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• 1.
KTH, School of Electrical Engineering (EES). Lund University, Sweden.
On the Physical Limitations of the Interaction of a Spherical Aperture and a Random Field2011In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 59, no 1, p. 119-128, article id 5618544Article in journal (Refereed)

This paper derives physical limitations on the interactions of antennas exciting TM or TE modes (but not both) and wireless propagation channels. The derivation is based on the spherical vector wave expansion of the electromagnetic field outside a sphere circumscribing the antennas. The result is an extension of the seminal work of Chu on the classical limitations on maximum antenna gain and radiation Q. Rather than maximizing antenna gain in a single direction we obtain physical limitations on the antenna gain pattern, which is directly translated to more condensed parameters, i.e., the instantaneous effective gain G(i) and the mean effective gain G(e) if instantaneous realizations or correlation statistics of the expansion coefficients of the electromagnetic field are known, spectively. The obtained limitations are on the maximum of G(i)/Q and G(e)/Q, which establish a trade-off between link gain and Q.

• 2.
Lund University.
Lund University. Lund University. Lund University. Lund University.
Spherical Vector Wave Expansion of Gaussian Electromagnetic Fields for Antenna-Channel Interaction Analysis2009In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 57, no 7, p. 2055-2067Article in journal (Refereed)

In this paper, we introduce an approach to analyze the interaction between antennas and the propagation channel. We study both the antennas and the propagation channel by means of the spherical vector wave mode expansion of the electromagnetic field. Then we use the expansion coefficients to study some properties of general antennas in thosefields by means of the antenna scattering matrix. The focus is on the spatio-polar characterization of antennas, channels and their interactions. We provide closed form expressions for the covariance of the field multimodes as function of the power angle spectrum (PAS) and the channel cross-polarization ratio (XPR). A new interpretation of the mean effective gains (MEG) of antennas is also provided. The maximum MEG is obtained by conjugate mode matching between the antennas and the channel; we also prove the (intuitive) results that the optimum decorrelation of the antenna signals is obtained by the excitation of orthogonal spherical vector modes.

• 3.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
Impact of Scattering Within a Multipath Simulator Antenna Array on the Ricean Fading Distribution Parameters in OTA Testing2014In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 62, no 6, p. 3257-3269Article in journal (Refereed)

In this paper, we investigate the unwanted scattering that exists within the multipath simulator (MPS) array antennas employed in over the air (OTA) testing of mobile terminals. The impact of scattering is evaluated in terms of the measurement uncertainty of the average received power and the Ricean K-factor. The maximum ratio combining diversity is investigated for a generic device under test comprising two half-wavelength dipole antennas. We provide closed-form expressions for the uncertainties of the average received power and the Ricean K-factor for a uniform circular array of MPS antennas and a 2-D uniformly distributed angle-of-arrival spectrum. We also derive the maximum number of MPS antennas and the minimum ring radius of the MPS system as a function of the separation between the most distant antenna elements if the device under test employs a uniform linear array. As a result, we provide design guidelines for MPS array in terms of the number antennas, the radius of the MPS array and the wavelength of the carrier frequency.

• 4. Bantavis, P. I.
KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering. KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
A Cost-Effective Wideband Switched Beam Antenna System for a Small Cell Base Station2018In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 66, no 12, p. 6851-6861, article id 8485638Article in journal (Refereed)

A wideband switched beam antenna array system operating from 2 to 5 GHz is presented. It is comprised of a 4 × 1 Vivaldi antenna elements and a 4 × 4 Butler matrix beamformer driven by a digitally controlled double-pole four-throw RF switch. The Butler matrix is implemented on a multilayer structure, using 90° hybrid couplers and 45° phase shifters. For the design of the coupler and phase shifter, we propose a unified methodology applied, but not limited, to elliptically shaped geometries. The multilayer realization enables us to avoid microstrip crossing and supports wideband operation of the beamforming network. To realize the Butler matrix, we introduce a step-by-step and stage-by-stage design methodology that enables accurate balance of the output weights at the antenna ports to achieve a stable beamforming performance. In this paper, we use a Vivaldi antenna element in a linear four-element array, since such element supports wideband and wide-scan angle operation. A soft condition in the form of corrugations is implemented around the periphery of the array, in order to reduce the edge effects. This technique improved the gain, the sidelobes, and helped to obtain back radiation suppression. Finally, impedance loading was also utilized in the two edge elements of the array to improve the active impedance. The proposed system of the Butler matrix in conjunction with the constructed array can be utilized as a common RF front end in a wideband air interface for a small cell 5G application and beyond as it is capable to simultaneously cover all the commercial bands from 2 to 5 GHz.

• 5. Bosiljevac, Marko
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Efficient Analysis of Aperture Antennas on Generally Shaped Convex Multilayered Surfaces Using a Hybrid SD-UTD Method2009In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 57, no 5, p. 1420-1431Article in journal (Refereed)

A novel hybrid method is described for analyzing convex multilayered conformal array antennas. The hybrid method is based on the spectral domain approach in combination with the ray-based uniform theory of diffraction (UTD) method. The analysis is divided in two parts. First, the spectral domain approach is accelerated by using an asymptotic extraction technique where the extracted term of the Green's function is calculated using UTD. It is shown that this new approach results in significant acceleration of the existing spectral domain algorithm without losing accuracy. The modified spectral domain method is then used in the second part where generally shaped convex multilayered surfaces are analyzed by using sets of canonically shaped surfaces (spheres and/or circular cylinders). Their radii are obtained using the UTD formulation, which contains important information such as distance and curvature of the generally shaped surface along each geodesic. The results obtained using the new algorithm are compared to the available results (calculated and measured) for different conformal antennas, showing very good agreement.

• 6. Braun, Christian
Allgon System AB, Sweden.
Evaluation of antenna diversity performance for mobile handsets using 3-D measurement data1999In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 47, no 11, p. 1736-1738Article in journal (Refereed)

An evaluation of two-branch antenna diversity on mobile telephones is performed in the presence of head and hand phantoms to model the influence of the user. Two mobile telephone prototypes for the 1800-MHz band are considered. The evaluation is done by measuring the three-dimensional (3-D) complex radiation patterns from each antenna with the prototypes positioned near the phantom head and hand (i.e., in talk position) and calculating the diversity performance for three different scenarios of the incoming field. The two-branch antenna setups on both prototypes produce a calculated diversity gain of about 9 dB for selection diversity at the 99% reliability level.

• 7.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Bespoke Lenses Based on Quasi-Conformal Transformation Optics Technique2017In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 65, no 5, p. 2256-2264Article in journal (Refereed)

In this paper, a systematic method to design a quasioptimum lens profile based on quasi-conformal transformation optics technique for a given excitation is presented. This method is applied to a number of conventional antennas, such as an aperture and a log-spiral slot. In all these configurations, an increase of the directivity is observed. Furthermore, using this method, a quasi-optimum graded index lens for a broadband enhanced leaky slot excitation is designed and the results are compared with a hyperhemispherical lens with and without matching layers. Our proposed methodology demonstrates to be able to increase the directivity, to reduce the sidelobes and the cross polarization in a broad bandwidth from 20 to 70 GHz. Due to the continuously changed dielectric constant of the lens profile, reflections are also reduced considerably inside the lens.

• 8.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
A Scan Blindness Model for Single-Polarized Tapered-Slot Arrays in Triangular Grids2008In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 56, no 9, p. 2937-2942Article in journal (Refereed)

A simple model is presented that predicts scan blindness in the E-plane for single polarized tapered-slot arrays where the elements are positioned in a triangular grid. The blindness occurs when the phase progression along the dielectric substrates is equal to the phase progression of a leaky mode in the structure. A simple equation is presented that estimates the frequency at which the scan blindness occurs for a given angle in the E-plane. This equation is derived from a simple representation of the phased array antenna, but includes the dominant effect of this particular class of scan blindness. The method is shown to agree well with computed results obtained with two different numerical codes.

• 9.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
A single polarized triangular grid tapered-slot array antenna2009In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 57, no 9, p. 2599-2607Article in journal (Refereed)

A triangular grid single polarized tapered-slot array antenna for radar applications is studied. Compared with a rectangular grid an equilateral triangular grid allows a larger unit cell without any onset of grating lobes. Since single polarized tapered-slots in triangular grids support guided modes, which cause scan blindness, the increase in unit cell size is smaller than the optimal 15%. The design presented in the paper is capable of scan angles out to 60° from broadside in the E and H planes. To improve the match over the radar band a local minimum in the active reflection coefficient is positioned at the most critical scan direction, resulting in a reflection coefficient that is less than -12 dB in the X-band. To reduce the radar cross section for the cross-polarization an absorbing layer is positioned above the ground plane, which affects some of the guided modes that lead to scan blindnesses. An experimental antenna with 16×16 elements was built, and it was found that the H-plane performance for large scan angles for the finite antenna deviates more than expected from the infinite array approximation. Otherwise both mutual coupling measurements and embedded element patterns agrees well with the numerical results.

A compact ultra-wideband multibeam antenna system2018In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 66, p. 125-131Article, review/survey (Refereed)
A Parallel Plate Ultrawideband Multibeam Microwave Lens Antenna2018In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 66, p. 4878-4883Article, review/survey (Refereed)
• 12.
KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
An Approximate Method for Calculating the Near-Field Mutual Coupling Between Line-of-Sight Antennas on Vehicles2015In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 63, no 9, p. 4132-4138Article in journal (Refereed)

Calculating the mutual coupling between antennas on vehicles using full-wave simulations requires a vast amount of computer resources due to the electrical size of the structures. We therefore propose an alternative and approximate method to determine mutual coupling between antennas on vehicles for the case where there is line-of-sight (LOS) between the antennas. The proposed method is based on approximating the mutual coupling between LOS antennas on vehicles as near-field transmission between antennas in free space. We begin the analysis with a brief review of four methods for calculating the near-field free-space transmission. Of the investigated methods, we demonstrate that a nonsingular form of the near-field transmission integral originally proposed by Yaghjian (1982) is the most suitable for LOS antennas on vehicles. We introduce a modification to this method, in order to only use the antenna far-fields and geometrical separation to determine the mutual coupling. The comparison with full-wave simulations indicates that the proposed method has a good accuracy for LOS antennas. This paper ends with a full-scale mutual coupling calculation for two monopoles on an aircraft under LOS conditions, demonstrating a root mean square (rms) accuracy of 6 dB for frequencies up to 5 GHz, as compared with full-wave simulations.

• 13.
KTH, School of Electrical Engineering and Computer Science (EECS), Micro and Nanosystems.
Institut National des Sciences Appliquées de Rennes: Rennes, Bretagne, FR. Université de Rennes 1: Rennes, Bretagne, FR. CNRS Délégation Bretagne et Pays de Loire: Rennes, Bretagne, FR. Université de Rennes 1: Rennes, FR. KTH, School of Electrical Engineering and Computer Science (EECS), Micro and Nanosystems.
A Low-Profile and High-Gain Frequency Beam Steering Subterahertz Antenna Enabled by Silicon MicromachiningIn: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221Article in journal (Refereed)

A very low-profile sub-THz high-gain frequencybeam steering antenna, enabled by silicon micromachining, is reported for the first time in this paper. The operation bandwidth of the antenna spans from 220 GHz to 300 GHz providing a simulated field of view of 56°. The design is based on a dielectric filled parallel-plate waveguide (PPW) leaky-wave antenna fed by a pillbox. The pillbox, a two-level PPW structure, has an integrated parabolic reflector to generate a planar wave front. The device is enabled by two extreme aspect ratio, 16 mm x16 mm large perforated membranes, which are only 30 μm thick, that provide the coupling between the two PPWs and form the LWA. The micromachined low-loss PPW structure results in a measured average radiation efficiency of −1 dB and a maximum gain of 28.5 dBi with an input reflection coefficient below −10 dB. The overall frequency beam steering frontend is extremely compact (24mm x 24mm x 0.9 mm) and can be directly mounted on a standard WM-864 waveguide flange. The design and fabrication challenges of such high performance antenna in the sub-THz frequency range are described and the measurement results of two fabricated prototypes are reported and discussed.

• 14. Gustafsson, Mats
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Physical Bounds and Optimal Currents on Antennas2012In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 60, no 6, p. 2672-2681Article in journal (Refereed)

Physical bounds on the directivity Q-factor quotient and optimal current distributions are determined for antennas of arbitrary shape and size using an optimization formulation. A variational approach offers closed form solutions for small antennas expressed in the polarizability of the antenna structure. Finite sized antennas are solved using Lagrangian parameters in a method of moments formulation. It is also shown that the optimal charge density for a small antenna can be generated by several current densities. Numerical examples for small and large antennas are used to illustrate the results.

• 15. Gustafsson, Mats
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Antenna Q and Stored Energy Expressed in the Fields, Currents, and Input Impedance2015In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 63, no 1, p. 240-249Article in journal (Refereed)

Although the stored energy of an antenna is instrumental in the evaluation of antenna Q and the associated physical bounds, it is difficult to strictly define stored energy. Classically, the stored energy is either determined from the input impedance of the antenna or the electromagnetic fields around the antenna. The new energy expressions proposed by Vandenbosch express the stored energy in the current densities in the antenna structure. These expressions are equal to the stored energy defined from the difference between the energy density and the far field energy for many but not all cases. Here, the different approaches to determine the stored energy are compared for dipole, loop, inverted L-antennas, and bow-tie antennas. We use Brune synthesized circuit models to determine the stored energy from the input impedance. We also compare the results with differentiation of the input impedance and the obtained bandwidth. The results indicate that the stored energy in the fields, currents, and circuit models agree well for small antennas. For higher frequencies, the stored energy expressed in the currents agrees with the stored energy determined from Brune synthesized circuit models whereas the stored energy approximated by differentiation of input impedance gives a lower value for some cases. The corresponding results for the bandwidth suggest that the inverse proportionality between the fractional bandwidth and Q-factor depends on the threshold level of the reflection coefficient.

• 16.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
On Methods to Determine Bounds on the Q-Factor for a Given Directivity2017In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 65, no 11, p. 5686-5696Article in journal (Refereed)

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.

• 17.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Decoupling of Multiple Antennas in Terminals With Chassis Excitation Using Polarization Diversity, Angle Diversity and Current Control2012In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 60, no 12, p. 5947-5957Article in journal (Refereed)

Excitation of the chassis enables single-antenna terminals to achieve good bandwidth and radiation performance, due to the entire chassis being utilized as the main radiator. In contrast, the same chassis excitation phenomenon complicates the design of multiple antennas for MIMO applications, since the same characteristic mode of the chassis may be effectively excited by more than one antenna, leading to strong mutual coupling and severe MIMO performance degradation. In this paper, we introduce a design concept for MIMO antennas to mitigate the chassis-induced mutual coupling, which is especially relevant for frequency bands below 1 GHz. We illustrate the design concept on a dual-antenna terminal at 0.93 GHz, where a folded monopole at one chassis edge excites the chassis' fundamental electric dipolemode and a coupled loop at the other chassis edge excites its own fundamental magnetic dipole mode. Since the two radiationmodes are nearly orthogonal to each other, an isolation of over 30 dB is achieved. Moreover, we show that the antenna system can be conveniently modified for multiband operation, such as in the 900/1800/2600MHz bands. Furthermore, by controlling the phase of the feed current on the folded monopole, the two antennas can be co-located on the same chassis edge with an isolation of over 20 dB. The co-located dual-antenna prototype was fabricated and verified in the measurements.

• 18. Li, Hui
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Equivalent Circuit Based Calculation of Signal Correlation in Lossy MIMO Antennas2013In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 61, no 10, p. 5214-5222Article in journal (Refereed)

Correlation coefficient of received signals across a pair of antennas is a key performance indicator for multiple-input-multiple-output (MIMO) systems. For multipath environments with uniform 3-D angular power spectrum, the signal correlation between two antennas can be exactly calculated from their 3-D radiation patterns. When radiation patterns are unavailable, a simplified approach that only requires the antennas' scattering parameters can be used instead. However, the simpler method assumes lossless antennas and thus only works well for antennas with high radiation efficiencies. To take into account the antenna loss, the idea of equivalent circuit approximation is used in this paper to analytically separate the lossy components (resistance or conductance) from the lossy antenna arrays, using known scattering parameters and radiation efficiencies. The simplified method using S parameters can then be applied to obtain the correlation coefficient of the equivalent lossless antennas. The effectiveness of the method has been verified on antennas operating at a single mode, such as dipole or patch at its lowest resonant frequency. Good results were also obtained for the measured case of a dual-antenna mobile terminal, consisting of a monopole and a PIFA.

• 19.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Characteristic Mode Based Tradeoff Analysis of Antenna-Chassis Interactions for Multiple Antenna Terminals2012In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 60, no 2, p. 490-502Article in journal (Refereed)

The design of multiple antennas in compact mobile terminals is a significant challenge, due to both practical and fundamental design tradeoffs. In this paper, fundamental antenna design tradeoffs of multiple antenna terminals are presented in the framework of characteristic mode analysis. In particular, interactions between the antenna elements and the characteristic modes and their impact on design tradeoffs are investigated in both theory and simulations. The results reveal that the characteristic modes play an important role in determining the optimal placement of antennas for low mutual coupling. Moreover, the ability of antenna elements to localize the excitation currents on the chassis can significantly influence the final performance. To demonstrate the effectiveness of the proposed approach, a dual-band, dual-antenna terminal is designed to provide an isolation of over 10 dB for the 900 MHz band without additional matching or decoupling structures. A tradeoff analysis of bandwidth, efficiency, effective diversity gain and capacity is performed over different antenna locations. Finally, three fabricated prototypes verify the simulation results for representative cases.

• 20.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
A Simple Compact Reconfigurable Slot Antenna With a Very Wide Tuning Range2010In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 58, no 11, p. 3725-3728Article in journal (Refereed)

A simple and compact slot antenna with a very wide tuning range is proposed. A 25 mm (roughly equal to lambda(H)/8, where lambda(H) corresponds to the highest frequency of the tuning range) open slot is etched at the edge of the ground. To achieve the tunability, only two lumped elements, namely, a PIN diode and a varactor diode are used in the structure. By switching the PIN diode placed at the open end of the slot, the slot antenna can resonate as a standard slot (when the switch is on) or a half slot (when the switch is off). Continuous tuning over a wide frequency range in those two modes can be achieved by adjusting the reverse bias (giving different capacitances) of the varactor diode loaded in the slot. Through optimal design, the tuning bands of the two modes are stitched together to form a very wide tuning range. The fabricated prototype has a tuning frequency range from 0.42 GHz to 1.48 GHz with S11 better than -10 dB, giving a frequency ratio (f(R) = f(u)/f(L)) of 3.52:1. The measured full spherical radiation patterns show consistent radiation characteristics of the proposed antenna within the whole tuning range.

• 21.
KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
Antenna and Sub-Millimeter Waves Section, European Space Agency, Noordwijk, 2200 AG, Netherlands. KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
Compact Multibeam Fully Metallic Geodesic Luneburg Lens Antenna Based on Non-Euclidean Transformation Optics2018In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 66, no 12, p. 7383-7388Article in journal (Refereed)

Non-Euclidean transformations have been recently proposed to produce a link between 3-D homogeneous surfaces and 2-D dielectric lenses. Therefore, the propagation in a geometrical surface has the same response of an equivalent refractive index distribution. By using this concept, we propose here a fully metallic Luneburg lens where the propagation is only in the air. Two metallic plates, following a curved shape, are employed to support the propagation mimicking the designed curvature. To reduce the height of the required curvature, the surface has been mirrored twice with respect to two z constant planes. The lens is fed by 11 waveguide ports spaced with an angle of 12.5° providing 1-D beam scanning over an angular range of ±62.5°. A prototype is manufactured and measured with a good agreement with the simulated results between 25 and 36 GHz to demonstrate the concept.

• 22.
KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Electromagnetic Engineering.
Univ Carlos III Madrid, Dept Commun & Signal Theory, Leganes 28911, Spain.. KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Electromagnetic Engineering.
Ka-Band Fully Metallic TE40 Slot Array Antenna With Glide-Symmetric Gap Waveguide Technology2019In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 67, no 10, p. 6410-6418Article in journal (Refereed)

Gap waveguide has recently been proposed as a low-loss and low-cost technology for millimeter-wave components. The main advantage of the gap waveguide technology is that the microwave components can be manufactured in two metallic pieces that are assembled together without electrical contact. The leakage through a thin air gap between the two pieces is prevented by a 2-D periodic structure offering an electromagnetic bandgap (EBG). This EBG is conventionally implemented with metallic pins. Here, we propose the usage of a holey glide-symmetric EBG structure to design a $4\times 4$ slot array antenna that is fed with a TE40 mode. The TE40 excitation is designed based on a TE10-TE20 mode converter whose performance is initially evaluated by radiation pattern measurements. The final antenna, the $4\times 4$ slot array antenna, was manufactured in aluminum by computer numerical control (CNC) milling. The antenna has a rotationally symmetric radiation pattern that could find application as a reference antenna as well as for 5G point-to-point communications.

• 23. Lin, Zhili
KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
On the Accuracy and Stability of Several Widely Used FDTD Approaches for Modeling Lorentz Dielectrics2009In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 57, no 10, p. 3378-3381Article in journal (Refereed)

A rigorous and comparative study on the approximation accuracy and stability limits of several widely used finite-difference time-domain (FDTD) approaches, namely the auxiliary differential equation (ADE) approach, the bilinear transform (BT) approach, the Z-transform approach (ZT) and the piecewise linear recursive convolution (PLRC) approach, for modeling dispersive Lorentz dielectrics is presented following the given updating equations between the electric flux density and the electric field intensity. We find the ZT approach with modified material parameters is much more accurate than the original ZT approach and the other three approaches for modeling Lorentz dielectrics. The stability limits of the ADE, ZT and PLRC approaches in simulating Lorentz dielectrics are also shown to be a bit more stringent than that of BT approach which preserves the Courant stability limit as previously reported.

• 24. Lindmark, B.
Chalmers University of Technology, Sweden.
Dual-polarized array for signal-processing applications in wireless communications1998In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 46, no 6, p. 758-763Article in journal (Refereed)

A novel dual-polarized antenna array designed for a spatial division multiple access (SDMA) system working in the 1850-1990-MHz band is designed and built. The antenna is designed to have similar element patterns, and measurements of S-parameters and radiation patterns are presented. The array signal processing performance of the array using all elements and with no compensation for mutual coupling or differences in element patterns is studied through direction-of-arrival (DOA) estimation using total least squares estimation of signal parameters via rotational invariance techniques (TLS-ESPRIT). The results show that the accuracy of the DOA estimates is quite acceptable for wireless communication applications.

• 25.
KTH, School of Electrical Engineering (EES), Signal Processing.
Comparison of mutual coupling compensation to dummy columns in adaptive antenna systems2005In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 53, no 4, p. 1332-1336Article in journal (Refereed)

Compensation for mutual coupling in antenna arrays by matrix multiplication is compared to the use of dummy elements. A least squares estimation of the coupling matrix is made, including co- and cross-polar coupling. We show that compensation for measurements off the phase center is important as well as a proper assumption of the ideal element pattern. We study the performance of a dual polarized patch array with respect to far-field phase error, signal-to-interference reduction, and cross-polarization level. In all aspects, the performance of the compensation method exceeds or equals the use of dummy elements.

• 26.
Saab Surveillance, SE-17588 Stockholm, Sweden..
Saab Surveillance, Elect Warfare Div, SE-17588 Stockholm, Sweden.. KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
On the Accuracy of Equivalent Antenna Representations-A Case Study2018In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 66, no 7, p. 3251-3260Article in journal (Refereed)

The accuracy of two equivalent antenna representations, near-field sources and far-field sources (FFSs), is evaluated for an antenna installed on a simplified platform. We show that the accuracy of the installed far-field and surface current for the investigated weakly scattering platform depends strongly on the configurations associated with the equivalent antenna representation. The root-mean-square error for the installed far-field error varies between 4.4%-8.4% for the considered configurations of near-field equivalent representations installed on the investigated platform. When using FFSs, the design parameters have an even larger influence of the achieved accuracy. There is also a varying accuracy depending on the type of numerical method used. Based on the results, some recommendations on the choice of subdomain for the equivalent antenna representation are given associated with the platform. In industrial antenna applications, the accuracy in determining, e.g., installed far-fields and antenna isolation on large platforms is critical. Equivalent representations can reduce the fine-detail complexity of antennas and thus give an efficient numerical description to be used in large-scale simulations. The results, in this paper, highlight to antenna designers and system engineers the accuracy challenges associated with the use of equivalent antenna representations.

• 27.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
A Circularly Polarized Cylindrical Dielectric Resonator Antenna Using a Helical Exciter2013In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 61, no 3, p. 1439-1443Article in journal (Refereed)

A novel circularly polarized cylindrical dielectric resonator antenna excited by an external tape helix is presented. The helix is fed by a coaxial line through a small hole on a finite size ground plane. The configuration offers a compact and easy to fabricate feeding network providing a 3 dB axial-ratio bandwidth of 6.4%. A prototype of the proposed configuration is fabricated and measured. Measured and simulated return loss, axial-ratio, radiation pattern, and realized gain are presented and discussed together with design guidelines.

• 28.
Ericsson Research, Ericsson AB, Stockholm, Sweden.
Ericsson Research, Ericsson AB, Stockholm, Sweden.
Smart antennas in a WCDMA Radio Network System Revisited: Modeling and Evaluations2006In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 54, no 11, p. 3302-3316Article in journal (Refereed)

A comprehensive study is given of smart antennas for wide-band code-division multiple access (WCDMA). It analyzes, evaluates, and presents methods and ideas for the deployment of smart antennas in WCDMA third-generation wireless systems and for future generations. Three major implementations are evaluated and analyzed in a dynamic system simulator: higher order sectorization (HOS), fixed beams (FBs) with secondary common pilot channel (S-CPICH), and FBs with primary common pilot channel (P-CPICH) as a phase reference. The system modeling integrates antenna and wave propagation modeling to evaluate and design smart antennas in wireless communications. Furthermore, this paper analyzes the impact of angular spread of various antenna configurations, the interaction and impact of radio resource management as power tuning of the common channel, scrambling code (SCO) allocation technique, and soft and softer handover algorithm. Moreover, an SCO allocation strategy and an adaptive load-dependent power-tuning algorithm of the P-CPICH are proposed. Furthermore, a signal-to-(interference plus noise) ratio derivation is given. Extensive simulation studies are carried out to evaluate the capacity gains of HOS and FB. The 12-sector sites yield the best system downlink capacity gain compared to three-sector sites equipped with a single antenna, slightly more than three-sector sites equipped with four FBs each. The six-sector sites equipped with two FBs each reduce the gap between HOS and FB systems to 10%.

• 29.
KTH, Superseded Departments, Electromagnetic Theory.
Calculating the mutual coupling between apertures on a convex circular cylinder using a hybrid UTD-MoM method2001In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 49, no 4, p. 672-677Article in journal (Refereed)

In this paper, the problem of calculating the mutual coupling between apertures on a perfectly conducting circular cylinder is considered. The mutual coupling is calculated by a combination of uniform theory of diffraction (UTD) and the method of moments (MoM). This hybrid method makes it possible to include higher order aperture modes in order to improve the accuracy. It is found that the UTD-MoM solution gives accurate results, even for small cylinders. The results are verified against results found in the literature, as well as against measurements performed at Ericsson Microwave System in Molndal, Sweden. For this purpose, a cylindrical test antenna with 54 apertures was built. Using a single-mode approximation, good agreement down to about -60 dB, comparisons to measured data has been obtained. If additional modes are added the results improve significantly both in amplitude and phase, Zn several cases, the agreement is good down to coupling levels about -80 dB. This requires that about six modes are included. However, the most important mode, besides the fundamental TE10 mode is the TM11 mode.

• 30.
KTH, Superseded Departments, Electromagnetic Theory.
Investigation of the mutual coupling between apertures on doubly curved convex surfaces: Theory and measurements2003In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 51, no 4, p. 682-692Article in journal (Refereed)

A hybrid method of high-frequency approximation (UTD) and method of moments is used to calculate. the mutual coupling between circular apertures on a doubly curved surface. This requires that the geodesics on doubly curved surfaces be found. To represent the aperture fields, a single waveguide mode approximation is used. In order to verify the theoretical results, a general paraboloid of revolution with circular waveguide-fed apertures was built at Ericsson Microwave Systems AB, Molndal, Sweden. This has given a possibility to verify the theoretical model for a general doubly curved surface against measurements. To the authors' knowledge, no such verification is available in the literature. It was found that measured and theoretical results in terms of mutual coupling agree very well for copolar coupling. As expected, the mutual coupling results are very dependent on the polarization. In fact, the definition of polarization directions and the heed for polarization control are important issues for doubly curved conformal array antennas.

• 31. Rajo-Iglesias, Eva
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Textile Soft Surface for Back Radiation Reduction in Bent Wearable Antennas2014In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 62, no 7, p. 3873-3878Article in journal (Refereed)

A textile soft surface is proposed to reduce back radiation of a textile patch antenna, and the performance is analyzed when the antenna is placed on a bent surface. This surface is assumed to be curved around cylinders with varying radii to emulate the real operation of the textile antenna when it is worn on the body, e. g., back, shoulders or arms. Two scenarios are considered for the evaluation of the performance of the antenna with the soft surface: a bent finite ground plane over an air cylinder and a more accurate model in which the electromagnetic properties of the body are included. In both situations the back radiation is reduced when compared to the same antenna without the soft surface. These results have been validated with experimental data which support this conclusion. This is the first textile implementation of a soft surface and the first demonstration that a soft surface can reduce the back radiation of a patch antenna in a conformal configuration.

• 32. Simovski, C. R.
KTH, Superseded Departments, Electromagnetic Theory.
Backward wave region and negative material parameters of a structure formed by lattices of wires and split-ring resonators2003In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 51, no 10, p. 2582-2591Article in journal (Refereed)
• 33.
Air Force Research Laboratory, Hanscom AFB.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. Saab Bofors Dynamics AB.
Spiral Elements for Broad-Band Phased Arrays2005In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 53, no 8, p. 2558-2562Article in journal (Refereed)

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.

• 34. Syrytsin, Igor
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Statistical Investigation of the User Effects on Mobile Terminal Antennas for 5G Applications2017In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 65, no 12, p. 6596-6605Article in journal (Refereed)

In this paper, the user effects on mobile terminal antennas at 28 GHz are statistically investigated with the parameters of body loss, coverage efficiency, and power in the shadow. The data are obtained from the measurements of 12 users in data and talk modes, with the antenna placed on the top and bottom of the chassis. In the measurements, the users hold the phone naturally. The radiation patterns and shadowing regions are also studied. It is found that a significant amount of power can propagate into the shadow of the user by creeping waves and diffractions. A new metric is defined to characterize this phenomenon. A mean body loss of 3.2-4 dB is expected in talk mode, which is also similar to the data mode with the bottom antenna. A body loss of 1 dB is expected in data mode with the top antenna location. The variation of the body loss between the users at 28 GHz is less than 2 dB, which is much smaller than that of the conventional cellular bands below 3 GHz. The coverage efficiency is significantly reduced in talk mode, but only slightly affected in data mode.

• 35.
KTH, Superseded Departments, Alfvén Laboratory.
The RCS of a clylindrical array antenna coated with a dielectric layer2004In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 52, no 7, p. 1851-1858Article in journal (Refereed)

The scattering properties of dielectric coated waveguide aperture antennas mounted on circular cylinders are investigated. Both the single element antenna and the array case are treated. The array antenna consists of 4 x 32 rectangular apertures placed in a rectangular grid on the surface of an infinitely long circular cylinder. The problem is formulated in terms of an integral equation for the aperture fields which is solved with the method of moments using rectangular waveguide modes as basis and test functions. An efficient uniform asymptotic technique is used to calculate the excitation vector and the backscattered far-field. The asymptotic solution is valid for large cylinders coated with thin dielectric layers away from the paraxial (i.e. near axial) region. A similar asymptotic solution is used to calculate the mutual coupling in the nonparaxial region. For the self coupling terms and for the mutual coupling in the paraxial region a planar approximation is used with a corresponding spectral domain technique. Numerical results are presented as a function of frequency, angle of incidence, cylinder radius, and electrical thickness of the coating.

• 36.
KTH, School of Electrical Engineering (EES).
KTH, School of Electrical Engineering (EES).
Broad-band fragmented aperture phased array element design using genetic algorithms2005In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 53, no 10, p. 3280-3287Article in journal (Refereed)

In this paper, a synthesis procedure to design thin broad-band fragmented aperture array elements is described. The arrays are assumed to be infinite periodic and the elements consist of a conducting pattern etched on a dielectric backed by a groundplane. A genetic algorithm (GA) is used to design the conducting pattern, relative permittivity, and thickness of the dielectric substrate with respect to array scan and bandwidth performance. The fitness function in the GA is evaluated using a finite-difference time-domain code with periodic boundary conditions. For a substrate thicker than about 0.1 lambda(L) (lambda(L)=wavelength at the lowest frequency in the frequency band investigated), it was found that a bandwidth of at least one octave can be obtained for arrays scanned within 45 degrees from broadside.

• 37. Valerio, Guido
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Accurate Equivalent-Circuit Descriptions of Thin Glide-Symmetric Corrugated Metasurfaces2017In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 65, no 5, p. 2695-2700Article in journal (Refereed)

Thin artificial surfaces that act as high frequency bandgap structures have been recently studied for the design of gap waveguides, hard surfaces, and planar lenses. Here, we propose a circuit-based method to analyze glide-symmetric corrugated metasurfaces that are embedded in a thin parallel plate waveguide. Our closed-form solution is based on rigorous analytical derivations. It achieves remarkable agreement with full-wave solvers, even when the waveguide thickness is extremely thin. In contrast, classical homogenization approaches are shown to be inaccurate for thin waveguides due to the interaction of higher order Floquet modes between the surfaces. Numerical results validate our theoretical analysis and show the utility of the proposed method.

• 38. Vukomanovic, Mladen
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Gap Waveguide Leaky-Wave Antenna2016In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 64, no 5, p. 2055-2060Article in journal (Refereed)

A novel leaky-wave antenna, realized using gap waveguide concept, is here proposed. Both, the feeding structure and the antenna are realized in groove gap waveguide technology, thus simplifying the whole structure. A complete design procedure, starting from a given groove waveguide to the final radiation mechanism, is described in this communication. The results are supported with measurements of a prototype that operates in the X-band. The realized antenna has a gain around 18 dB between 9 and 11.5 GHz, demonstrating the potential of the proposed radiation technology.

• 39.
Department of Electrical Engineering and Computer Science, University of Newcastle, Australia.
Experimental and theoretical comparison of some algorithms for beamforming in single receiver adaptive arrays1991In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 39, no 1, p. 21-28Article in journal (Refereed)

Adaptive null steering in single receiver adaptive arrays is discussed. The single receiver structure allows only output power for a given set of weights to be measured. The problem, then, is to adaptively adjust the weights of the antenna array, based on output power measurements only, so as to reject interference signals while maintaining a fixed response in a given look direction. Current least-mean-square (LMS)-based iterative techniques use perturbations of the beamformer weights to obtain an estimate of the gradient of a given cost (power) function in order to update an initial guess. The key idea of this work is to determine the optimal beamformer weights in a single step, by estimating the covariance matrix of the array sensor outputs using a weight perturbation technique. Based on this covariance matrix estimate, three different approaches for finding the beamformer weights are studied. The first corresponds to a sample matrix inversion (SMI) scheme, with the sample covariance matrix replaced by the one obtained from the perturbation estimation method, while in the second approach the weights are determined using an eigenvalue decomposition of the covariance matrix estimate. In the third approach the directions-of-arrival (DOAs) of the incoming wavefronts are first estimated, and then, in a second step, the beamformer weights are calculated from the DOA estimates. The advantage of the third approach is that this method is not affected by correlation between the different sources. In comparison to iterative techniques, the approach offers much faster beamforming, albeit at the expense of increased susceptibility to unmodeled characteristics.

• 40.
Hamburg Univ Technol, Inst Electromagnet Theory, D-21079 Hamburg, Germany..
Tech Univ Cartagena, Dept Commun & Informat Technol, Cartagena 30202, Spain.. Univ Carlos III Madrid, Dept Commun & Signal Theory, Madrid 28911, Spain.. KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
Low-Dispersive Leaky-Wave Antenna Integrated in Groove Gap Waveguide Technology2018In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 66, no 11, p. 5727-5736Article in journal (Refereed)

In this paper, the use of a dispersive prism with a triangular shape is proposed to reduce the dispersive radiation nature of a leaky-wave antenna (LWA) in groove-gap waveguide technology. The operation of gap waveguide technology is based on the use of metallic pins that act as an artificial magnetic conductor, so the electromagnetic fields are confined and guided in the desired directions. To control a leaky-wave radiation of these confined fields is possible by tailoring the height of the pins, its periodicity, and the waveguide width. This radiation, as in any conventional LWA, is dispersive, leading to beam squint as the frequency is varied. Here, we mitigate this beam squint by using a prism made of dispersive pins and choosing appropriately their periodicity and height. With this prism, the leaky-wave radiation is focused into one single direction in a wide frequency band. This concept is demonstrated with a prototype designed to radiate at phi = 41 degrees with a central frequency of 12 GHz and the high gain of 16.5 dBi. A 22% frequency bandwidth for the 3 dB realized gain at phi = 41 degrees is achieved, and the main radiating direction, with half-power beamwidth of 5 degrees, steers only +/- 0.5 degrees from 11.4 to 13.4 GHz.

• 41.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Circularly Polarized Compact LTSA Array in SIW Technology2017In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 65, no 6, p. 3247-3252Article in journal (Refereed)

Typical linearly tapered slot antenna (LTSA) usually features a wideband and a high gain. However, its geometry size is large, especially very long in the tapering direction, which limits its application in compact antenna and array systems. This communication has minimized the tapered length to less than 0.19 lambda(0) when the aperture width is less than 0.35 lambda(0) both for horizontally and vertically polarized LTSA arrays. Moreover, the typical polarization of the LTSA is linear, which is perpendicular to the slot. In this communication, an X-band circularly polarized 1x8 LTSA array, made of cross-LTSAs, is proposed with the feeding of a substrate integrated waveguide horn. All the three different polarized LTSA arrays are designed and investigated both in simulation and experiment, and the simulation results agree well with the measured results.

• 42. Xiong, J.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
A Broadband Low Profile Patch Antenna of Compact Size With Three Resonances2009In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 57, no 6, p. 1838-1843Article in journal (Refereed)

An E-shaped patch antenna fed by a folded L-shaped probe is proposed. The shorted wall loading is employed to make the antenna compact, and the lateral size is only 0.196 lambda(L) x 0.221 lambda(L), where lambda(L) IS wavelength at the lower cutoff frequency of the measured operating bandwidth in the free space. Three adjacent resonances are utilized to achieve a broad band while keeping the antenna very low profile. The air substrate is only 4 mm (i.e., 0.029 lambda(L)) thick, and the measured impedance bandwidth is about 19.8% (with a central frequency of 2.378 GHz). The procedure of optimal design is also described.

• 43.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. Zhejiang University, China.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. Zhejiang University, China.
Power Density Measurements at 15 GHz for RF EMF Compliance Assessments of 5G User Equipment2017In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 65, no 12, p. 6584-6595Article in journal (Refereed)

In this paper, different measurement schemes are studied in order to investigate the possibilities and limitations of scalar-and vector-based measurement systems for radio frequency electromagnetic fields compliance assessments of fifth generation mobile communication user equipment (UE). Two UE antenna array designs, transmitting at 15 GHz and employing patch and notch antenna elements, are considered for different phase excitations. Using free space power density as the exposure metric, the maximum permissible transmitted power of UE, compliant with the maximum permissible exposure limits specified by the U.S. Federal Communications Commission (FCC) and the basic restrictions of the International Commission on Non-Ionizing Radiation Protection, is determined. The accuracy of different measurement schemes is assessed using numerical simulation. Verifying measurements is carried out in a semiane-choic chamber. The results indicate that, for UE employing array antennas and intended to be used in immediate vicinity of the human body, scalar measurement systems used in combination with straightforward field combination techniques will lead to overly conservative results. A more accurate and less conservative approach for these products is to conduct separate measurements for different excitations in order to span the space of possible excitations. This will result in a more complicated measurement setup and increase the measurement time, which points to a need for very fast measurement systems.

• 44.
KTH, School of Electrical Engineering (EES), Signal Processing.
Measurement investigation of tap and cluster angular spreads at 5.2 GHz2005In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 53, no 7, p. 2156-2160Article in journal (Refereed)

In this paper, we present indoor measurement results on the cluster angular spread (AS), the tap AS and its variation with different channel bandwidths. A frequency domain space alternating generalized expectation maximization (FD-SAGE) algorithm is employed to estimate the multipath components from the measured data. We then manually identify the clusters of the multipaths and calculate the tap and cluster ASs for each identified cluster. It is found that for the 100 MHz channels, the average tap AS is just few degrees less than the cluster AS and the difference diminishes for less channel bandwidth.

• 45.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Reduction of the envelope correlation coefficient with improved total efficiency for mobile LTE MIMO antenna arrays: Mutual scattering mode2013In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 61, no 6, p. 3280-3291Article in journal (Refereed)

A mutual scattering mode is introduced in this paper. Utilizing this mode, the correlation of a lossy long-term evolution (LTE) multiple-input and multiple-output (MIMO) antenna array can be reduced efficiently, even down to zero, by increasing the Q factors of the MIMO antenna elements. In practice, the Q factors can be straight forwardly tuned through different input impedance matching. The zero correlation occurs at a Q factor higher than that resulting from the conjugate input impedance matching. On one hand, if the inter-element distance is larger than a certain distance (what we denominate as the Critical Distance), the total efficiency can also be improved in addition to reducing the correlation. On the other hand, when the inter-element distance is less than the critical distance, a reference MIMO antenna with high correlation and high total efficiency is obtained. This antenna can well be proposed for over-the-air (OTA) measurement applications. The introduced scattering mode is investigated for dual monopoles on a large lossy ground plane and for various mobile terminal MIMO antenna designs. A wideband MIMO antenna, with multiple resonances, covering the band 746-870 MHz is proposed with the envelope correlation coefficient and total efficiency less than 0.5 and higher than 50% (-3 dB), respectively. Measurements and simulations agree well for all the fabricated prototypes. The envelope correlations and the multiplexing efficiencies of the prototypes are also investigated in propagation channels with Gaussian distributed angle of arrivals.

• 46.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Reducing Mutual Coupling for an Extremely Closely-Packed Tunable Dual-Element PIFA Array Through a Resonant Slot Antenna Formed In-Between2010In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 58, no 8, p. 2771-2776Article in journal (Refereed)

An efficient mutual coupling reduction method is introduced for an extremely closely packed tunable dual-element planar inverted-F antenna (PIFA) array. High isolation can be achieved through a lambda(0)/2 folded slot antenna formed by a slot on the ground plane and the neighboring edges of the two PIFAs. Direct coupling is blocked by the slot antenna through radiating the coupling power into free space. A measured isolation of more than 36.5 dB can be achieved between the two parallel individual-element PIFAs operating at 2.4 GHz WLAN band with an inter-PIFA spacing of less than 0.063 lambda(0) (center to center) or 0.0147 lambda(0) (edge to edge). Since there is only a narrow slot antenna formed between the PIFAs in the present method, the distance can be further reduced to less than 0.0016 lambda(0) (edge to edge) with the maximal isolation of better than 40 dB. Both measured and simulation results show the effectiveness of the present mutual coupling reduction method.

• 47.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Closely-Packed UWB MIMO/Diversity Antenna With Different Patterns and Polarizations for USB Dongle Applications2012In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 60, no 9, p. 4372-4380Article in journal (Refereed)

A closely-packed ultrawideband (UWB) multiple-input multiple-output (MIMO)/diversity antenna (of two elements) with a size of 25 mm by 40 mm is proposed for USB dongle applications. Wideband isolation can be achieved through the different patterns and polarizations of the two antenna elements. Moreover, the slot that is formed between the monopole and the ground plane of the half slot antenna is conveniently used to further enhance the isolation at the lower frequencies and to provide an additional resonance at one antenna element in order to increase its bandwidth. The underlying mechanisms of the antenna's wide impedance bandwidth and low mutual coupling are analyzed in detail. Based on the measurement results, the proposed antenna can cover the lower UWB band of 3.1-5.15 GHz, and within the required band, the isolation exceeds 26 dB. The gains and total efficiencies of the two antenna elements are also measured. Furthermore, a chassis mode can be excited when a physical connection is required between the ground planes of the two antenna elements. Without affecting the performance of the half slot element, the monopole can now cover the band of 1.78-3 GHz, apart from the UWB band. The proposed antenna structure is found to provide good MIMO/diversity performance, with very low envelope correlation of less than 0.1 across the UWB band.

• 48.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Mutual Coupling Reduction of Two PIFAs With a T-Shape Slot Impedance Transformer for MIMO Mobile Terminals2012In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 60, no 3, p. 1521-1531Article in journal (Refereed)

An efficient technique is introduced to reduce mutual coupling between two closely spaced PIFAs for MIMO mobile terminals. The proposed mutual coupling reduction method is based on a T-shape slot impedance transformer and can be applied to both single-band and dual-band PIFAs. For the proposed single-band dual PIFAs, the 10 dB impedance bandwidth covers the 2.4 GHz WLAN band (2.4-2.48 GHz), and within the WLAN band an isolation of over 20 dB is achieved. Moreover, the dual-band version covers both the WLAN band and the WiMAX band of 3.4-3.6 GHz, with isolations of over 19.2 dB and 22.8 dB, respectively. The efficiency, gain and radiation patterns of the two-PIFA prototypes are verified in measurements. Due to very low pattern correlation and very good matching and isolation characteristics, the capacity performances are mainly limited by radiation efficiency. The single-band and dual-band PIFAs are also studied with respect to their locations on the ground plane. An eight-fold increase in the bandwidth of one PIFA is achieved, when the single-band PIFAs are positioned at one corner of the ground plane, with the bandwidth of the other PIFA and the good isolation unchanged.

• 49.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Adaptive Quad-Element Multi-Wideband Antenna Array for User-Effective LTE MIMO Mobile Terminals2013In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 61, no 8, p. 4275-4283Article in journal (Refereed)

An LTE MIMO antenna array, which is adaptive to the effects of the user's body, is presented for mobile terminals. The bands of 750-960 MHz and 1700-2700 MHz are covered with a good efficiency in free space. Three kinds of user effects are studied, namely, "SAM head and PDA hand," "PDA hand," and "dual hands." The array is formed by selecting the best two elements out of four, with the two ports and grounding points of the other two elements open. The user effects on MIMO channel capacity are reduced through the adaptive selection of two optimal antenna elements. The total efficiency, envelope correlation coefficient, and multiplexing efficiency are presented for the three kinds of user effects. In the lower band, the decreased correlation due to the optimal selection improves the multiplexing efficiency. The underlying physical mechanisms and some general rules are discussed. The specific absorption rate (SAR) in the "SAM head and PDA hand" case is studied. Experiments for the three kinds of user effects are also carried out.

• 50.
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
KTH, School of Education and Communication in Engineering Science (ECE). KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
Channel Characteristics and User Body Effects in an Outdoor Urban Scenario at 15 and 28 GHz2017In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 65, no 12, p. 6534-6548Article in journal (Refereed)

The effect of a user's body on channel characteristics for single user downlink transmission in an urban scenario for the fifth generation (5G) systems is investigated with ray-tracing at 15 and 28 GHz. Three different designs of user equipment (UE) antennas are fabricated and integrated into a mobile phone prototype, and their 3-D radiation patterns are measured both with and without a user. The user remains in Cellular Telephone Industries Association (CTIA) standard data mode and talk mode during measurements. The results show that the user's body will cause a strong shadowing loss and generate a large fluctuation on the received signal strength of the UE at both 15 and 28 GHz, which is crucial to channel modeling studies at frequencies above 6 GHz. In addition, the user's body effect on a linear array system in an UE is presented, and the main challenges for the future work are also addressed.

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