Elementary Magnetic Dipoles Research Articles

Wideband low‐profile endfire circularly polarized antenna using mode superposition technique

In this article, a wideband low-profile endfire circularly polarized (CP) antenna using magnetoelectric dipole is proposed. Designing a wideband electric dipole element is easy and well-studied in the literature. However, designing a compact wideband magnetic dipole element is still a challenge. In this paper, the impedance bandwidth (IBW) of the magnetic dipole is enhanced by reducing the resonance frequency of higher-order mode half-TE310 with the help of rectangular slots placed at the null of mode half-TE310. By combining the resonance frequency of mode half-TE110 and mode half-TE310, the magnetic dipole can yield a larger IBW (6.86–7.67 GHz, 11.14%) with smaller planar dimensions of 1.259λ0 × 0.363λ0 × 0.038λ0 (λ0 is the free space wavelength at the centre frequency). To generate a wideband endfire CP wave, a horizontally polarized electric dipole with a feed line is combined with the vertically polarized magnetic dipole. The measured IBW and axial ratio bandwidth (ARBW) are 11.65% and 12.02%, respectively, with a measured peak endfire gain of 5 dBic. The overall dimension of the proposed wideband low-profile endfire CP antenna is 1.11λ0 × 0.66λ0 × 0.033λ0.

Experimental Exposure Evaluation From the Very Close Near- to the Far-Field Using a Multiple-Multipole Source Reconstruction Algorithm

Exposure assessment of wireless devices operating at ≥6 GHz requires measurement of power density (PD) from a distance of 2 mm ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\geq \lambda $ </tex-math></inline-formula> /25) from the surface of the device (reactive near field) to the far field with minimal uncertainties. The challenges are field distortion and backscattering by the probe, spatial resolution, sensitivity, isotropy, and phase accuracy. In addition, the surfaces of the device may not be planar. Today, methods based on phaseless amplitude measurements are widely applied using plane-to-plane phase reconstruction (PR) to obtain the field phasors. The obtained phasors in conjunction with Maxwell’s equations allow to evaluate the fields in the whole volume of interest. Although these methods meet the PD compliance criteria for flat surfaces at distances as close as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda /5$ </tex-math></inline-formula> , they are unsuited for curved surfaces and closer distances. To overcome these limitations, we present a method based on a multiple-multipole expansion for accurate PD evaluation in all field regions using sparse phaseless electric field measurements. We construct a physically relevant vectorial basis in terms of elementary electric or magnetic dipoles and optimize their amplitudes such that the measured fields are reproduced with minimal error. The proposed procedure yields current distributions that resemble the physical radiation source and accurately reconstruct the electromagnetic field from the (reactive) near-field to the far-field regions with an uncertainty of <0.6 dB. Our approach does not pose any geometric requirements to the measurement or evaluation surfaces and thus supports conformal assessments, providing a potential solution for accurate PD evaluation of 5G devices and beyond.

Highly-Directive Cross-Polarized Backscatterers Integrated With a Ground Plane

Cross-polarized backscatterers integrated with a ground plane are a key element of different applications, including chipless RFIDs, sensing, ambient backscattering communications, and data storage. The use of a cross-polarized signal removes the difficulties arising from the strong copolar signal reflected by the ground plane. Concurrently, the cross-polarized signal is typically very weak, hindering the effectiveness of crosspolarized backscatterers in noisy and uncontrolled environments. Herein, the generalized optical theorem is used to derive upper bounds on the cross-polarized backscattering cross section of systems integrated with a ground plane and identify viable strategies to design them based on highly directive scattering systems. Popular antenna configurations, including elementary electric and magnetic dipoles and their combinations, such as Huygens sources and endfire arrays, are examined to ascertain which ones can be used to realize highly directive cross-polarized backscattering performance when they are integrated with a ground plane. Realistic single-antenna elements that attain the upper-bound performance are identified. A two-element endfire array based on crossed electric dipole elements also achieves the upper bound performance, further confirming the efficacy of the derived cross-polarized backscattering bounds.

A Novel 1 Bit Wide-Angle Beam Scanning Reconfigurable Transmitarray Antenna Using an Equivalent Magnetic Dipole Element

A novel 1 bit reconfigurable transmitarray antenna (RTA) using an equivalent magnetic dipole element for wide-angle beam scanning at Ku -band is presented. The RTA element consists of a rectangular patch and a side-shorted patch with an equivalent magnetic dipole radiation. A 1 bit phase difference is achieved by controlling two p-i-n diodes on the side-shorted patch based on the current reversal mechanism. The half-power beamwidth of the presented antenna element is 142° in E-plane, which is significantly larger than other conventional elements. A RTA prototype with $16\times16$ elements is designed, fabricated, and tested. The measured results show good beam scanning capabilities of the 1 bit RTA. The measured gain in the broadside direction is 21.4 dBi, and the scan gain loss is 3.6 dB for the 60° scanned beam in E-plane.

Reconstruction of the Human Heart Functional Structure Based On a Few-Channel Magnetocardiogram

The new method of magnetocardiography data analysis is proposed. The method is based on the Fourier transform of prolonged time series and on the massive inverse problem solution for all spectral components. Magnetocardiograms (MCG) were registered in the plane above the subject’s chest in the nodes of the “rectangular” (6×6) grid with the step 40 mm at usual laboratory conditions without any additional magnetic shielding. The 9-channel MCG-system “MAG-SCAN-09” with dc-SQUID-based axial second order gradiometers was used. The MCG-recording was performed in four positions of investigated subjects under the instrument to get all 36 MCGs. For each of four positions of the MCG-recording the partial functional tomogram was calculated, which is the spatial distribution of elementary magnetic dipoles, observed in this position. The complete functional tomogram of the thorax was obtained by the summation of four partial functional tomograms, containing the data about the same object, observed from various positions. Filtering and contrasting of the complete functional tomogram made it possible to extract the 3D-object, representing the functional structure of the heart. The method was used for five subjects and provided consistent results. It is possible to use this method in cardiography, because the functional tomogram contains all measured information about individual heart.

Diffractive Hyperbola of a Skin Layer

Based on an analysis of physics of the phase transition from the quasistatic state field to the running wave field of elementary electric and magnetic dipoles located in absorbing media, it is concluded that the skin layer is formed at the boundary of this phase transition. The possibility is considered of obtaining the diffractive hyperbola of the skin layer and its subsequent application for sensing of objects in strongly absorbing media.

An Insight into Creeping Electromagnetic Waves around the Human Body

The electromagnetic wave propagation around human body torso is modelled by considering elementary electric and magnetic dipoles over an infinite muscle-equivalent cylinder. The poles in the spectral domain Green’s function with smallest imaginary part are found to correspond to creeping wave propagation coefficients which predict the general trend in propagation around human body. In addition, it was found that axial magnetic field component is crucial for communication via creeping waves since it generates modes with smaller field decay compared to axial electric field. The developed model may thus serve as a practical guideline in design of on-body wearable antennas. The theoretical considerations are verified with simulations and measurements on the prototype of PIFA antenna placed on the human body.

28 GHz Compact Omnidirectional Circularly Polarized Antenna for Device-to-Device Communications in the Future 5G Systems

An omnidirectional circularly polarized (OCP) antenna operating at 28 GHz is reported and has been found to be a promising candidate for device-to-device (D2D) communications in the next generation (5G) wireless systems. The OCP radiation is realized by systematically integrating electric and magnetic dipole elements into a compact disc-shaped configuration (9.23 mm $^{3} =0.008~\lambda _{0}^{3}$ at 28 GHz) in such a manner that they are oriented in parallel and radiate with the proper phase difference. The entire antenna structure was printed on a single piece of dielectric substrate using standard PCB manufacturing technologies and, hence, is amenable to mass production. A prototype OCP antenna was fabricated on Rogers 5880 substrate and was tested. The measured results are in good agreement with their simulated values and confirm the reported design concepts. Good OCP radiation patterns were produced with a measured peak realized RHCP gain of 2.2 dBic. The measured OCP overlapped impedance and axial ratio bandwidth was 2.2 GHz, from 26.5 to 28.7 GHz, an 8 % fractional bandwidth, which completely covers the 27.5 to 28.35 GHz band proposed for 5G cellular systems.

Computational imaging with a frequency-diverse metasurface aperture

We demonstrate image formation with radio frequency (RF) waves using a frequency-diverse metasurface antenna. The metasurface antenna consists of a planar cavity—formed from a double-layer, copper-clad printed circuit board—fed from one side by an RF source, and with a series of irises patterned into the opposite side. At any particular frequency, the cavity mode excited by the feed in turn excites the irises, each of which radiates as a magnetic dipole element. The cavity walls (fabricated using vias patterned into the circuit board) are irregular, so that the excited cavity modes exhibit considerable variation as a function of frequency. Thus, the radiated fields from the cavity-backed metasurface tend to have a series of randomly directed lobes, the number and directions of which vary significantly as a function of frequency. These radiation patterns can be used to form measurements of a scene, with the scene then estimated using computational imaging techniques. We have applied the metasurface aperture in a prototype imaging system capable of acquiring high resolution images of human-scale targets. The metasurface aperture design and reconstruction methods are general, and can be easily adapted for acoustic imaging.

Time-Domain Magnetic Dipole Model of PCB Near-Field Emission

A magnetic dipole array based time-domain (TD) modeling of electromagnetic near-field (NF) radiated by printed circuit boards (PCB) is introduced. The behavioral equivalent model of radiated emission using elementary magnetic dipoles compliments the previously described TD model using electric dipoles. The equivalent dipole model parameters are determined by the matrix inversion method from the time-dependent magnetic NF data. The proposed modeling approach was implemented in MATLAB. As a proof of concept, the TD magnetic NF radiated by a passive microstrip test PCB exited by a nanosecond-duration signal was considered. The modeled-magnetic NF maps were compared with the reference data in the observation planes situated at some centimeters above the tested PCB, showing good agreement. The comparison between magnetic and electric dipole models was performed, showing the advantage of the latter.

Omnidirectional Square Loop Segmented Antenna

A square loop antenna of circumference about one free-space wavelength displays an omnidirectional radiation pattern when its conductor is partitioned into five segments that are interconnected by four small capacitances. The optimization process for determining the capacitances values is described in detail. The required four capacitances are created by overlapping the conductor segments on the opposite sides of the substrate. The far field of the antenna resembles the one of an elementary magnetic dipole. At the center frequency of 956 MHz, the measured maximum gain is 2 dBi, and the measured antenna efficiency is 94%.

A Super-Resolving Near-Field Electromagnetic Holographic Method

In this work, we study the problem of characterizing electromagnetic radiation from given near-field electromagnetic measurements on an arbitrarily shaped surface. Due to the evanescent waves present in the near-field data the inversion procedure requires special treatment in order to recover a stable physical solution. Our proposed approach to this challenging inverse problem is to seek a sparse decomposition of the fields into elementary electric and/or magnetic dipole on an equivalent source surface. As we show in this paper the incorporation of a sparsity constraint during the inversion process yields impressive stability and accuracy. Additionally the method exhibits a remarkable ability to image and identify point like dipole sources even when spaced closer than a fraction of the wavelength from each other. The proposed inversion procedure is very simple to implement, is easily adaptable to arbitrary surfaces, and is relatively fast even with dense grids. These features make the proposed method suitable for a complete on-the-spot electromagnetic radiation analysis; a capability in which many applications rely on. Proof of concept for this work is established through simple numerical experiments, and from physical measurements taken for a slotted radiating cylinder.

A 2-D-Based Analytical Method for Calculating Permanent Magnetic Ring Bearings With Arbitrary Magnetization and Its Application to Optimal Bearing Design

This paper investigates the optimal design of passive permanent magnetic bearings in terms of maximum stiffness per magnet volume ratio. Examining the influence of design parameters on maximum material utilization, we present general conclusions regarding cross section dimension, type of magnetization, and magnetic pole number. Furthermore, we provide generalized design plots for selected configurations, which can be used for rapid dimensioning of bearings with adequate raw material utilization. The analytical computation method used for all calculations is based on the interaction energy of elementary magnetic dipoles and thus supports any kind of magnetization, including when strength and direction may vary within the cross section of the magnetic rings. Although the design problem is 3-D and the analytical method 2-D, the resulting error is negligible for most of the practical applications.

A compact beam focusing and steering element using quadrupoles with independently excited poles

Beam steering elements for accelerator beam transport are conventionally and conveniently incorporated into beamlines by fitting magnetic dipole elements around the vacuum tube of the line. Two steerers in each plane (X and Y) together with a quadrupole doublet constitute a module providing full control of the direction, position and focus of the beam. In some installations however, there may be insufficient space on the beamline to mount separate steerer elements. To provide steering capabilities in such a situation we have used a magnetic quadrupole doublet with the coils of each pole independently excited to synthesise the desired combination of quadrupole, horizontal dipole and vertical dipole fields. This paper describes the quadrupole steerer and its multichannel power supply and presents calculated magnetic field distributions together with raytracing simulation of its performance.

Superdirective Magnetic Dipole Array as a First-Order Probe for Spherical Near-Field Antenna Measurements

The theory as well as numerical and experimental results are presented for a superdirective array composed of closely spaced electrically small resonant magnetic dipole elements. The array operates on a metal ground plane and can exhibit a maximum directivity of 11.5 dBi, 15.2 dBi, and 17.8 dBi (including 3 dB due to the ground plane), for 2, 3, and 4 magnetic dipoles, respectively. The array is self-resonant and is directly excited by a 50-ohm coaxial cable through the ground plane. The array radiates essentially the |μ| = 1 spherical modes, which, despite a narrow bandwidth, makes it an excellent first-order probe for spherical near-field antenna measurements at low frequencies.

Estimation of the Effects of Microscopic Misalignments on the Magnetization Process in Thin Films by Energetic Modeling

In the framework of an energetic model, which is based on a stochastic description of the macroscopic magnetization process, we want to explain how misalignments of microscopic magnetic dipoles influence the magnetization curve. In contrast to previous work, the orientation of the elementary magnetic dipoles is modeled via statistical distribution functions that are characterized by a mean orientation and a corresponding variance that accounts for misalignments. This nonuniform orientation can result from structural defects in the crystal lattice, for example. The model is applied to thin-film permalloy samples that are used in anisotropic magnetoresistive sensor elements. Here, the variance of the angular distribution function is a measure for inhomogeneities in the film. Furthermore, it can be shown how technological parameters of the sputter process influence the alignment of the magnetic moments and, thus, determine the magnetoresistive characteristics of the sensor element.

Optimal Characteristics of an Arbitrary Receive Antenna

Several fundamental questions about the operation of receiving antennas are addressed, such as ldquoWhy does a receiving antenna scatter an incident field?rdquo and ldquoUnder what conditions does a receive antenna capture all of the available incident power?rdquo A new method is described by which the received power can be maximized for an arbitrary receiving antenna. The technique is first illustrated for two-dimensional infinite receiving arrays of electric and/or magnetic dipole elements, which result in simple plane waves for the scattered (re-radiated) fields. Optimal results (for maximum received power) are derived for several cases, and it is established that half the available incident power may be received by an array of electric (or magnetic) elements in free space, and that all available incident power may be received by an array that combines electric and magnetic elements, or one that incorporates a ground plane. Next, an arbitrary finite three-dimensional antenna enclosed by a mathematical spherical surface is treated using spherical vector wave functions. It is shown that half the available incident power can be received by such an antenna consisting of either TM or TE only elements, while all available incident power can be received when both TM and TE elements are used. It is also shown that the absorption efficiency for any optimal arbitrary antenna is 50%.

Optimal Polarized Beampattern Synthesis Using a Vector Antenna Array

Using polarized waveforms increases the capacity of communication systems and improves the performance of active sensing systems such as radar. We consider the optimal synthesis of a directional beam with full polarization control using an array of electromagnetic vector antennas (EMVA). In such an array, each antenna consists of p ges 2 orthogonal electric or magnetic dipole elements. The control of polarization and spatial power patterns is achieved through carefully designing the amplitudes and phases of the weights of these dipole antennas. We formulate the problem in a convex form, which is thus efficiently solvable by existing solvers such as the interior point method. Our results indicate that vector antenna arrays not only enable full polarization control of the beampattern, but also improve the power gain of the main beam (over the sidelobes), where the gain is shown numerically to be linearly proportional to vector antenna dimensionality p. This implies that EMVA not only offers the freedom to control the beampattern polarization, but also virtually increases the array size by exploiting the full electromagnetic (EM) field components. We also study the effect of polarization on the spatial power pattern. Our analysis shows that for arrays consisting of pairs of electrical and magnetic dipoles, the spatial power pattern is independent of the mainbeam polarization constraint.

A New Physics-Based Approach for Estimating a Buried Object's Location, Orientation and Magnetic Polarization from EMI Data

A new physics-based expression is presented for determining a buried object’s location, orientation and magnetic polarizibility. The approach assumes the target exhibits a dipolar response and requires only three global values: a magnetic field vector H, a vector potential A and a scalar magnetic potential ψ, all at a single location in space. Among these values, only the scattered magnetic field, H, is measurable with current electromagnetic induction sensors. Therefore, in order to estimate the scattered magnetic scalar and vector potentials from data, a numerical technique called the normalized surface magnetic source (NSMS) method is employed. Originally, in the NSMS model, the scattered magnetic field outside the object is reproduced mathematically by equivalent magnetic charges distributed on a three-dimensional (3-D) closed surface. Here, a two-dimensional (2-D) implementation of the NSMS that uses elementary magnetic dipoles, instead of magnetic charges distributed on a planar surface placed under...

Power Absorption of near Field of Elementary Radiators in Proximity of a Composite Layer

Near-field behavior of elementary electric and magnetic dipoles close to a plane layer (or layers) of engineered composite materials is analyzed using the rigorous analytical approach. Some results of computations are represented for composite media containing conductive inclusions. These composites provide shielding mainly due to absorption of electromagnetic energy. The effect of conductivity of inclusions and their geometry (through their aspect ratio) on the absorption and radiation efficiency of a radiator near composite layers is analyzed.