnetics is exploited for diagnostic and therapeutic purposes by a multitude of medical devices. The biomedical engineering society strives to develop and design new methods, as well as, to improve existing methods for diagnosis and therapy. In addition, electromagnetic compatibility of both electromagnetic and non-electromagnetic medical devices must be assessed. These tasks can be complicated since electromagnetic measurements in the human body can be difficult and in some cases harmful to the patient. Furthermore, the human body is highly heterogeneous, which makes predictions of its interaction with electromagnetic fields demanding.In this thesis, these problems are mitigated by means of accurate, unbiased, and automatized electromagnetic modeling that feature a number of disciplines: (i) detailed electromagnetic modeling based on Maxwell’s equations; (ii) mathematics with particular emphasis on numerical analysis and optimization; and (iii) large-scale parallel computations on computer clusters. Progress in these three areas enables larger and more difficult problems to be addressed.In particular, this methodology is applied to three biomedical problems in this thesis. First, the electromagnetics of pacemaker lead heating in MRI is modeled with emphasis on the multi-scale characteristic of the problem. The results show the resonant nature of the problem and that detailed modeling is essential to accurately describe this phenomenon. Second, a method for optimization of sensor positions in magnetic tracking systems is proposed. The method uses powerful mathematics to alleviate the difficulties and computational burden associated with experimental or computational trial-and-error procedures. Third, the estimation procedure in EEG-based source localization is facilitated by exploiting electromagnetic reciprocity during the modeling. This reduces the demands for tailored estimation procedures and removes one obstacle for real-time source localization.
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| 14. |
- Talcoth, Oskar, 1980, et al.
(författare)
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Electromagnetic Modeling of Pacemaker Lead Heating During MRI
- 2011
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The electromagnetic part of pacemaker lead heating during magnetic resonanceimaging (MRI) is a resonant phenomenon which is complicated by, among otherfactors, the wide range of length scales involved in the problem. In this work, themulti-scale part of the problem is taken into special consideration during the modelingprocess. The model incorporates a radio frequency coil, a human body phantom,and a highly detailed model of a pacemaker system with a bipolar lead.Several configurations of pacemaker systems exposed to MRI are modeled and theresults clearly show the importance of detailed lead modeling. Furthermore, modelingof resonant structures is investigated by a comparison between different modelingtechniques.
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| 15. |
- Talcoth, Oskar, 1980, et al.
(författare)
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Modeling of a multi-scale electromagnetic problem: pacemaker lead heating in MRI
- 2012
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Ingår i: Proceedings of AntennEMB 2012. ; , s. 24-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Modeling of so-called multi-scale problems is challenging due to large differences between the length scales ofthe problem, which can be associated with large computational costs. For example, modeling of pacemaker leadheating by a 1.5 T magnetic resonance imaging (MRI) system requires length scales differing by a factor 1000 tobe resolved. Lead heating is the main cause of the contraindication of pacemakers for MRI [1]. The human bodysets the size of the largest scale whereas the helically shaped conductors of the pacemaker lead constitute thesmallest scale.To this date, modeling of these length scales and the heterogeneous body tissue has not been performedconcurrently. Neufeld et al. [2] exploited the finite-difference time-domain technique (FDTD) and modeled theheating caused by a single helix. However, the associated computational cost prevented them from examiningmore complex leads. Nevertheless, modern pacing leads often include two helically shaped conductors,consisting of several filars each, whose winding scheme has been shown experimentally to have significantimpact on the heating [3].Therefore, we devote special attention to the multi-scale part of the problem and exploit the frequency-domain method of moments to model an MRI radio frequency coil and a homogeneous human body phantom with animplanted pacemaker system. The pacemaker lead consists of two helically shaped conductors modeled as thinwires, insulation, and electrodes modeled by surfaces.We exploit the model to assess the effect on the heating by different factors, such as the presence of a pacemakerunit. Figure 1 shows the amplification of the absolute value of the electric field with respect to the fields in anempty phantom. Furthermore, we study the accuracy of the thin-wire approximation for densely wound helicesby comparing it to a surface discretization of the helix. Figure 2 shows the maximum value of the inducedcurrent on a straight helix which is illuminated by a plane wave polarized along the helix axis. The waveimpinges at a straight angle to the helix axis.[1] Götte et al. Magnetic resonance imaging, pacemakers and implantable cardioverter-defibrillators: currentsituation and clinical perspective. Netherlands heart journal, 18(1):31–7, January 2010.[2] Neufeld et al. Measurement, simulation and uncertainty assessment of implant heating during MRI. Physicsin medicine and biology, 54(13):4151–69, July 2009.[3] Bottomley et al. Designing passive MRI-safe implantable conducting leads with electrodes. MedicalPhysics, 37(7):3828–3843, 2010.
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| 16. |
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| 17. |
- Talcoth, Oskar, 1980, et al.
(författare)
-
Optimization of Sensor Positions in Magnetic Tracking
- 2011
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- In recent years, magnetic tracking has been applied in many biomedical settings dueto the transparency of the human body to low-frequency magnetic fields. One wayto improve system performance and/or reduce system cost is to optimize the sensorpositions of the tracking system.In this work, the sensor positions of a magnetic tracking system are optimized byexploiting an analytical model where the transmitting and sensing coils of the systemare approximated by magnetic dipoles.In order to compare different sensor array layouts, two performance measures basedon the Fisher information matrix are discussed and compared for the optimization ofthe sensor positions of a circular sensor array. Furthermore, the sensor positioningproblem is formulated as an optimization problem which is cast as a sensor selectionproblem. The sensor selection problem is solved for a planar sensor array by theapplication of a convex relaxation. Several transmitter positions are considered andgeneral results are established for the dependence of the optimal sensor positions onthe transmitter’s position and orientation.
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| 18. |
- Talcoth, Oskar, 1980, et al.
(författare)
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Sensitivity Optimization for Electromagnetic Measurement Systems by Sensor Selection
- 2013
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Ingår i: Proceedings of the 7th European Conference on Antennas and Propagation, EUCAP 2013. Göteborg, 8-12 April 2013.
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Konferensbidrag (refereegranskat)abstract
- We attempt to maximize the sensitivity of a sensor system, where the system sensitivity is expressed by a goal function formulated in terms of the Fisher information matrix. Given a large set of sensor candidates, we formulate the problem as a sensor selection problem by means of introducing a weight for each sensor candidate. Such a weight corresponds to the fraction of measurements that is performed by a specific sensor and it is allowed to vary continuously, which yields a convex optimization problem.
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| 19. |
- Talcoth, Oskar, 1980, et al.
(författare)
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Sensor selection in magnetic tracking based on convex optimisation
- 2013
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Ingår i: Electronics Letters. - : Institution of Engineering and Technology (IET). - 1350-911X .- 0013-5194. ; 49:1, s. 15-16
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Tidskriftsartikel (refereegranskat)abstract
- The performance of a magnetic tracking system is substantially influenced by the positions of its sensors. To optimise these sensor positions, exploited is a performance metric based on the Fisher information matrix and a convex relaxation of sensor selection problem. Optimised sensor positions are presented for a magnetic tracking system with a planar sensor array and a measurement domain consisting of one point.
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| 20. |
- Wang, Enlin, 1998, et al.
(författare)
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A 50 dBi E-Band Dual-reflector Antenna for 5G Backhauling with Auto-beam-tracking Function
- 2024
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Ingår i: IEEE Transactions on Antennas and Propagation. - 0018-926X .- 1558-2221. ; 72:6, s. 4874-4887
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Tidskriftsartikel (refereegranskat)abstract
- An ultra-high-gain E-band dual-reflector antenna for 5G backhaul with beam tracking function is proposed in this paper. Beam tracking is achieved by defocussing the feed away from the focus point of the sub-reflector, while keeping the interface with the transmit-receive unit stationary by applying a movable gap-waveguide configuration. The antenna has been prototyped and measured. The measured reflection coefficients remain below –10 dB for both horizontal and vertical polarizations over 71 – 86 GHz, almost independent from the offset positions of the feed. The measured radiation patterns agree well with the simulated ones, with a peak gain of about 50 dBi in the band and less than 1 dB gain drop for beam steering within ±2°. Finally, a low-cost and environmentally friendly mechanical tracking system is proposed, prototyped, and tested. Measurements have verified that the tracking system functions well for slow sways.
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| 21. |
- Wang, Enlin, 1998, et al.
(författare)
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Applications of Contactless Characteristics of Gap Waveguides in Mechanical Reconfigurability
- 2023
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Ingår i: 2023 IEEE International Symposium on Antennas and Propagation, ISAP 2023.
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Konferensbidrag (refereegranskat)abstract
- Three reconfigurable transmission lines (TLs) (a sheath rectangular waveguide, a movable rectangular waveguide, and a universal-joint circular waveguide) have been proposed in this work to extend the applications of gap waveguide technology in the domain of mechanical reconfigurability. These three reconfigurable TLs enable movements of waveguide ports through mechanisms of sheathing, displacement, and rotation without wave propagation leakage, and thereby provide possibilities for antenna mechanical reconfigurability.
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