Short Dipole Research Articles

DESIGN AND DEVELOPMENT OF COMB-SHAPED ME DIPOLE ANTENNA

This paper presents a novel comb-shaped electric dipole and a quarter-wave shorted magnetic dipole in the magneto-electric (ME) dipole antenna. This antenna has been excited by cascaded 1 shaped feed line to exhibit broadside radiation pattern. A wide impedance bandwidth of 24%, with 8.1 dBi peak gain has been achieved for the frequency range 2.2 GHz-2.8 GHz. The antenna also exhibits low side lobe radiations and almost identical E-plane and H-plane radiation patterns in the given frequency range. The 3-dB E-plane and H-plane beamwidth of 101.4° at 2.5 GHz has also been reported. The antenna has a wide potential for various applications in the S band.

The Extended Manifold for Antenna Arrays

The paper investigates a general parametric model for the manifold of polarization sensitive arrays based on representing each antenna by a collection of short dipoles. The manifold has a simple form consisting of an extended isotropic manifold, vertical and horizontal direction vectors and a constant non-square coupling matrix. The resulting manifold captures the effects of coupling, polarization and antenna characteristics. It provides an accurate representation of the analytic manifold which is based on electromagnetic theory. Three methods for computing the Coupling Matrix are described based on: (i) the current distributions in the antennas, (ii) its approximation by a sum of Kronecker products, and (iii) least-squares fit of the analytic and extended manifolds. The accuracy of these manifolds is illustrated by numerical examples.

Electrically Long Dipoles in a Crossed Pair for Closed-Form Estimation of an Incident Source’s Polarization

Using two electrically long dipoles to estimate the polarizations of impinging sources, this paper pioneers signal-processing algorithms in closed forms. The existing literature on crossed-dipoles polarimetry are restricted to electrically short dipoles with an electrical length of $(L/\lambda) \le (1/10)$ , which have subpar efficiency in radiation and in reception. In contrast, this paper allows electrically long dipoles—two of them here are perpendicularly oriented but may be collocated or may be separated by a known displacement. Using such a pair of electrically long dipoles for polarization estimation, this paper proposes new closed-form formulas and derives the associated Cramer–Rao bounds.

Numerical Calculations of Multi-Channel Receiver Coil Array with High-Pass Spoke Coil for Parallel MRI

The proposed study reports the design and testing of spoke coil (SC), that compensates for the degradation in the g-factor that is caused by the insufficient sensitivity of the coil array along the main magnetic field direction. Electromagnetic (EM) simulations were performed to evaluate the performance of the proposed coil in a 7-T MRI system. 25-channel surface loop array and short dipole array containing SC or circular loop coil (LC) on the top of the phantom are 3D modeled. The SENSE g-factor and SNR of coil arrays are calculated and evaluated, and the quality factor (Q), scaling factor (α), and noise-corelated resistance (R) are considered. The directed effective magnetic field (B1−) generated by the SC improves the g-factor by compensating for the insufficient sensitivity information gathered by the array. However, this arrangement should be optimized to increase the available sensitivity information, depending on the type and structure of the coil elements in the array.

An efficient SPICE-compatible circuit model for transmission line response excited by an electrically short dipole inside a metallic cavity

ABSTRACTFor the transmission line response excited by an electrically short dipole inside a rectangular cavity, a SPICE-compatible broadband Foster type equivalent circuit is presented. The electric field inside a cavity is represented by the eigenmode decomposition of Helmholtz equation, while the current of transmission line is expanded using the eigenmode of transmission line equation. The field-to-line coupling effect is described by the Agrawal formula. The convergence of the circuit is accelerated by the extraction of quasi-static inductances and capacitances. The validity and efficiency of the presented method is verified both in frequency and time domain. Also, the effects of lossy filling material are discussed. Besides, in order to show the aggravation effect of metallic cavity on the electromagnetic interference, the induced currents on the transmission line with and without a metallic casing are given and compared.

Performance of Electrically Small Conventional and Mechanical Antennas

Antennas that operate in the very low frequency (LF) band and below are useful for a number of applications, including long-distance and underwater communication. When constrained in size, the antennas are electrically small and very inefficient. This has motivated the need for novel approaches to LF antenna design. Here, we present concepts for antennas that generate electromagnetic signals from mechanical motion. We first review the generated fields and efficiency of conventional magnetic and electric dipole transmitters. This is then extended to their mechanical counterparts for comparison. Our results show that the motion of magnets or electrets (the electrical analog of a magnet) can efficiently radiate electromagnetic energy when coupled to a low-loss electromechanical suspension. Mechanical antennas, with spatial dimensions on the order of a meter, can theoretically exceed the performance of conventional short dipole and coil transmitters by more than eight orders of magnitude for frequencies of 1 kHz and below. This paper is intended to lay the foundation for future development involving the implementation of efficient, small form-factor, mechanically actuated antennas.

Electric Field Probe for Time-Domain Monitoring of Radio Frequency Exposure During Development and Evaluation of MRI-Conditional Medical Devices at 3 T

This paper presents the design and validation of a tuned time-domain electric field probe for mapping of radio frequency (RF) exposures used during testing of magnetic resonance imaging (MRI) conditional medical devices. The probes were 5 and 10 mm short dipole antenna, developed as a tradeoff between spatial resolution, linearity, and sensitivity. The probes were tuned and matched at a center frequency of 127.6 MHz, which corresponds to the RF frequency for 3T MRI scanners. To improve the accuracy and sensitivity, an RF low noise amplifier with high gain and very low noise figure was developed, followed by distributed $\boldsymbol {\lambda /4}$ baluns along a triaxial cable to reduce the electric field pickup in the MRI environment. The probe was fabricated on a double-sided printed circuit board, FR4 thickness of 1.57 mm and a copper thickness of $35~\mu \text{m}$ . Theoretical analysis was performed to calculate the exposed electric field from the real-time receive signals. To verify the probe performance finite-difference time-domain method simulations were compared to the actual measured electric fields. Developed probe was tested in a commercially available 3T RF exposure system to determine the probe dynamic range and linearity.

A Novel Interpretation of the Electromagnetic Fields of Lightning Return Strokes

Electric and/or magnetic fields are generated by stationary charges, uniformly moving charges and accelerating charges. These field components are described in the literature as static fields, velocity fields (or generalized Coulomb field) and radiation fields (or acceleration fields), respectively. In the literature, the electromagnetic fields generated by lightning return strokes are presented using the field components associated with short dipoles, and in this description the one–to-one association of the electromagnetic field terms with the physical process that gives rise to them is lost. In this paper, we have derived expressions for the electromagnetic fields using field equations associated with accelerating (and moving) charges and separated the resulting fields into static, velocity and radiation fields. The results illustrate how the radiation fields emanating from the lightning channel give rise to field terms varying as 1 / r and 1 / r 2 , the velocity fields generating field terms varying as 1 / r 2 , and the static fields generating field components varying as 1 / r 2 and 1 / r 3 . These field components depend explicitly on the speed of propagation of the current pulse. However, the total field does not depend explicitly on the speed of propagation of the current pulse. It is shown that these field components can be combined to generate the field components pertinent to the dipole technique. However, in this conversion process the connection of the field components to the physical processes taking place at the source that generate these fields (i.e., static charges, uniformly moving charges and accelerating charges) is lost.

What We Can Learn from the Electric Field and Conductivity Measurements in Auroral Atmosphere

AbstractIn this paper we present measurements of the electric field and electric conductivity of air performed with the Short Dipole Antenna (SDA) in the auroral atmosphere. The observations were carried out during two stratospheric balloon flights in the northern Sweden in winter of 2011. Two‐fold objectives were attained as the outcome of these flights and the data analysis. First aimed at understanding the mechanisms that control the electric properties of the auroral atmosphere and at estimation of some key parameters from the measured data, such as the charge and current density, the mobility of the small‐ions, their concentration and generation rate, as well as the radiation dose rate. Second objective was to test the SDA instrument performances in a radiation environment close to that we expect to encounter near the Mars surface.

Fano resonances in symmetric gold nanorod trimers

A symmetrical gold nanorod trimer structure consisting of a short center nanorod and two long nanorods on both sides is proposed. The scattering spectra, electromagnetic field and current density vector distributions across the central cross section of the nanorod trimer are calculated by the finite difference time domain method, and the effects of structural parameters and dielectric environment on Fano resonance characteristics are theoretically investigated in detail. The results show that the Fano resonance can be generated mainly due to the interference between the bonding electric dipole mode in lower energy and the antibonding electric dipole mode or antiphase magnetic dipole mode in higher energy. The Fano dip is blue-shifted with the decrease in the short nanorod length, the size of whole trimer structure with constant displacement, or the refractive index of dielectric medium in the gaps between the central nanorod and two side nanorods; the resonance intensity on both sides of the Fano dip also changes. Meanwhile, the bonding mode on the red side of the Fano dip is gradually dominated by the electric dipole mode of two side nanorods, and the spectral intensity increases, while the antibonding mode on the blue side gradually evolves into the short nanorod-dominated antiphase magnetic dipole mode, and the spectral intensity becomes weaker. The increase in the inter-rod spacing also leads the Fano dip to be blue-shifted, and a similar change in the spectral intensity occurs on both sides of the Fano dip, due to the degeneration of bonding and antibonding modes caused by the decrease of near-field coupling between the short nanorod and two side nanorods, which finally degenerate into the electric dipole modes generated by the short nanorod or the two side nanorods, respectively. In addition, the Fano dip is insensitive to the change of the side nanorod length, but the relative resonance intensity on both sides of the Fano dip also changes. Furthermore, it is found that the spectral contrast ratio of the Fano resonance first increases and then decreases by varying the above-mentioned structural parameters or dielectric environment. These results are expected to be used for guiding the design of Fano controllable nanostructures and also for developing the applications of specific micro-nano photonics.

A Novel Ultrawideband Transmitarray Design Using Tightly Coupled Dipole Elements

One of the key challenges in transmitarray (TA) designs is its narrow bandwidth. To overcome this limitation, this paper proposes a novel design of ultrawideband TA antenna based on tightly coupled dipole arrays (TCDAs). The array consists of $20 \times 10$ unit cells, and each unit cell is composed of a pair of short dipoles connected with a section of meandered transmission line. The size of each unit cell is 20 $\text {mm} \times 10$ mm, which is about $0.2\lambda \times 0.1\lambda $ , where $\lambda $ is the wavelength in free space at the lowest working frequency of the TA. By utilizing the strong coupling between adjacent units of the TA, the bandwidth of the TA is improved significantly. To verify the design concept, one prototype of the proposed TA is fabricated and measured. The prototype demonstrates good performance over a bandwidth of 104%, i.e., from 3.0 to 9.5 GHz, which is significantly wider than that of other TAs reported in the literature. Within the working band of the TA antenna, the radiation pattern is stable and no distortion or splitting of the main beam of the antenna is observed. This is the first time that the TA based on TCDA is reported.

Vector Cross Product-Based 2D-DOA and Polarization Estimation With “Long” Electric-Dipole Quint

Dipole antenna elements whose physical length L is greater than or equal to one-tenth of the wavelength λ are generally denoted as long dipoles. Such radiators are more useful in practical polarization sensitive arrays (PSAs) for electromagnetic sensing applications because their radiation efficiency is greater than that of short dipole radiators for which L <; λ /10. Thus, research focused on estimating the parameters of unknown electromagnetic sources using long dipoles has attracted considerable attention from numerous researchers in recent years. However, the development of PSAs and algorithms for this purpose remains in its infancy. In this paper, we propose a PSA composed of five spatially non-collocated long dipoles in a centrosymmetric configuration and apply an algorithm based on the vector cross product for estimating the 2-D direction of arrival (2D-DOA) and polarization parameters of unknown electromagnetic sources. First, the time-domain rotational invariance of the received data facilitates the use of the ESPRIT algorithm for estimating the steering vector. Second, the relationships between the entries of the steering vector are employed to obtain highly accurate, albeit cyclically ambiguous estimates of the directional cosines of the x- and y-axes. Then, the coarse 2D-DOA parameter estimation is conducted using the vector cross product algorithm. Finally, the course 2D-DOA parameter estimations are employed as a reference to obtain a set of highly accurate and unambiguous directional cosine estimates from the previously determined set of cyclically ambiguous estimates. The effectiveness of the proposed algorithm is verified by the simulation results.

Light-dressed states under intense optical near fields

We studied the excitation dynamics of a finite quantum system with an intense optical near field (ONF) from a perspective of light-dressed states. A simple model consisting of a single electron and a nano-sized short dipole source were employed. By calculating the time-dependent wave function subjected to the ONF, we demonstrated that the optical responses involved not only the first- and third-order but also the second-, fourth-order harmonic generations that were not obtained from conventional spatially homogeneous laser fields. In order to elucidate the origins of the exotic high-order harmonic generations, the dressed states altered by the ONF were explored. The result showed that the spatial distribution of the dressed states were significantly influenced by the ONF, making forbidden optical transitions to be allowed by parity breaking. This was caused by the spatial inhomogeneousness of the ONF, specifically its asymmetry, that was inherited to the dressed states.

Using Electrical Resistivity Tomography (ERT) to Delineate Subsurface Structures at Siloam Hot Spring in the Soutpansberg Basin, South Africa

Electrical resistivity tomography (ERT) was used to delineate subsurface structures at Siloam hot spring in the Soutpansberg Basin of South Africa. The spring water was reported to be hottest in South Africa. Groundwater with temperature ranging between 63.3 &amp;deg;C and 67.5&amp;deg;C was observed flowing from the spring. Relatively wet areas were noted to the south and north of the spring. Electrical resistivity arrays with Dipole-dipole and Wenner-Schlumberger configurations were used. Current injection and potential dipole separations of 5 m and 10 m were applied across the spring. Inversion results for arrays with the shorter dipole separation of 5 m had better resolution. Resistivity sections were generated from inversion of ERT data. The resistivity values for the low and high resistive zones were &amp;rho;a &amp;lt; 10 and &amp;rho;a &amp;gt; 70 Ωm respectively. Sub-horizontal resistivity zones were obtained from the inversion of Wenner-Schlumberger array survey data. High resistivity zones associated with dykes or sills were delineated on Dipole-dipole derived depth sections. Near vertical structures associated with water bearing faults or fractures were delineated at the Siloam hot spring.

Dual-Wideband Magnetoelectric Dipole Antenna With Director Loaded

In this letter, a dual-band magnetoelectric (ME) dipole antenna with wide operation bands is proposed for WiMAX and WLAN applications. A full-metal rectangular director is loaded right above the ME dipole antenna and it resonates at around 3.6 GHz due to the excitation of the long electric dipole (LE-dipole). Then, the lower operation bandwidth is extended significantly. This added director also allows our antenna to work as a quasi-Yagi antenna at specific frequencies, and thus, the proposed antenna is capable to achieve better unidirectional radiation performance and higher gain. In order to satisfy WLAN communication, a short trapezoid electric dipole is designed beneath the LE-dipole, which maintains the overall antenna dimensions as unchanged. Besides, such design strategy avoids the distortion of the radiation pattern at 5 GHz as well. As for the ground, a hornlike reflector is adopted instead of traditional planar structures. The measured results show that our proposed antenna achieves two wide bandwidths, 2.3–3.7 GHz (46.7%) and 5–6.25 GHz (22.2%), and high stable gain (8.5 ± 0.9 and 9 ± 1 dB) within these two operation bands.

Theory and Realization of a Pattern-Reconfigurable Antenna Based on Two Dipoles

A front-to-back pattern-reconfigurable antenna based on two dipoles is investigated. The theory for a front-to-back pattern reconfiguration is analyzed. The front-to-back ratio (F/B) is improved by using two folded dipoles. The pattern-reconfigurable antenna is realized by switching two PIN diodes with two broadband baluns. Two parasitic elements are introduced for the improvement of impedance bandwidth and pattern bandwidth. A shorter dipole is added to each folded dipole for further enhancing the pattern bandwidth. Experimental results show that the pattern-reconfigurable antenna achieves a bandwidth of 31.4% for return loss >10 dB and a pattern-bandwidth of 20.8% for F/B > 10 dB and 11.5% for F/B > 15 dB.

Solid-State Conductive-Bridging Reconfigurable RF-Encoding Particle for Chipless RFID Applications

We report the design and first results of a reconfigurable RF-encoding particle (REP) for chipless RFID tag, based on a Nafion conductive-bridging random-access memory cell. The developed tag consists of a modified shorted dipole as REP, equipped with a copper-Nafion-aluminum metal-insulator-metal (MIM) nonvolatile ionic-bridging switch. The MIM switch is similar to an RC parallel network, which tunes the electrical length of the resonator. The chipless tags switch between two different resonant frequencies, in the set and reset states of the switch. An electrical model is also developed for the REP to explain the mechanism of switching. The experimental results are in close agreement with the simulation results using commercial electromagnetic simulators. This letter is a proof of concept of electronically reconfigurable chipless tags based on integrated switches which have the potential to be directly printed along with the tag.

Frequency-Domain Diagnosis Methods for Quality Assessment of Nb3Sn Coil Insulation Systems and Impedance Measurement

In recent years, the superconducting Nb3Sn cable material became the privileged mature candidate for the high-field magnets in new projects like high-luminosity LHC (HL-LHC) accelerator at CERN, Geneva, Switzerland. The technology in 2017–2021 needs to be deployed through an unprecedented magnet series production with dedicated online quality control. The key fabrication stage of the vacuum pressure impregnation (VPI) after the heat treatment reaction of Nb3Sn coils, as on the new 11-T dispersion region dipole, enhances both the structural integrity and the dielectric strength of the winding packs. The global vacuum impregnation pressure method exhibits various merits in insulation performance and high dielectric strength reliability, which is strongly dependent on the success of the resin filling cycle. This online capacitive measurement method enables one to derive comparative master trend curves of various impregnated coils and possibly optimize the curing cycle. Ultimately, a combination of the above methods with a dielectric frequency response can bring insights on the impregnation process, the impacts from the resin choice and insulation material quality on the degree of curing, and the coil assembly geometry. The frequency impedance measurement of the first short dipole models DP101-102 provides the distributed lumped circuit fitting electrical parameters for the transient characterization of produced magnets.

Shadow-free multimers as extreme-performance meta-atoms

We generalize the concept of parity-time symmetric structures with the goal to create meta-atoms exhibiting extraordinary abilities to overcome the presumed limitations in the scattering of overall lossless particles, such as non-zero forward scattering and the equality of scattering and extinction powers for all lossless particles. Although the forward scattering amplitude and the extinction cross section of our proposed meta-atoms vanish, they scatter incident energy into other directions, with controllable directionality. These meta-atoms possess extreme electromagnetic properties not achievable for passive scatterers. As an example, we study meta-atoms consisting of two or three small dipole scatters. We consider possible microwave realizations in the form of short dipole antennas loaded by lumped elements. The proposed meta-atom empowers extraordinary response of a shadow-free scatterer and theoretically enables most unusual material properties when used as a building block of an artificial medium.

Investigating Equations Used to Design a Very Small Normal-Mode Helical Antenna in Free Space

A normal-mode helical antenna (NMHA) has been applied in some small devices such as tire pressure monitoring systems (TPMS) and radio frequency identification (RFID) tags. Previously, electrical characteristics of NMHA were obtained through electromagnetic simulations. In practical design of NMHA, equational expressions for the main electrical characteristics are more convenient. Electrical performances of NMHA can be expressed by a combination of a short dipole and small loops. Applicability of equations for a short dipole and a small loop to very small normal-mode helical antennas such as antennas around 1/100 wavelengths was not clear. In this paper, accuracies of equations for input resistances, antenna efficiency, and axial ratios are verified by comparisons with electromagnetic simulation results by FEKO software at 402 MHz. In addition, the structure of the antenna equal to 0.021 λ is fabricated, and measurements are performed to confirm the design accuracy.