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This work reports experimental investigation and numerical validation of millimeter-sized Spherical Bragg Resonators (SBRs) fabricated using 3D printing technology. The frequency dependencies of the reflection and transmission coefficients were analyzed, and eigenfrequency values were calculated to examine the density of photonic states in air/PLA-polylactide SBRs, showing the appearance of an eigenmode and an increase in the local density of states in the core of a defect cavity. A decay rate enhancement of ∼ 10 2 was obtained for a dipole placed in the core of the defect SBR. The study also investigated the influence of the source position on the resonator's electromagnetic wave energy. Scattering efficiencies up to order twelve of the multipole electric and magnetic contribution in a 10-layer SBR were calculated to validate the presence of the resonant modes observed in the scattering measurements performed for parallel and perpendicular polarizations. The results demonstrate that SBRs can act as omnidirectional cavities to enhance or inhibit spontaneous emission processes by modifying the density of electromagnetic states compared to free space. This finding highlights the potential of SBRs engineering spontaneous electromagnetic emission processes in various applications, including dielectric nanoantennas, optoelectronics devices, and quantum information across the entire electromagnetic spectrum. [ABSTRACT FROM AUTHOR]

We present a multifilament current method (MFCM) to accurately and efficiently determine the cutoff wavenumbers (cutoff frequencies or eigenfrequencies) of homogeneous anisotropic material-filled metallic waveguides with arbitrary shapes. A set of $z$ -directed filamentary line sources placed outside of a waveguide are responsible for generating the simulated fields within the anisotropic material-filled waveguide in light of the derived 2-D anisotropic dyadic Green’s functions. Instead of solving an excitation-free eigenvalue problem, a substantial line source presented as an excitation is incorporated into the MFCM to formulate an eigenmode analyses technique without spurious eigenmodes. The internal field intensity response versus free space wavenumber are subsequently simulated, and a high magnitude response can be observed when an eigenmode is excited. In this case, the cutoff wavenumbers of physical eigenmodes are revealed by maximum locations of simulated field intensity response. Several numerical examples with different configurations on a waveguide are investigated. The simulated results are compared with those obtained from commercial software packages and an excellent agreement is achieved. In addition, the study of the oscillating phenomenon on magnitudes of filamentary sources and the ill-conditioning issue of constructed impedance matrix with respect to the placement of filamentary sources are also discussed. [ABSTRACT FROM AUTHOR]

We present a multifilament current method (MFCM) incorporated with generalized sheet transition condition (GSTC) to simulate 2-D cylindrical metasurfaces with arbitrary cross sections. The GSTC casts in the form of an impedance-type boundary condition to relate the tangential fields on two sides of a cylindrical metasurface. The MFCM is subsequently introduced and formulated by employing the GSTC. The doublet-current unit, including both electric and magnetic filamentary sources, is introduced in order to capture both co- and cross-polarized scattered and transmitted fields from the GSTC characterized cylindrical metasurfaces. Three illustrative examples are presented to validate the proposed simulation technique. The behaviors of filamentary sources in terms of scattered field singularities and the ill-conditioning issue of the constructed matrix are also discussed in detail. In comparison to existing numerical methods, the proposed technique has its merits on simplicity and conciseness. [ABSTRACT FROM AUTHOR]

The TM shielding effectiveness (SE) analyses of slotted carbon fiber reinforced polymer (CFRP)-based shells are presented in this letter. The multifilament doublet current method (MFDCM) is deployed to simulate the scattered and penetrated fields in the considered situation. The inner and outer regions of a slotted shell are separated by a hybrid boundary, which contains the material and slot parts. The material part is constituted of multilayered CFRP materials, whereas the slot part is a virtual boundary. A tensorial boundary condition (TBC) is then constructed to stand for the slotted shell with a chain matrix. The MFDCM is subsequently formulated by employing the TBC to estimate the EM performances of interests. Different slot angles of the shell and conductivity of the CFRP material are considered in our numerical calculations. Only transverse magnetic (TM) case is considered herein, yet the transverse electric (TE) situation can be handled straightforwardly with a similar formulation. [ABSTRACT FROM AUTHOR]

This article presents a new application of microwave microscopy as a diagnostic of the painted lightning strike protection (LSP) mesh used in composite aircraft skins. A new flexible probe is presented. This probe can sweep over the surface of the composite skin and detect breaks in the LSP lying underneath. It is demonstrated that shielding of the probe feeding line leads to better balancing, and consequently highly improved immunity to the presence of nearby objects. With this new approach, it is possible to localize faults as small as one cut strand under the paint. Also, this technique features the possibility to measure the paint thickness with a resolution on the order of micrometers. [ABSTRACT FROM AUTHOR]

Implementation of the multifilament current method (MFCM) to analyze the scattering from homogeneous anisotropic cylinders is introduced. The ${z}$ -directed electric and magnetic line sources are used to associate with TM and TE polarizations, respectively. The radiation fields of a line source in unbounded space occupied with an anisotropic material are derived in detail. With the help of the derived fields, the application of the MFCM is expanded to anisotropic materials. The scattering field is then obtained in terms of these line sources. Monostatic and bistatic scattering widths of cylinders with and without sharp edge are investigated under different incident wave polarizations and material characteristics. Apart from homogeneous anisotropic cylinders, the scattering from an anisotropic shell-coated PEC cylinder is also straightforwardly analyzed without additional considerations in the formulation. The oscillation phenomenon of filament currents in representing the scattered fields and the ill-conditioned issue of the constructed matrix are also discussed in detail. Numerical results obtained with this proposed scheme are also in good agreement with published ones, whereas the proposed technique has its merits on simplicity and conciseness in comparison to the existing numerical methods. [ABSTRACT FROM AUTHOR]

The design, simulation, and measurement of a beam steerable slotted waveguide antenna operating in X band are presented. The proposed antenna consists of a standard rectangular waveguide (RWG) section with longitudinal slots in the broad wall. The beam steering in this configuration is achieved by rotating two dielectric slabs inside the waveguide and consequently changing the phase of the slots excitations. In order to confirm the usefulness of this concept, a nonresonant 20-slot waveguide array antenna with an element spacing of $d = 0.6\lambda _{0}$ has been designed, built, and measured. A 14° beam scanning from near broadside ($\theta = 4^{\circ }$) toward end-fire ($\theta = 18^{\circ }$) direction is observed. The gain varies from 18.33 to 19.11 dBi and the antenna efficiency from 95% to 79%. The sidelobe level is −14 dB at the design frequency of 9.35 GHz. The simulated and measured co-polarized realized gains are in good agreement. [ABSTRACT FROM AUTHOR]

We present in this paper, a multifilament doublet current method (MFDCM) combined with the tensorial impedance boundary condition (TIBC) to simulate and analyze scattering and shielding performances of carbon fiber composite (CFC) material-based cylindrical shells. The TIBC of a shell made up with the multilayered CFC material with an arbitrary fiber orientation in each layer is introduced and derived. The MFDCM is subsequently formulated by employing the TIBC to handle numerical calculations with the anisotropic lossy material, such as a multilayered CFC material, for the first time. Monostatic and bistatic echo widths are investigated with reference to different layer numbers, fiber orientations, and incident wave polarizations. These different scenarios are also taken into consideration in the shielding effectiveness (SE) prediction. Both TE and TM modes are excited within the cylindrical shell under arbitrary incident plane waves over a high frequency range. Results are in excellent agreement with published ones. The proposed method has its merits on simplicity and conciseness. Also it can be applied to handle shells of arbitrary smooth cross sections. [ABSTRACT FROM AUTHOR]

A new type of frequency scanning slot array antenna using composite right/left‐handed (CRLH) waveguide as a feeding structure is presented in this study. The waveguide is loaded periodically with short‐circuited air‐filled double‐ridge stubs, which permit the CRLH propagation. The feeding structure is highly dispersive in order to scan a wide angular sector with a narrow frequency sweep. It also easily allows amplitude tapering of the array elements. An eight‐element longitudinal slot array designed for X‐band operation with −22 dB sidelobe level has been built and tested. Good agreement between simulations and measurements was obtained. The structure exhibits 31° of scanning over a frequency bandwidth of only 6%, and low return loss in its band of operation. [ABSTRACT FROM AUTHOR]

A focused beam and a shaped beam four-layer transmitarray (TA) antenna operating in the $K$ -band have been designed, fabricated, and measured. Four-layer unit cells allowing a transmission phase range of 360° are used. The gain of the beam-focusing TA is 30.3 dB at 19 GHz with an aperture efficiency of 34.9%. Compared with TAs proposed in the literature, the presented antenna bandwidth is almost two times larger with a −1 dB gain bandwidth of 18% and a −3 dB gain bandwidth of 25.5%. A second TA radiating a shaped beam was designed, and its measured pattern correlates well with theoretical calculations. The sensitivity of the proposed unit cells to fabrication errors and the effect of oblique incidence are also studied. [ABSTRACT FROM PUBLISHER]

Circular polarization selective surfaces (CPSS) based on two variants of the original Pierrot cell are proposed. For the first variant, which uses a load in the middle segment, it is possible to equalize the frequencies at which the reflection and transmission performance for the two orthogonal circular polarizations is optimal. The second variant is a double Pierrot cell having a twofold rotation symmetry, which leads to an increased bandwidth and less sensitivity to variations in the angle of incidence. Experimental results in good agreement with full wave simulations are presented. The printed circuit board (PCB) fabrication process used with flexible substrates allows more freedom for adjusting the cells in order to improve the performance and the bandwidth of the CPSS. [ABSTRACT FROM PUBLISHER]

A generalized methodology to design low-profile transmitarray (TA) antennas made of several stacked layers with nonresonant printed phasing elements is presented. A study of the unit cell bandwidth, phase-shift range and tolerances has been conducted considering different numbers of layers. A structure with three metalized layers with capacitive and inductive elements enabling a phase range of nearly 360° and low insertion loss is introduced. A study of the four-layer structure shows improvement in the performance of the unit cells in terms of bandwidth from 2% to more than 20% and a complete phase coverage. Implementations on a flexible substrate of TAs with progressive phase shift operating at 19 GHz are used for validation. [ABSTRACT FROM PUBLISHER]

A new technique to realize a circular polarization selective surface (CPSS) implemented with Pierrot cells is proposed. This technique uses a folded flexible substrate, which enables the implementation of the 3D Pierrot cells on a single metal layer defined with precision printed circuit board techniques, without the need for metallized via holes. A left-hand CPSS is designed to illustrate the benefits of the proposed approach. For the fabricated prototype, the measured characteristics at normal incidence are in good agreement with simulations results obtained with HFSS. It is shown that compared to other Pierrot cell designs presented in the literature, this new approach simplifies the fabrication process while giving similar performance. [ABSTRACT FROM PUBLISHER]

This paper proposes a circular polarization selective surface (CPSS) design consisting of two meander-line circular polarizers and a linear polarizer. No vias are required. These polarizers are individually designed with ANSYS-HFSS (High Frequency Structure Simulator) software whereas the response of the overall cascade CPSS is computed with the GSM approach. A cascade CPSS which includes three-layer meander-line polarizers is designed and fabricated. Good agreement is shown between experimental and simulated results, with only the TE00 and TM00 modes in the GSM analysis. Finally, analysis of the circular polarizers based on simple filter theory is done to better understand how the performance of the whole structure can be improved. [ABSTRACT FROM PUBLISHER]

This paper presents the effects of a continuous wave electromagnetic interference (EMI) on a discrete microwave phase-locked loop (PLL). Susceptibility of the voltage-controlled oscillator (VCO) is first studied as a standalone circuit before being integrated into the PLL. The effects observed on the VCO alone and when it is integrated into the PLL are analyzed as a function of the frequency and power of the interference signal. Most of the effects observed are due to intrinsic nonlinearities of the VCO. Some of them, like intermodulation and injection pulling can be predicted by the small-signal perturbation theory, while others such as a phase unlocking condition generated by strong EMI injection cannot. Finally, the consequence of using such an EMI affected PLL used as a local oscillator in a QPSK receiver, is examined. In all cases, the EMI is injected at the output of the VCO through inductive coupling. [ABSTRACT FROM PUBLISHER]

This paper presents measurements, simulations, and a theoretical basis to support the observation of spurious peaks on the output spectrum of a microwave phase-locked loop (PLL) exposed to a continuous wave electromagnetic interference (EMI). It is shown that high-power EMI combined to nonlinear characteristics of the PLL may induce subharmonic oscillation phenomena, which provoke the appearance of spurious peaks on the output spectrum. Measurements show that in addition to their dependence on the EMI power and frequency, these spurious peaks depend also on the natural frequency of the PLL and on its stability. Transient circuit simulations of the PLL allowing the prediction of these spurious peaks are presented. Finally, a theoretical explanation based on an analysis of a second-order PLL is given. [ABSTRACT FROM PUBLISHER]

In this paper, a new compact monopole surrounded by four switched parasitic elements having the ability to work in the directional (beam steering) and omnidirectional modes is presented. The antenna is designed to operate over a 6% bandwidth in the both modes. In the directional mode, the beam is steered in the azimuth plane and a front-to-back ratio greater than 10 dB is achieved over the bandwidth of interest. To steer the beam, the parasitic elements surrounding the driven element are connected and disconnected to the ground plane through switches. The performance of the antenna with both ideal and non-ideal switches is discussed based on the simulation and measurement results. To overcome the adverse effect of the non-ideal switch on the antenna radiation pattern and transmission coefficient, a transformer network is used in the switch circuit. It is shown that the accurate design of this transformer network can reduce the adverse effects significantly. [ABSTRACT FROM PUBLISHER]

A novel two-dimensional fan-beam lens is presented. The principle of operation of the lens is based on Gaussian optics. The focal point of the circular interface between the dielectric and air is coincident with a focal point of a hyperbolic reflector, while the feeding source is located in the other focal point. The ray tracing method along with physical optics and full wave finite element method (Ansys HFSS) are used to simulate the structure and study the effect of the feed blockage. The two-dimensional lens is implemented in a parallel-plates waveguide and a technique for matching the feeding structure is proposed. Measurement results are presented and confirm the predicted performance. [ABSTRACT FROM PUBLISHER]

In this paper, an adaptation of Hurst's method to take into account the presence of array guided surface waves in the calculation of the scattered field of finite frequency selective surfaces in free space is presented. Numerical approaches to determine the guided wave's wavenumber needed in this calculation, namely Prony's method and the discrete Fourier transform (DFT), are examined and compared. The results show that the scattered fields obtained with the proposed method, which is based on numerically extracted diffraction coefficients, are in very good agreement with those obtained with the method of moments. [ABSTRACT FROM PUBLISHER]

It is known that for reflectarrays designed to radiate in an off-specular direction, part of the scattered field may still be directed in the specular direction. In this paper, a reflectarray made of single-layer patch elements is considered. The dependence of the unwanted specular reflection on the size of the patches is examined. Moreover, in a global analysis of the reflectarray, we show that this specular reflection can be due to a periodic error occurring in the applied phase shift. It is found that this periodic error is also responsible for the occurrence of a parasitic lobe in backward directions. [ABSTRACT FROM PUBLISHER]

In this paper, a method to build equivalent models of radiating printed circuit boards based on complex E-field measurements taken in the close vicinity of the device under test is explored. It is shown that the inverse problem to be solved to retrieve the currents from the field data is ill-posed. An innovative regularization approach implementing a penalty on abrupt spatial variations of the currents is proposed to alleviate this difficulty. Various schemes to mesh the equivalent current distribution are also explored. Combining these with measurements, it is shown that accurate estimation of equivalent current models can be achieved, thereby allowing the identification of the emission sources. The method is tested for different circuit configurations with both synthetic and real data. Obtained results demonstrate the efficiency of the method. [ABSTRACT FROM AUTHOR]

A novel type of short monopole antenna is proposed. Its structure consists of a periodically slotted coaxial line radiating in linear polarization (LP). The structure can be easily modified by the addition of a thin layer of dipoles in order to generate circular polarization (CP). The number of slots along the structure, their periodicity and the tuning of the terminating load allow control of the antenna main beam' angle and width. One advantage of the proposed design over conventional monopoles is that the antenna does not require a ground plane. Both left-hand and right-hand CP are possible with the same design, simply by inverting the symmetry of the printed dipoles. Design details for the annular slotted coaxial line and for the LP and CP mast antennas are given. Prototypes in both LP and CP were fabricated and their characteristics were measured. Simulations and measurement are in good agreement. The proposed CP mast antenna is compliant with the requirements of the XM digital satellite radio system, with a frequency bandwidth of more than 1.7% in which VSWR < 2, and an AR < 4 dB in the elevation range of ±30° < θ < ± ± 70°. [ABSTRACT FROM AUTHOR]

A new design of two-dimensional cylindrical Luneberg lens is introduced based on TE10 mode propagation between parallel plates, with special focus on ease of manufacturing. The parallel plates ace partially filled with low cost polymer material (Rexolite ϵr =2.54) to match Luneberg's law. A planar linear tapered slot antenna (LTSA) is inserted into the air region between the parallel plates at the edge of the Luneberg lens as a feed antenna, with fine positioning to the focal point of the Luneberg lens to optimize the antenna system performance. A combined ray-optics/diffraction method is used to obtain the radiation pattern of the system and results are compared with predictions of a time domain numerical solver. Measurements done on a 10-cm Luneberg lens designed for operation at 30 GHz agree very well with predictions. For this prototype, 3-dB E- and H-plane beamwidths of 6.6° and 54° respectively were obtained, and the sidelobe level in the E-plane was -17.7-dB. Although the parallel plate configuration should lead to a narrow band design due to the dispersion characteristics of the TE10 mode, the measurement results demonstrate broadband characteristics with radiation efficiencies varying between 43% and 72% over the tested frequency hand of 26.5-37 GHz. The designed cylindrical Luneberg lens can be used to launch multiple beams by implementing an arc array of planar LTSA elements at the periphery of the lens, and can be easily extended to higher mm-wave frequencies. [ABSTRACT FROM AUTHOR]

The design of a C-band electronically beam scanning reflectarray is presented. The reconfigurable reflectarray element consists of a microstrip patch, printed on a flexible membrane substrate, aperture-coupled to a transmission line loaded with two varactor diodes. The designed element allows continuous tuning of the reflected signal's phase over a 360° range with a maximum loss of 2.4 dB at 5.4 GHz. The measured results on a 30-element reflectarray breadboard show that by varying the bias voltages on each element the main beam can be steered to large angles, up to 40° from broadside in the H-plane. A loss analysis of the proposed reflectarray is also presented. [ABSTRACT FROM AUTHOR]

This paper presents a new approach to handle the nonlinear microwave tomography equations for dielectric materials. The electromagnetic tomography equations are rewritten in the framework of an equivalent wire grid model loaded with resistors and capacitors to represent the permittivity and conductivity of the material. The validation of the model is performed using near-field measurements at a frequency of 2.45 GHz on devices under test made of dielectric plates of various shapes surrounded by an unbounded free-space medium. The reconstruction algorithm is based on the contrast source inversion (CSI) technique. Here, we introduce an enhanced version of the CSI cost function by adding extra regularization terms; in addition, minimization is carried out using a logarithmic barrier constraint in order to avoid nonphysical permittivity and conductivity values. The distributions of those physical properties retrieved with experimental data, for nonhomogeneous dielectric structures of known composition, are in good agreement with the expected ones. The imaging resolution is limited by the density of the wire grid meshing. [ABSTRACT FROM AUTHOR]

Demonstrates the potential of equivalent magnetic current (EMC) in the prediction of low-directivity printed antenna radiation patterns based on the measurements in the reactive near-field. Near-field measurement system; EMC modeling of printed radiators; Accuracy of the far-field patterns obtained after processing the near-field measurements.

A novel switch topology is proposed in which p-i-n diodes are embedded in a transformer network free of via holes. The topology is designed to achieve cancellation of diode parasitics and optimization of insertion loss. Prototypes for 15 and 29.75 GHz operation showing very good insertion loss (<0.5 dB) are presented. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 28: 28–32, 2001. [ABSTRACT FROM AUTHOR]

Discusses design considerations and experimental investigations of an integrated active antenna for space power combining that makes use of unidirectional dielectric radiators (UDR). Implementation of UDR feed circuit; Advantages of distributing the devices into an array; Factors that caused a non-negligible negative shift of antenna's resonance frequency.

Focuses on a study which investigated the performance of a neural network-based direction-finding system. Antenna array and radio frequency combining network; Information on beamforming network; Experimental results; Conclusions.

Presents a concept for exciting slots with a coplanar waveguide line based on inductive coupling. Antenna impedance tuning; Description of single elements and arrays designed for 5- and 25-GHz operation; Simulation and measurement of characteristics.

Investigates the multifilament current method's (MFCM) ability to solve electromagnetic scattering and/or absorption problems involving inhomogenous cylindrical structures. Numerical difficulties of MFCM when dealing with a medium characterized by a high loss tangent or large electrical conductivity.

In this work, we present a bilateral reflectionless filter topology. A low‐pass filter prototype and its theoretical frequency responses are given. The frequency responses are obtained by means of ABCD matrices, thereby showing that the proposed prototype is reciprocal, symmetrical, and reflectionless at both ports. A first‐order bandpass filter with the reflectionless characteristic has been designed by applying the conventional low‐pass‐to‐bandpass transformation to the proposed low‐pass filter topology. It is verified by measurement results that the proposed bandpass filter can have less than −20 dB reflection coefficient over the passband and stopband at both ports with the same transmission response of a conventional first‐order bandpass filter. [ABSTRACT FROM AUTHOR]

A simple and low-cost single substrate layer Luneburg lens-based beamforming network (BFN) operating in $W$ -band is proposed. A multibeam antenna is then realized using the novel BFN and fabricated using a low-cost printed circuit board process. The Luneburg lens is realized by drilling holes in a substrate, and to extend the refractive index variation, a high dielectric constant substrate is embedded at the lens center. The lens is excited by radially placed open-ended substrate integrated waveguides (SIWs) at the lens circumference, feeding a subset of antenna ports placed on the opposite side of the lens. The proposed BFN thus provides half-power beamwidth (HPBW) and beam steering angle flexibility for multibeam antennas. An antenna generating seven beams was prototyped at 79.5 GHz to validate this BFN, providing measured beam steering of approximately ±41° with a broadside HPBW of 10°. The bandwidth of the proposed BFN is determined by the single-mode operation of the SIW. The fabricated multibeam antenna impedance bandwidth is, however, about 2% at 79.5 GHz because of the radiating elements. The radiating elements are series fed slot arrays placed on SIWs. [ABSTRACT FROM AUTHOR]

In this paper, the problem of optimizing probes used in modulated scattering systems is examined. Instead of the usual empirical or blind numerical optimization, a simple analytical formulation is developed, uniquely based on the impedance states of the modulating devices. This formulation allows obtaining some bounds on system performances and assessing the sensitivity to frequency and component characteristics variations. In particular, an optical modulation component, providing an advantage in terms of low invasiveness, is compared with the more common p-i-n diode-based modulators. The validity of the proposed approach, for high-resolution electrically small probes is demonstrated numerically and experimentally. [ABSTRACT FROM PUBLISHER]

In this communication, a new broadband antenna with a very low profile is proposed. The antenna inherits the wide bandwidth of the blade antenna, while its height is considerably reduced to 0.03\lambda0, where \lambda0 is the wavelength of the lowest operating frequency. This new design consists of a top loaded monopole with several shorting pins and an inductive loading. The antenna resonates at two distinctive frequencies determined by the shorting pins and inductive loading and the wideband characteristic is achieved by the control of the resonance separations. The inductive loading is created by a shorted metallic disk embedded into the antenna structure. This new method of inductive loading facilitates the antenna fabrication while the physical size of the antenna is kept compact. A prototype was fabricated and its VSWR and far field radiation pattern were measured. The antenna provides a bandwidth of 17% for VSWR smaller than 3. Also, a minimum realized gain of 1 dBi was achieved over the frequency bandwidth in which the antenna is well matched. [ABSTRACT FROM PUBLISHER]

The microwave inverse problem is addressed using a wire grid model representation with capacitors loaded in parallel with resistors to respectively replace the permittivity and the conductivity of the device under test. A new approach is presented to embed the properties of isotropy and positiveness of the constitutive parameters without additional penalty terms or weighting parameters. An edge-preserving regularization technique is used to better estimate the discontinuities present in the device under test (DUT) and to decrease the sensitivity to noise during the reconstruction process. The optimization algorithm makes use of the conjugate gradient method to minimize the objective function. Synthetic data are used to assess the reconstruction speed of the new method. Simulation results show a five-fold reduction of the computation time compared to what had been presented previously. Experimental near-field measurements at 2.45 GHz on thin plate DUTs are used to assess the validity of the proposed reconstruction method. Satisfactory results are obtained and a spatial resolution of \lambda/20 is achieved. [ABSTRACT FROM AUTHOR]

A system to measure the direction of arrival (DOA) of a signal within a 45-degree conical sector is demonstrated. The system is compact and uses only four circularly polarized patch elements. A printed beamforming network is used to create a set of partially overlapping beams allowing DOA estimation without ambiguity. Neural networks are used to first classify the antenna signals and then estimate the DOA. The proposed system was validated experimentally in C band and, in spite of the highly disturbed beams caused by the finite size of the antenna platform, it was shown that DOA estimation errors in the order of one degree were achievable under signal-to-noise ratios of 10 dB. [ABSTRACT FROM AUTHOR]

The equivalent magnetic currents method can be used to characterize radiating structures and to obtain information about the current distribution on the radiating device under test (DUT). In order to do so, the system is assumed to be linear and inversion from near-field measurements can be carried out using numerical techniques such as the singular value decomposition. However this approach is very sensitive to modeling and measurement errors, which is a consequence of the ill-conditioned nature of the inversion process. Here, we propose to account for the ill-conditioning by using a Tikhonov regularization technique to perform the inversion. This technique requires a regularization parameter to be set to an appropriate value, and we show that a generalized cross-validation approach yields adequate determination of the regularization parameter directly from the measured data. The proposed method can be tailored to account for specific characteristics of the DUT. This point is illustrated by the estimation of the current distribution of a bidimensional radiating structure for which prior information about the orientation of radiating edges is incorporated into the inversion method. [ABSTRACT FROM AUTHOR]

An efficient modes analyses technique for isotropic or anisotropic material filled 2D metallic waveguides with an arbitrary contour using the multifilament current method (MFCM) is presented. The ideal PEC boundary of a 2D waveguide is replaced by a shell with a high conductivity and electrical small thickness. The thin lossy shell not only can well approximate the boundary condition of PEC waveguide wall therefore without altering the initial waveguide modes, but also can let the external excitation penetrate through to excite the inside modes, resulting in a high internal field intensity at the frequency of each mode. In this case, the modes are revealed by the peaks of field intensity responses, and the spurious modes which existed in traditional source‐free modes determination techniques can be completely avoided. Based on this idea, a generalized impedance boundary condition (GIBC) is formulated to represent the lossy waveguide wall and further utilized in the MFCM for simulating the internal field intensity over frequency. Three different configurations of a 2D waveguide are considered. The computed modes are compared with that obtained from commercial software, and an excellent agreement is achieved, yet an competitive advantage on simulation performances is observed by using the proposed technique. [ABSTRACT FROM AUTHOR]

A novel feed system for circular arrays enabling beam steering over 360° in the azimuth plane with a stable scanned beam is proposed. It consists of rotating and fixed parts. The rotating part is a two-dimensional offset fed parabolic reflector which is inside a circular parallel plate waveguide. The power coming from the rotating part is coupled with the elements of the circular array through a fixed coupling structure. The rotating part is light and its moment of inertia is significantly less than the conventional rotating antenna systems for beam scanning. Therefore, the beam can be steered much faster than the current rotating antenna systems. The details of the structure including the rotating and the fixed parts are presented and a design procedure for the fixed coupling section is suggested. The performance of the feed system is investigated by simulations and measurements. It is shown that the simulations and experimental results are in good agreement. [ABSTRACT FROM AUTHOR]

This paper presents the design of a meander-line circular polarizer optimized for oblique incidence and for a direction off from the principal planes (\phi=\text0^\circ, \text90^\circ, \text180^\circ, or \text270^\circ). This polarizing screen converts linearly polarized (LP) waves into circularly polarized (CP) waves and vice versa. To illustrate the design process, a three-layer meander-line circular polarizer is designed for an oblique incidence angle of (\theta=\text40^\circ, \phi=\text45^\circ) and its performance is examined in detail over the 18–20 GHz band. Measurements and simulations are in good agreement. Moreover, the sensitivity of the results to frequency and incidence angle variations is studied. The results show that the bandwidth of the meander-line circular polarizer is maximum when it is designed for normal incidence or for an oblique incidence in the two principal planes because coupling between orthogonal modes is weaker. [ABSTRACT FROM PUBLISHER]

In this paper, several conformal leaky-wave antennas (CLWAs) are designed, which utilize backward radiating waves to focus the beam. They employ a metal strip grating (MSG) in order to introduce the required phase constant ($\beta$) on a curved aperture to compensate for the defocusing due to the curvature. The required $\beta$ is obtained by modulating the distance between adjacent strips/slots to scan the beam toward broadside. Using backward waves with a modulated MSG on a smoothly curved CLWA is a novel design. Using a backward wave allows for a doubling of the aperture size as compared to the forward-wave case. This, in turn, leads to a doubling of the directivity. A simple two-dimensional (2-D) Fourier-series model is used to explain the radiation physics. Full-wave analyses using the FEKO and HFSS software packages are carried out to design and analyze these CLWAs in 2-D and three-dimensional (3-D), and to verify the concept. A CLWA with a focused backward wave, supported by a modulated MSG, is fabricated and tested. The calculations and experimental results confirm that directive beaming is possible with a backward wave on a curved, modulated MSG. [ABSTRACT FROM PUBLISHER]

This paper presents the design of a dual-surface reflector (DSR) antenna working in dual circular polarization (CP). This DSR consists in a linearly polarized dual-gridded reflector (DGR) that is made of a solid parabolic reflector on which a reflectarray that reflects horizontal polarization (HP) and is transparent to the vertical polarization (VP) is affixed. The conversion from linear polarization (LP) to CP of this antenna is realized by introducing a three-layer meander-line circular polarizer optimized for oblique incidence. The antenna system was designed to generate two main beams in different directions and orthogonal CPs. This antenna has been fabricated and characterized. For the two polarizations, the realized gain is higher than 30 dB with a cross-polarization discrimination (XPD) higher than 27 dB in the 18–20 GHz frequency band. [ABSTRACT FROM PUBLISHER]

An alternative formulation of the microwave tomography problem is proposed, in order to reduce the computation time of the inversion procedure. The well-known multiplicative regularized contrast source inversion method is used as reference algorithm. Our formulation is based on a truncated singular value decomposition of the matrix involved in the most computationally intensive operations of the inversion algorithm. After theoretical and practical investigation of the behavior of the singular values, it is shown that the corresponding approximation does not yield any significant degradation of the quality of the solutions even for large complex permittivity contrasts, while the computation times are reduced by a factor ranging between 1.1 and 6 with respect to the original method. [ABSTRACT FROM AUTHOR]