Dipole In Free Space Research Articles

Quantum limits of position and polarizability estimation in the optical near field

Optical near fields are at the heart of various applications in sensing and imaging. We investigate dipole scattering as a parameter estimation problem and show that optical near fields carry more information about the location and the polarizability of the scatterer than the respective far fields. This increase in information originates from, and occurs simultaneously with, the scattering process itself. Our calculations also yield the far-field localization limit for dipoles in free space. Published by the American Physical Society 2024

A THz Slot Antenna Design Using Analytical Techniques

Slot antennas are very popular microwave antennas, and slotted waveguides are used for high frequency radar systems. A thin slot in an infinite ground plane is the complement to a dipole in free space. This was described by H.G. Booker, who extended Babinet’s principle from optics to show that the slot will have the same radiation pattern as a dipole such that the E and H fields are swapped. As a result, the polarization is rotated, so that radiation from the vertical slot is polarized horizontally. In this work, we show how this straightforward analytical technique can be used for the design of high-frequency THz slot antennas. The analysis is then corroborated by using a numerical simulation which validates the performance parameters predicted by the analytical technique. We show by simulation that, despite the simplicity of our classical approach, we obtained useful results, even at THz frequencies. We show that gradually moving the slot position from the centerline improves the antenna’s performance.

Interference of the scattered vector light fields from two optically levitated nanoparticles.

We experimentally study the interference of dipole scattered light from two optically levitated nanoparticles in vacuum, which present an environment free of particle-substrate interactions. We illuminate the two trapped nanoparticles with a linearly polarized probe beam orthogonal to the propagation of the trapping laser beams. The scattered light from the nanoparticles are collected by a high numerical aperture (NA) objective lens and imaged. The interference fringes from the scattered vector light for the different dipole orientations in image and Fourier space are observed. Especially, the interference fringes of two scattered light fields with polarization vortex show the π shift of the interference fringes between inside and outside the center region of the two nanoparticles in the image space. As far as we know, this is the first experimental observation of the interference of scattered vector light fields from two dipoles in free space. This work also provides a simple and direct method to determine the spatial scales between optically levitated nanoparticles by the interference fringes.

Far-field calculation of an arbitrarily oriented electric dipole in horizontal layered confined space

The far-field calculation of arbitrarily oriented electric dipole in a stratified confined space is of great significance in analyzing electromagnetic properties in the lightning return stroke, submarine communication, over-the-horizon ground-wave radar, etc. Based on the mirror image method and the far-field approximation of an electric dipole in free space, a three-layer horizontal confined space model with an arbitrarily oriented dipole is established in this work. Through novel vector operations, the expression of the far field generated by an arbitrarily oriented electric dipole in the confined space model is derived, where the direct wave from the source point to the observation point and the waves reflected by the upper boundary and the lower boundary are all comprehensively considered. On this basis, the radiation characteristics of the electric dipole with frequencies of 100 kHz, 6 MHz and 10 MHz at different positions in the Earth-ionosphere waveguide are compared and analyzed, which are taken for example. Three different orientations of electric dipoles,i.e. vertical direction, horizontal direction, and 45º tilt are taken into account and the corresponding radiation patterns are presented. The results show that the radiation characteristics of electric dipoles in the Earth-ionosphere waveguide will change greatly with their frequencies, orientations and positions. For the electric dipole source at the same location, the higher the frequency, the more the number of radiation lobes is. In addition, when the source frequency keeps unchanged, the farther the dipole source is from the bottom boundary, the more the radiated lobes are. The proposed expression can conveniently and accurately consider the direct wave of a dipole source and its primary reflection from the upper interface and the lower interface in a confined space, and can also be further extended to solving the contribution of multiple reflections from the interfaces.

Coordinate Transformations-Based Antenna Elements Embedded in a Metamaterial Shell with Scanning Capabilities

In this work transformation electromagnetics/optics (TE/TO) were employed to realize a non-homogeneous, anisotropic material-embedded beam-steerer using both a single antenna element and an antenna array without phase control circuitry. Initially, through theory and validation with numerical simulations it is shown that beam-steering can be achieved in an arbitrary direction by enclosing a single antenna element within the transformation media. Then, this was followed by an array with fixed voltages and equal phases enclosed by transformation media. This enclosed array was scanned, and the proposed theory was validated through numerical simulations. Furthermore, through full-wave simulations it was shown that a horizontal dipole antenna embedded in a metamaterial can be designed such that the horizontal dipole performs identically to a vertical dipole in free-space. Similarly, it was also shown that a material-embedded horizontal dipole array can perform as a vertical dipole array in free-space, all without the need of a phase shifter network. These methods have applications in scanning for wireless communications, radar, beam-forming, and steering.

Waveguide-induced dispersion interaction between two two-level atoms with orthogonal in-transverse-plane dipoles

We study the dispersion interaction between two ground-state two-level atoms near a cylindrical vacuum-clad optical waveguide. We focus on the case where the electric-dipole matrix-element vectors of the atoms are perpendicular to each other and to the interatomic axis. When these atoms are in free space, the dispersion interaction between them vanishes. In the presence of a waveguide aligned parallel to the interatomic axis, the energy of the dispersion interaction between the atoms may become nonzero and comparable to the average energy of the dispersion interaction between two atoms with arbitrarily oriented dipoles in free space. This waveguide-induced dispersion interaction is a consequence of the anisotropy of the medium around the atoms.

Emission of Electric Dipole Radiation in Between Parallel Mirrors

An oscillating electric dipole in free space emits its energy along straight lines. When the dipole is located in between parallel mirrors, this mechanism is significantly altered. Interference between the electric field of the dipole and the reflected magnetic field by the mirrors leads to a four-vortex structure in the emission pattern. The strength of the vortices depends on the separation between the mirrors and the distance of the dipole to one of the mirrors through a universal function.

Compressed Dipoles Resonating at Higher Order Modes With Enhanced Directivity

A compressed dipole operating at higher order modes is proposed and investigated for the enhancement of boresight directivity. Compared with conventional dipoles in free space, the length of the proposed dipole is compressed although the currents on the compressed dipole still sinusoidally distribute. The traces of calculated compressed dipole’s directivity against compressibility show that the directivity of a compressed dipole operating at higher resonant modes is larger than conventional dipoles for the well selected compressibility. This paper shows that printing the dipole onto a piece of dielectric substrate can achieve the desired compressibility. The relationship between the compressibility and relative permittivity of the dielectric slab provides the guidance to realize the selected compressibility. As an example, a thin linear compressed dipole resonating at the third mode is designed, fabricated, and measured to validate the method. The measured broadside gain of the proposed compressed dipole reaches up to 4.6 dBi, 75% or 2.45 dBi higher than a conventional dipole.

On the Momentum Transported by the Radiation Field of a Long Transient Dipole and Time Energy Uncertainty Principle

The paper describes the net momentum transported by the transient electromagnetic radiation field of a long transient dipole in free space. In the dipole a current is initiated at one end and propagates towards the other end where it is absorbed. The results show that the net momentum transported by the radiation is directed along the axis of the dipole where the currents are propagating. In general, the net momentum P transported by the electromagnetic radiation of the dipole is less than the quantity U / c , where U is the total energy radiated by the dipole and c is the speed of light in free space. In the case of a Hertzian dipole, the net momentum transported by the radiation field is zero because of the spatial symmetry of the radiation field. As the effective wavelength of the current decreases with respect to the length of the dipole (or the duration of the current decreases with respect to the travel time of the current along the dipole), the net momentum transported by the radiation field becomes closer and closer to U / c , and for effective wavelengths which are much shorter than the length of the dipole, P ≈ U / c . The results show that when the condition P ≈ U / c is satisfied, the radiated fields satisfy the condition Δ t Δ U ≥ h / 4 π where Δ t is the duration of the radiation, Δ U is the uncertainty in the dissipated energy and h is the Plank constant.

Magnetic Induction-Based Positioning in Distorted Environments

Ferrous and highly conductive materials distort low-frequency magnetic fields and can significantly increase magnetoinductive positioning errors. In this paper, we use the image theory in order to formulate an analytical channel model for the magnetic field of a quasi-static magnetic dipole positioned above a perfectly conducting half-space. The proposed model can be used to compensate for the distorting effects that metallic reinforcement bars (rebars) within the floor impose on the magnetic field of a magnetoinductive transmitter node in an indoor single-story environment. Good agreement is observed between the analytical solution and numerical solutions obtained from 3-D finite-element simulations. Experimental results indicate that the image theory model shows improvement over the free-space dipole model in estimating positions in the distorted environment, typically reducing positioning errors by 22% in 90% of the cases and 26% in 40% of the cases. No prior information on the geometry of the metallic distorters was available, making this essentially a “blind” technique.

The giant dipole vortex

The field lines of energy flow of radiation emitted by an oscillating electric dipole in free space are either straight lines (linear dipole) or they form a vortex (rotating dipole). When the dipole is embedded in a material, the properties of the medium affect the direction of energy flow. Damping due to the imaginary part of the relative permittivity makes the field lines curve for the case of a linear dipole, and for a rotating dipole, the shape of the vortex is altered. In addition, a negative value of the real part of has the effect that the rotation direction of the vortex reverses for the case of a rotating dipole. The value of the relative permeability has in general not much effect on the redistribution of the direction of energy propagation. We show that a dramatic effect occurs when the embedding material is near-single-negative (both and approximately real, and the real parts of opposite sign). The curving of field lines is in general a sub-wavelength phenomenon. For near-single-negative materials, however, this curving extends over large distances from the dipole. In particular, the small free-space vortex of a rotating dipole becomes a vortex of enormous dimensions when the radiation is emitted into a near-single-negative material.

Compact Single-Feed Dual-Mode Antenna for Active RFID Tag Application

This communication proposes a compact single-feed dual-mode antenna for active radio-frequency identification (RFID) tag application. The proposed antenna consists of a slotted patch on an FR4 substrate, a feeding microstrip line, and a foam spacer with a volume of only $60\;\text{mm} \times 50.3\;\text{mm} \times 6\;\text{mm}$ ( $0.48{{\lambda}_0} \times 0.40{{\lambda}_0} \times 0.048{{\lambda}_0}$ , where ${{\lambda}_0}$ is the wavelength at the center frequency). It operates as a dipole in free space and as a patch when placed on a metallic surface. A prototype that can provide an impedance bandwidth of 390 MHz (2.28–2.67 GHz) in free space and 470 MHz (2.32–2.79 GHz) on a metallic surface is built and measured. The measured radiation patterns, peak gains, and radiation efficiencies in these two scenarios are provided.

Radiation From an Arbitrarily Oriented Hertzian Dipole in the Presence of a Gyroelectric Slab

Numerical analysis for the radiation of an elementary dipole is presented when it is located over a half-space gyroelectric medium, a grounded gyroelectric slab, and inside the slab. It is revealed that the direction of the maximum radiated field for a dipole over a half-space gyroelectric medium corresponds to the critical angles where total internal reflection occurs. Usually 6-dB enhancement can be obtained by placing a dipole above the anisotropic region, either over a half-space or grounded anisotropic slab. The effects of the frequency and the biasing magnetic field are studied for the radiation of a dipole over a grounded gyroelectric slab. In addition, it is discovered that more than 6-dB enhanced radiation along the broadside direction, compared with the radiation of the dipole in free space, is achievable if the vertically oriented dipole is placed inside the grounded gyrolectric slab with the proper adjustment of the thickness of the slab and the magnitude of the biasing magnetic field. This thickness is significantly less than the optimum thickness required to achieve the directive emission along the broadside direction using the horizontal dipole inside the grounded isotropic plasma slab without the biasing magnetic field.

The input impedance and the resonance length of two parallel dipoles with a rectangular screen

The induced emf method and the method of uniform geometrical theory of diffraction are used for the development of efficient fast algorithms and programs for calculation of the input impedance and the resonance length of two coupled parallel equiphase dipoles placed parallel to a rectangular metal screen. It is found that shortening of the resonance dipole length is larger than in the case of two parallel dipoles in free space but is lesser than in the case of one dipole with a screen. Starting from the screen dimensions equal to the wavelength, the resonance length of the dipoles is independent of the screen dimensions and, for the half-wavelength spacing between the dipoles, is coincident with the resonance length of the dipole in free space.

Analysis of the Far field pattern of Ungrounded Embedded Dipole in different dielectric slabs

The characteristics of dipoles in free space are reported by many researchers. However such characteristics are not considered by many. The data on Far-field patterns of embedded dipoles is generated in the present work. The dipoles are considered to be embedded in different dielectrics, and also they are unique.

The Radiation $Q$-Factor of a Horizontal $\lambda/2$ Dipole Above Ground Plane

Closed-form equations are given for radiation quality factor of a thin-wire $\lambda/2$ dipole above an infinite electric ground plane. Particular results of interest include a simple formula for a small separation between the dipole and the ground and for one dipole in free space.

Propagation of the Lissajous singularity dipole in free space

The propagation properties of a pair of Lissajous singularities with opposite singularity indexes called the Lissajous singularity dipole (LSD) in free space are studied analytically and illustrated numerically. It is shown that the motion, creation, annihilation and change in the degree of polarization of the LSD, and change in the shape of Lissajous figures take place by suitably varying the waist width, off-axis distance or propagation distance. In particular, the creation and shift to infinity of a single Lissajous singularity may appear. A comparison with the free-space propagation of an optical vortex dipole and a C-dipole is also made.

시간영역에서 수직 다이폴의 전파경로손실 특성 해석

In this paper, we analyze the radio propagation path loss characteristics for the vertical electric dipole radiation over the perfect electric conductor(PEC) ground plane. Most research have been performed about the electromagnetic analysis of vertical electric dipole in free space for time domain or frequency domain. But this paper present the radio propagation path loss over PEC ground plane in time domain under the assumption of the vertical electric dipole as a base station. From the simulated results, the ground plane effect can change the location of near field from transmitting antenna and the transient responses at mobile are calculated. The results of this paper can be applied to surface radar or signal processing applications.

Propagation of Electric Dipole Radiation through a Medium

When electromagnetic energy propagates through a material medium, the paths of energy flow may be altered, as compared to propagation in free space. We consider radiation emitted by an electric dipole, embedded in a medium with permittivity and permeability . For a linear dipole in free space, the field lines of energy flow are straight, but when the imaginary part of is finite, the field lines in the material become curves in the near field of the dipole. Therefore, the energy flow is redistributed due to the damping in the material. For a circular dipole in free space, the field lines of energy flow wind around the axis perpendicular to the plane of rotation of the dipole moment. When has an imaginary part, this flow pattern is altered drastically. Furthermore, when the real part of is negative, the direction of rotation of the flow lines reverses. In that case, the energy in the field rotates opposite to the direction of rotation of the dipole moment. It is indicated that in metamaterials with a negative index of refraction this may lead to an observable effect in the far field.

Emission properties of an oscillating point dipole from a gold Yagi-Uda nanoantenna array

We investigate numerically the interaction of an oscillating point dipole with a periodic array of optical Yagi-Uda nanoantennas in the weak coupling limit. A very strong near-field enhancement of the dipole emission by the resonant plasmon mode in the feed element is predicted in this structure. It is shown that the enhancement strength depends strongly on the dipole position, the direction of the dipole moment, and the oscillation frequency. The radiative intensity of the point dipole from appropriate places next to one feed element may exceed the radiative intensity of an equivalent dipole in free-space by a factor of hundred. In spite of only one director used in each nanoantenna of the array, the far-field emission pattern is highly directed. The radiative efficiency (the ratio of the radiative to the full emission) appears to be around 20%.