Near-field Radiation Patterns Research Articles

Near-Field Gain Expression for Tapered Circular Aperture Antennas

Near-field electromagnetic wave, which is radiated by tapered circular aperture antennas, has found more engineering applications in many diverse fields. Through the simulation of a reflector antenna whose efficiency is up to 51.4%, it can be found that the aperture distribution cannot be regarded as uniform distribution but as tapered distribution. Thus, expressions for near fields of uniform circular apertures cannot be applied to tapered circular aperture antennas in practical engineering. To alleviate such problem, near-field calculation formulas are proposed based on the Sommerfeld integrals with steepest descents technique, which ensures the accurate and efficient analysis of the near field of the tapered circular aperture antenna. And near-field characteristics of tapered circular aperture antennas are displayed. The maximum difference of normalized near-field amplitudes between tapered and uniform distribution is 0.35V/m. Three-dimensional near-field gain of circular aperture antennas is developed from the near-field radiation pattern. The proposed expression extends the range of calculation from the axial to more directions compared with the previous expression. Also, this expression is valid in both the near field and the far field. Using full-wave simulations, results obtained from the proposed expression are compared with the simulated results. And the deviation in near-field gain is less than 0.3dB, which indicates the convenience and effectiveness of our expression. The proposed near-field gain expression can be applicable to the near-field measurement and design of millimeter or terahertz radar systems that work in the near-field region.

A Single-Layer Series-Fed SIW Slot Array Antenna Generating Two Orbital Angular Momentum Modes

A low-profile antenna capable of generating two coaxially propagating orbital angular momentum (OAM) modes is presented. It consists of two series-fed traveling-wave uniform circular arrays (UCA) of sequentially-rotated radiating slots and operates at 10190 MHz. The smaller and larger radius arrays, with 4 and 8 radiating elements, produce OAM modes with indices <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l^OAM</i> = 0 and +1, respectively. They are designed using a unique simulation-based step-by-step technique and implemented on a single layer of the substrate-integrated waveguide, resulting in a simple structure. The antenna's performance is evaluated using simulations and measurements, with a high agreement between the results. Both modes work in left-hand circular polarization (LHCP), with the first mode having a gain and axial ratio measured bandwidth of about 5.9 and 3.9 percent, respectively. In addition, the measured impedance bandwidth for the first and second modes is around 8.2 and 7.9 percent, correspondingly. Also, the orthogonality bandwidth of two identical antennas set 28 cm apart in front of each other is roughly 4.5 percent. The antenna's far-field and near-field radiation patterns are also investigated.

Inverse MoM Approach to Near-Field Prediction and RFI Estimation in Electronic Devices With Multiple Radiating Elements

An inverse method of moments (MoM) approach is developed to estimate the equivalent electric/magnetic current sources to predict radio frequency interference (RFI) in electronic devices. The proposed method is designed to reconstruct the noise sources in electronic devices containing multiple radiating elements by using a finite metal object carrying electric surface currents. The surface current elements are further applied to calculate the electromagnetic fields on arbitrary observation planes near the device under test (DUT). The method is validated through numerical simulation examples, including electric-type CPU noise, magnetic-type non-planar HDMI connector noise, printed circuit board (PCB) with Wi-Fi and PIFA antennas, and heat sink embedded in the DUT. The results confirm less than 0.4 dB difference in the predicted pattern at the antenna operating frequency when a triangular segmentation is applied in the vicinity of the noise source area, and the equivalent surface current is calculated at the edges of the Rao-Wilton-Glisson (RWG) meshes. The proposed method is a promising ’source reconstruction’ candidate for predicting the near-field radiation characteristics of electronic devices with multiple peaks and nulls in the near-field radiation pattern. The method helps estimate the level of RFI by summing up the contributions of all the dipoles of the equivalent noise source derived by solving forward and inverse problems. The RFI results based on the inverse MoM approach are validated by comparing them with those obtained from CST simulation, and good agreement is achieved between the two.

Design of a low-frequency miniaturized piezoelectric antenna based on acoustically actuated principle

An acoustically actuated piezoelectric antenna is proposed for low frequency (LF) band in this paper. The proposed antenna is theoretically calculated, numerically optimized by the finite element method (FEM), and experimentally analyzed. The measurement results show that the near-field radiation pattern of the piezoelectric antenna is similar to that of the electric dipole antenna. The radiation efficiency of the piezoelectric antenna is 3–4 orders of magnitude higher than that of electrically small antenna (ESA), with their sizes being the same size, and the maximum transmission distance obtained experimentally is 100 cm, which can be improved by increasing the input power. In addition, the gain, directivity, and quality factor of piezoelectric antenna are also analyzed. In this paper, traditional antenna parameters are creatively used to analyze the performance of piezoelectric antenna. The research conclusions can provide reliable theoretical basis for realizing LF antenna miniaturization.

Microwave Magnetic Field Pattern Evaluation of Horn and Patch Antennas Using Cs Atomic Sensor

In this study, microwave (9.2 GHz) magnetic field radiation patterns of a horn antenna in the near-field (10 cm) and of a patch antenna in the near-field (3 mm, 2 cm) and far-field (10 cm) are measured using a Cs atomic sensor with $7\pi $ and $8~\sigma $ Zeeman double radio optical resonance (DROR) spectra at 6S1/2(F = 3, $\text{m}_{f}$ ) – 6S1/2 (F = 4, $\text{m}_{f}$ ) - 6P3/2(F = 4) transition by applying a 6 Gauss external magnetic field. Radiation patterns of a horn antenna and a patch antenna are characterized by measuring the DROR amplitude, while rotating the antennas 360 degrees relative to the atomic sensor, and the results are compared to patterns measured in the anechoic chamber using traditional methods. Radiation patterns in both polarizations are measured in a single full rotation, by measuring the $\pi $ and $\sigma $ Zeeman DROR fluorescence amplitudes at the same time. In addition, near-field microwave magnetic field radiation patterns of the patch antenna at orthogonal polarizations are measured at 3 mm distance by moving the antenna ±12.5 mm on the horizontal plane. Advantages and possible applications of small-sized atomic sensors in radiation pattern measurements at different polarizations and radiation analysis of complicated emitters such as circuit boards are discussed.

Compact Flexible Wideband Antenna for On-Body Electromagnetic Medical Diagnostic Systems

A flexible on-body antenna that is compact, unidirectional, and wideband is presented for electromagnetic (EM) biomedical diagnostic systems. To achieve a low operating-frequency band with compact size, an annular ring antenna is loaded with a capacitive gap, a shorting post at its end, and a shorted circular parasitic element. The antenna is designed and fabricated on a purposely built, highly flexible, low-loss, low-cost room-temperature-vulcanizing silicone (RTV) substrate with a tremendously compact size of $0.055\lambda _{0}\times 0.055\lambda _{0}\times 0.009\lambda _{0}$ , where $\lambda _{0}$ is the free-space wavelength at the lowest frequency of 0.55 GHz. The antenna exhibits a measured bandwidth of 0.55–3 GHz at different locations of the human body. The measured near-field radiation patterns demonstrate that the antenna attains unidirectional near-field radiations with more than 10 dB front-to-back-ratio. The received dominant signals of the antenna also exhibit a low distortion level with more than 90% fidelity factor. The achieved features of the proposed antenna confirm its merits for EM biomedical diagnosis applications.

Computation of Near-Field and Far-Field Radiation Characteristics of Acoustic Transducers for Underwater Acoustic Imaging

This paper proposes a novel approach through analytical formulation for the determination of the radiation pattern of acoustic transducers at any arbitrary observation distance which covers both the near-field as well as the far-field. The proposed near-field/far-field analysis is generic and can be applied to a wide variety of acoustic apertures. The radiation pattern of the acoustic transducer at any arbitrary point of observation (both the near-field and far-field) is obtained by the vector summation of the fields radiated by the array of point sources located on the aperture of the acoustic transducer. The validity and novelty of the proposed approach have been proved by treating the far-field as a special case of near-field with the relevant distance criterion. The simulation studies show that near-field radiation patterns of the acoustic transducer exhibit changes in the profile of the amplitude patterns when the distance of observation is varied. The formation of a well-defined main beam of the radiation pattern occurs only after a distance of separation of about R=D2/λ from the radiating aperture. The presented analytical formulation has an attribute of wider generality because of its applicability to a variety of geometrical configurations of the aperture of an acoustic transducer.

A Portable Very Low Frequency (VLF) Communication System Based on Acoustically Actuated Magnetoelectric Antennas

A novel very low frequency (VLF) communication system using one pair of magnetoelectric (ME) antennas has been proposed. The ME antennas are strain-mediated acoustic resonators operating at their electromechanical resonance in the VLF band. The measured near-field radiation pattern reveals ME antennas are equivalent to dipole antennas. The magnetic field radiated by the ME transmitter has been predicted along with distance ranging from 1 mm to 100 km. The measured magnetic field distribution coincided well with the prediction, and the maximum communication distance of 120 m has been achieved. With 80 V driving voltage, the power consumption of the ME transmitter has been measured as 400 mW. Furthermore, the direct antenna modulation (DAM) has also been successfully demonstrated on the ME antennas.

Scattering of surface modes propagating along metamaterial interface

The problem of surface mode scattering from an iris diaphragm is studied by the integral equation technique. The theory is illustrated by the example of scattering of mode, propagating along metamaterial interface. The characteristics of radiation modes excited by the diaphragm are considered. The surface mode reflection and transmission coefficients, the near-field distribution and far-field radiation pattern are calculated.

EHF Band Linear Antenna Array Based on Surface Wave Transformation

The experience of using of experimental samples of diffraction radiation antennas shows the need to expand the list of options and modifications of the technical solutions for such antennas. One of variants is the modification of a linear dielectric waveguide for rigidly fixation of the dielectric rod. H-shaped surface-wave waveguides in the form of combination of dielectric rod and rigid metal elements have been proposed for a basic element of diffraction radiation antennas designed for operation in conditions of increased mechanical loads and vibrations at frequencies above 80 GHz. The results of experimental studies of the near-field distribution, radiation patterns, gain and energy losses of the antenna are presented. The obtained results show the effectiveness of the implemented constructive approach. The configuration of the linear antenna grating of diffraction radiation with a modified dielectric waveguide is proposed, which can be applied for the development of scanning antennas in the 80–100 GHz frequency range.

An Orbital Angular Momentum (OAM) Mode Reconfigurable Antenna for Channel Capacity Improvement and Digital Data Encoding

For purpose of utilizing orbital angular momentum (OAM) mode diversity, multiple OAM beams should be generated preferably by a single antenna. In this paper, an OAM mode reconfigurable antenna is proposed. Different from the existed OAM antennas with multiple ports for multiple OAM modes transmitting, the proposed antenna with only a single port, but it can be used to transmit mode 1 or mode −1 OAM beams arbitrary by controlling the PIN diodes on the feeding network through a programmable microcontroller which control by a remote controller. Simulation and measurement results such as return loss, near-field and far-field radiation patterns of two operating states for mode 1 and mode −1, and OAM mode orthogonality are given. The proposed antenna can serve as a candidate for utilizing OAM diversity, namely phase diversity to increase channel capacity at 2.4 GHz. Moreover, an OAM-mode based encoding method is experimentally carried out by the proposed OAM mode reconfigurable antenna, the digital data are encoded and decoded by different OAM modes. At the transmitter, the proposed OAM mode reconfigurable antenna is used to encode the digital data, data symbol 0 and 1 are mapped to OAM mode 1 and mode −1, respectively. At the receiver, the data symbols are decoded by phase gradient method.

How Ultranarrow Gap Symmetries Control Plasmonic Nanocavity Modes: From Cubes to Spheres in the Nanoparticle-on-Mirror

Plasmonic nanocavities with sub-5-nm gaps between nanoparticles support multiple resonances possessing ultra-high-field confinement and enhancements. Here we systematically compare the two fundamentally different resonant gap modes: transverse waveguide (s) and antenna modes (l), which, despite both tightly confining light within the gap, have completely different near-field and far-field radiation patterns. By varying the gap size, both experimentally and theoretically, we show how changing the nanoparticle shape from sphere to cube alters coupling of s and l modes, resulting in strongly hybridized (j) modes. Through rigorous group representation analysis we identify their composition and coupling. This systematic analysis of the Purcell factors shows that modes with optical field perpendicular to the gap are best to probe the optical properties of cavity-bound emitters, such as single molecules.

The Characterization of a Ridged Half-Mode Substrate-Integrated Waveguide and Its Application in Coupler Design

The miniaturization of microwave and millimeter-wave components is necessary to meet demand for increasingly smaller integrated circuits. This paper explores the technique of capacitively loading the open side of a half-mode substrate-integrated waveguide (HMSIW) with a continuous ridge in order to lower the first-mode cutoff frequency and allow for substantial miniaturization. The full analytical calculation for the design of the first-mode cutoff frequency is presented, along with the extraction of the complex propagation constant and the analysis of the tradeoff between radiation and conductor losses. A comparison between the performance of the ridged HMSIW (RHMSIW) and HMSIW is simulated and measured, where the insertion loss of the RHMSIW is shown to be lower for frequencies up to 1.45 times the cutoff frequency $f_{c}$ , with a total width approximately 0.45 times the width of the HMSIW. The near-field radiation pattern is also measured, clearly illustrating the reduced radiation loss of the RHMSIW. A 3-dB coupler using the RHMSIW technology is designed, fabricated, and measured. The RHMSIW coupler’s performance is shown to be comparable to similar HMSIW and SIW designs in the literature, but with substantially improved miniaturization. The isolation of two parallel RHMSIWs in different configurations is also studied and compared with the HMSIW for use in applications requiring high-density routing or interconnection. It is shown that the capacitive ridge helps to reduce the crosstalk between neighboring transmission lines, allowing for the possibility of greater circuit densities, and further improving the miniaturization of microwave systems using the RHMSIW technology. To the best of our knowledge, the RHMSIW is the smallest waveguide-based transmission line with respect to wavelength presently in the literature.

Performance of Directional and Omnidirectional Antennas in Wideband Head Imaging

Researchers have proposed numerous wideband antennas with directional or omnidirectional radiations to meet the requirements of microwave-based head imaging systems. This letter aims to study the effect of directionality on image quality of those systems. Hence, both directional and omnidirectional wideband antennas are designed and prototyped. Both antennas cover a wide bandwidth (1.25–2.4 GHz) that is typically used in microwave head imaging and attain a boresight average gain of 3.5 dBi. The antennas’ near-field radiation patterns are measured and analyzed. The antennas’ transient pulse performance in the near-field region along boresight direction is also studied. It is observed that without the effects of multipath inside the head, the directional antenna provides 20% higher impulse fidelity and 4 dB more peak transient response than the omnidirectional antenna. An experimental verification is examined by imaging a realistic artificial head phantom with an emulated brain injury. Quantitative analysis of the reconstructed images demonstrates that directional antenna yields more focused images with low artifacts than omnidirectional antenna.

Characteristics of Electromagnetic Radiation of a Railgun at the Final Firing Stage

The navy railgun prototype propels its projectile with Lorentz force and is capable of delivering $\sim $ Mach 7 speed for $\sim 15$ -kg projectiles. The source of the Lorentz force is a gigawatt-class pulsed power supply (PPS) that delivers $\sim 1$ MA current with $\sim 10$ kV voltage into the railgun load. The energy efficiency of the railgun is usually less than 40%: a considerable amount of energy is released as heat, and a small amount as electromagnetic energy. The electromagnetic radiation of the railgun, although mostly shielded by its metallic cover before firing, may leak out at the final stage when the projectile leaves the barrel and can be a potential electromagnetic compatibility issue for onboard electronics. We analyze the electromagnetic (EM) radiation coming from the rails and armature of the railgun with commercial software using the multilevel fast multipole method or higher order basis functions in the method of moment and evaluate the effects on electromagnetic radiation by adding one key component at a time in four models. Three field probes are set up 10 m from the muzzle, and four near fields are plotted on top of the muzzle. The near-field intensities and far-field radiation patterns at the final firing stage are presented and the frequency response is simulated. We found that most far-field patterns are directed toward the breech direction due to impedance mismatch between the PPS and railgun, and $E$ -field probes show stronger intensity on the $z$ -axis. Outer metal casing helps concentrate the electromagnetic energy inside the gun, and dielectric material absorbs the field within. The $E$ -field inside the gun is well below the breakdown threshold of common dielectric insulators. Strong radiation may occur if the railgun voltage spectrum contains a large megahertz component.

RC‐loaded planar half‐ellipse antenna for impulse radar application

Combined with the resistor loading and capacitive loading technique, a planar printed ellipse antenna is designed to achieve ultra-bandwidth and upper radiation efficiency. To accomplish loading, four resistor lumps bridge the metallic strip against the antenna arm on the top layer and rectangular conducting cavity. Simulations are performed for various characteristics of the antenna such as current distribution, near-field radiation pattern and time waveform. Comparison of antenna characteristics between the antenna with resistor loading (type-I) and the antenna with resistor and capacitive loading (type-II) is made using simulations and measurements. The proposed antenna is constructed and measured. It is shown that the antenna achieves an ultra-bandwidth over 8:1 (1–8 GHz) with a return loss less than −10 dB. In addition, experimental verification has been carried out to confirm the improved properties of the antenna. The capacitive loading produced at the end of the antenna arm has improved the peak of the radiation waveform at least 0.62 dB without the cost of increased size.

Wideband symmetric near‐field radiation pattern of sleeve dipole antenna by connecting additional ferrite‐loaded wire

A scaled-down laboratory experiment has been performed in a well-controlled situation to find an enhanced rule for detecting a man-made underground tunnel by employing a cross-borehole frequency-swept radar system. A sleeve dipole antenna, fed by a coaxial cable passing inside its upper arm, is employed to easily put and pull it along an air-filled thin and long glass tube in scaled-down experiments. The tube-guided wave along the coaxial cable can be suppressed by ferrite-loading along the coaxial cable. However, the dipole antenna fed by the ferrite-loaded coaxial cable provides an asymmetrical radiation pattern in the near-field region. To improve the symmetry of its near-field radiation pattern, an additional ferrite-loaded wire is connected to the lower arm of the sleeve dipole antenna. According to the scattering measurement by an air-filled circular cylinder, the additional ferrite-loaded wire improves the symmetry of the electric field by more 30 dB in the frequency range of 1.5–3 GHz.

Lasing on surface states in vertical-cavity surface-emission lasers.

We report experimental observation of lasing on surface states, in the form of standing waves at the termination of a defect-free photonic crystal on top of vertical-cavity surface-emission lasers. Direct images of lasing modes at the truncated periodic potential, along one side of a square lattice, are demonstrated by collecting near-field radiation patterns, as well as in numerical simulations. Our results provide a step toward realizing surface and edge states in optical cavities.

Research on radiation characteristic of plasma antenna through FDTD method.

The radiation characteristic of plasma antenna is investigated by using the finite-difference time-domain (FDTD) approach in this paper. Through using FDTD method, we study the propagation of electromagnetic wave in free space in stretched coordinate. And the iterative equations of Maxwell equation are derived. In order to validate the correctness of this method, we simulate the process of electromagnetic wave propagating in free space. Results show that electromagnetic wave spreads out around the signal source and can be absorbed by the perfectly matched layer (PML). Otherwise, we study the propagation of electromagnetic wave in plasma by using the Boltzmann-Maxwell theory. In order to verify this theory, the whole process of electromagnetic wave propagating in plasma under one-dimension case is simulated. Results show that Boltzmann-Maxwell theory can be used to explain the phenomenon of electromagnetic wave propagating in plasma. Finally, the two-dimensional simulation model of plasma antenna is established under the cylindrical coordinate. And the near-field and far-field radiation pattern of plasma antenna are obtained. The experiments show that the variation of electron density can introduce the change of radiation characteristic.

Near-field radiation pattern distortion of antenna attached to wall in through-the-wall radar imaging

Near-field radiation pattern distortion of antenna attached to wall in through-the-wall radar imaging