Due to the limitation of low-resolution radar system and the influence of background clutter in the detection process, it is hard for low-resolution radars to classify and identify aircraft targets. To solve the above problems, a classification method for aircraft based on Ensemble Empirical Mode Decomposition (EEMD) and multifractal is proposed, in which the intrinsic modes are obtained by EEMD, and the waveform entropy in the Doppler domain is used to screen and reconstruct the intrinsic modes. The multifractal feature of the target echo data is extracted from the reconstructed signal, and then the aircraft target classification and recognition experiment is carried out with support vector machine. The experimental results show that the feature data extracted by ensemble empirical mode decomposition and multifractal analysis can be used for the classification and identification of civil aircraft and fighter aircraft, and the accuracy rate is about 98.5%, which is higher than that of time-domain multifractal method.
A conformal quasi-isotropic dielectric resonator antenna (DRA) is first investigated for wireless capsule endoscope (WCE) application under the 5.8-GHz industrial, scientific, and medical (ISM) standard. The probe-fed hemispherical DRA (HDRA) is studied to match the shape of the spherical dome end, and the characteristic mode analysis (CMA) tool is applied to analyze the resonant modes of the proposed antenna to reveal the intrinsic behavior of the dielectric resonator. It is found that the quasi-isotropic radiation pattern can be achieved by combining HDRA's TE111sinφ mode which radiates like a magnetic dipole and a small ground plane's TM10 mode that radiates like an electric dipole. In order to reach the requirement of 5.8 GHz in ISM, a ceramic hemispherical dielectric resonator with dielectric constant of 21.984 is investigated. The radius of the hemisphere is set to 5.35 mm. In free space, the measurement results show that the proposed antenna achieves 3.25% bandwidth, 86% maximum efficiency and 7.2 dB gain deviation. The antenna is also measured in pork to approximate human body environment. The measurement results demonstrate that the antenna achieves 3.20% bandwidth, 8.15% maximum efficiency and 9.0 dB gain deviation. Accordingly, the proposed antenna is suitable for WCE application at 5.8 GHz ISM standard.
Minimum variance distortionless response (MVDR) beamformer is an one of the well-known space-time antijamming techniques for global navigation satellite system (GNSS). It can jointly utilize spatial filter and temporal filter to suppress interference signals. However, the computational complexity is usually so high that it is difficult to apply in engineering problems. In order to solve this problem, a novel MVDR algorithm based on rank-reducing transformation (RRT) and multistage wiener filter (MWF) is proposed for reducing the computational complexity, named as RRT-MWF-MVDR algorithm. Via the characteristics of the oppressive jamming environment and the steering vector of satellite signal, a rank-reducing transformation is given. By the rank-reducing transformation, a rank reduction is realized for the high dimensional received data. Taking these received data with reduced rank as the input of the MWF, the forward decomposition and backward iteration are accomplished. Then the equivalent reduced rank matrix and equivalent weight vector of MWF can be given, respectively. Finally, the space-time two-dimensional antijamming weight vector is given by the mathematical relationship between the reduced-rank matrix and the weight vector.The proposed method can effectively avoid the inverse of high-dimensional matrix. The proposed method offers a number of advantages over the existing algorithms. For example, (1) it has less computational load and is easier to be executed in practical application. (2) It can maintain higher output signal-to-interference-noise ratio (SINR). Simulation results verify the effectiveness of proposed method.
Space-time adaptive processing (STAP) algorithms can provide effective interference suppression potential in global navigation satellite system (GNSS). However, the performance of these algorithms is limited by the training samples support in practical applications. This paper presents an effective STAP based on atoms extension (named as AE-STAP) algorithm to provide better anti-jamming performance even if within a very small number of snapshots. In the proposed algorithm, a spatial-temporal plane is constructed firstly by the sparsity of received signals in the spatial domain. In the plane, each grid point corresponds to a space-time steering vector, named as an atom. Then, the optimal atoms are selected by searching atoms that best match with the received signals in the spatial-temporal plane. These space-time steering vectors corresponding to the optimal atoms are used to construct the interference subspace iteratively. Finally, in order to improve the estimation accuracy of interference subspace, an atoms extension (AE) method is given by extending the optimal atoms in a diagonal manner. The STAP weight vector is obtained by projecting the snapshots on the subspace orthogonal to the interference subspace. Simulation results demonstrate that the proposed method can provide better interference suppression performance and higher output signal-to-interference-plus-noise ratios (SINRs) than the previous works.
In this article, the drawbacks of Li's formula is rectified and extended to compute accurately the resonant frequency of a rectangular patch antenna in multi-dielectric layers. Computed results employing the present model are compared with experimental and simulation results. The present model shows excellent improvement in accuracy compared to the previously reported investigations.
In this paper, a novel three-dimensional (3D) bandpass frequency selective surface (FSS) is presented based on a square waveguide structure using 3D printing technology. The proposed 3D FSS is composed of a periodic array of the square waveguides with dumbbell slots embedded in waveguide walls. The square waveguide of the unit cell provides a propagation path, which can excite two resonant modes, leading to a bandpass response with one transmission pole and one transmission zero below the cutoff frequency of the square waveguide. To explain the operating principle of the proposed 3D FSS, the electric field distributions at the frequencies of transmission pole/zero are analyzed, and an equivalent circuit model is also established. For validation, a practical example is manufactured simply and rapidly, by using 3D printing technology. To verify the performance of the proposed 3D FSS, the frequency selective characteristics of the implemented 3D FSS for both TE and TM polarizations under different incident angles are measured. The measurement results show that the proposed structure exhibits dual polarizations and provides good frequency stability under incident angles from 0° to 40°.
The purpose of this paper is to provide simple analytical homogenization methods for composite materials containing a metallic wire grid. Estimating their effective electrical properties facilitates the numerical simulation of composite structures for shielding applications in the automotive industry. The presented methods are based on surface impedance approaches and effective media theory. The obtained results show that the shielding properties of the described wire grid composites can be accurately estimated and bounded, using the proposed theories in the low frequency range. The frequency limits vary according to the studied sample. For the presented materials, the validity of the results is shown to be up to a few megahertz. The experimental validation is done by measuring the shielding effectiveness of composite samples using a near-field test bench.
Different global and national electromagnetic regulatory standards have been developed based upon significantly diversified premises, developmental backgrounds and objectives to safeguard life. Some standards aim at minimizing short duration thermal effects, some try to mitigate non-thermal effects over prolonged duration and rest have adopted precautionary limits. As a consequence, these global and national electromagnetic standards substantially differ from each other. Moreover, in spite of lossy dielectric nature of plant tissues, electromagnetic energy absorption rate level estimations for a complete plant model have neither been reported in literature nor been considered while preparing safety standards. To this end, Specific Absorption Rate levels have been estimated for a typical Catharanthus roseus plant model --- typical geometric shape of the plant prototype has been modelled considering the most practical scenario. Detailed analyses on variation of Specific Absorption Rate levels due to wide discrepancy among the existing electromagnetic regulatory standards have been reported in a quantitative manner. This particular work encompasses dielectric properties measurement of different Catharanthus roseus plant samples, modelling a typical Catharanthus roseus plant containing leaves, flower and twig with appropriate dielectric properties defined, and finally the simulation-based investigations to estimate the variation in Specific Absorption Rate levels based on the contrasting electromagnetic exposure standards. Specific Absorption Rate levels have been reported at five different telecommunication bands as per two occupational and four public exposure scenarios. Variations among the estimated Specific Absorption Rate levels have been noted to be significant and presented in detail in this article. A total of thirty rigorous simulations have been carried out along with one hundred and twenty Specific Absorption Rate data evaluations to ensure accurate comparison among different electromagnetic standards. Noted vast variations among estimated Specific Absorption Rate levels based on contrasting electromagnetic standards over the frequencies indicate the necessity of re-evaluating all existing guidelines and also call for the need of maintaining a global uniformity among the existing electromagnetic standards worldwide.
The propagated fields within and radiated fields outside a rectangular dielectric cylinder are represented as guided and radiation modes respectively. These fields of the cylinder are related with incident, backward scattered fields at x=0 and transmitted fields at x=a by Mode Matching technique. The expressions for guided and radiation mode amplitudes are derived by applying the orthogonal property of the modes. The unknown functions (mode amplitudes) in each of these equations that are defining discrete functions of the guided modes field and angular spectrum for the radiation field are determined numerically. The powers due to discrete guided modes (even and odd) are calculated. The integrals related with the backward and forward scattered fields and the powers associated with them are approximately evaluated by the method of steepest descents.
An accurate rigorous modal theory has been applied to investigate the propagation characteristics in a rectangular waveguide filled with multilayer left-handed and right-handed metamaterials. It is shown that such a waveguide supports different passbands below the waveguide's cutoff frequency, and the number of passbands is related to the corresponding layers of different left-handed metamaterials (LHMs) filled in the waveguide. The rigorous modal analysis of this structure reveals in detail how the waveguide geometry and left-handed metamaterial parameters may be selected to design rectangular waveguides supporting double or triple below-cutoff passbands. The efficient power transmissions in these below-cutoff passbands are validated by using the full-wave simulation software HFSS. These structures supporting multiple below-cutoff passbands could find applications in waveguide components requiring miniaturization and multiband properties, such as miniaturized multifunctional antennas and filters.
Wireless communication plays a vital role in transmitting information from one point to another. Wireless devices have to be smart, intelligent, compact in size and cost effective to meet the demand of wireless communication. A multi-layered, Split Ring Resonator (SRR), negative permeability material inspired antenna has been designed, analyzed, fabricated, and measured. The developed antenna resonates at 1.13 GHz, 2.47 GHz, and 2.74 GHz frequencies with gain of 3.73 dBi, 6.18 dBi, 1.35 dBi, and bandwidth of 2.10%, 2.81%, and 2.09%, respectively. The structure utilizes FR4 material as a substrate. The engineered model has applications in navigation, WiFi, and satellite communication applications.
This paper presents a fifth-generation (5G) wireless smart antenna for performing both power substation communication (in space domain beam-steering) and electrostatic discharge (in time domain Ultra-high Frequency ``UHF'' impulse) detection. The same smart antenna used to communicate with other wireless antennas in the switchyard, as well as with the control room is utilized to cyclically gather data from power apparatus, busbars and switches where electrostatic discharge (ESD) may occur. The ESD poses a major threat to electrical safety and lifetime of the apparatus as well as the stability of the power system. The same smart antenna on which beam rotation in space-domain is designed by implementing an artificial neural network (ANN) is also trained in time-domain to identify any of the received signals matching the ultra-high frequency band electrostatic discharge pulses that may be superimposed on the power frequency electric current. The proposed system of electrostatic discharge detection is tested for electrostatic pulses empirically simulated and represented in a trigonometric form for the training of the Perceptron Neural model. The working of the system is demonstrated for electrostatic discharge pulses with rising times of the order of one nanosecond. The artificial intelligence system driving the 5G smart antenna performs the dual role of beam steering for 5G wireless communication (operating in the space domain) and for picking up any ESD generated UHF pulses from any one of the apparatus or nearby lightning leaders (operating in the time domain).
In this paper, a substrate integrated waveguide (SIW) quasi-uniform leaky-wave antenna (LWA) is proposed for a dynamically steerable beam design at a single frequency through the use of a thin layer of nematic liquid crystal (LC) underneath the substrate. The orientation of the LC molecules, and therefore the effective dielectric properties of the LC cell, is controlled via an externally low-frequency, low-strength bias voltage. The radiation occurs through a series of closely placed transverse slots etched on the top plane of the SIW. This antenna was designed to operate based on the fundamental space harmonic (n=0) in the frequency range between 24.25 GHz and 29 GHz, which covers one of the future 5G frequency bands to be deployed in some parts of the world. This novel antenna design concept was verified numerically using a commercial software based on the Finite Element Method (FEM), and the results are presented and discussed herein.
For the first time, a real sized complex target that is coated with an absorber material is discriminated from the uncoated one using an aspect independent discrimination method based on natural resonances. This resonance based technique provides a real-time, accurate and aspect independent solution for stealth target discrimination. First, the discrimination is studied for a complex shaped aircraft of electrical size 1.5λ. The Perfectly Electrically Conducting (PEC) target is coated uniformly with sintered nickel-zinc-ferrite, a magnetic Radar Absorbing Material (RAM) with complex dielectric and magnetic properties. The resonant range Radar Cross Section (RCS) of the aircraft for different coating thicknesses is computed using the Method of Moments (MoM). The resonances contained in the RCS are extracted using the vector fitting method, and the dominant resonances representing the target are determined by applying the power criteria. The variation in the pole placements with the increasing coating thickness is also studied. A one number quantifier of discrimination --- ``Risk'' in dB is defined to express the amount of mismatch between the compared targets. Further, the discrimination technique is also studied for an aircraft of electrical length, 7λ. A Risk value of 2 dB and more is obtained in this study at all aspects. This demonstrates the capability of the algorithm to discriminate between targets of identical structure but with different material compositions.
In this article, a meander line dipole antenna for radio frequency identification (RFID) tag is presented. The loaded meander antenna has a simple meander line structure with a spiral inductor at the end for size miniaturization, a T-match structure and an inductively coupled parasitic element for impedance matching with the tag IC. The antenna is designed to operate in North American UHF RFID frequency band of 915 MHz. The size of the proposed tag antenna is 50 mm × 12 mm and has good impedance matching with Alien Higgs IC chip of 13.5-j111 Ω at the desired frequency band. The proposed tag antenna provides omnidirectional radiation pattern with a maximum read range of 3.5 m at an effective isotropic radiated power of 4 W. Simulation results are in good agreement with measurement results.
A novel computational technique is proposed for heat conduction analysis. The heat transfer equation is expanded in the complex frequency domain and solved using the finitedifference method (FDM). The results in the complex frequency domain are transformed into the time domain via fast inverse Laplace transform. In the proposed approach, the instantaneous temperature at a specific time can be easily obtained. Moreover, the computational time for the conventional explicit FDM is reduced by employing the time-division parallel computing method.
A meshless method for fast solution of the electromagnetic scattering problem related to arbitrary shaped radially inhomogeneous cylinders is proposed. This is an important problem since radially inhomogeneous circular cylinders are common in various engineering applications, and deformations such as notches, grooves and noncircular holes on such cylinders are required for different purposes. This approach is basically an extension of the previously proposed method, which is based on Fourier series representation of the electric field on boundaries. In the original method, a multilayer cylinder with arbitrary shaped homogeneous layers is considered, and accordingly, the general solution of the cylindrical wave equation in homogeneous medium is used. Here we modify the method by considering the general solution in radially inhomogeneous medium, and derive compact expressions for the field.
As ionosphere is one of the most prominent sources of error, ionospheric TEC and scintillation studies are necessary for improving the performance of a navigation system. In this paper, the behavior of the correlation coefficient (ρ) between Rate of TEC Index (ROTI) and amplitude scintillation index (S4) over low latitude station Hyderabad (Latitude: 17.44° (deg.) N, Longitude: 78.74° (deg.) E) for different seasons is analyzed. Also, the analysis is extended for nearly same longitude stations like Trivandrum, Bangalore, Bhopal, Delhi and Shimla for the higher values of total Kp index for 60 days (most disturbed 5 days per month). For Trivandrum (lowest latitude station), it is observed that both S4 and ROTI are high as compared to Bangalore, Bhopal, Delhi, and Shimla. It is found that there is a good correlation between the temporal variations of ROTI with S4 after post sunset hours. The confidence intervals for computed correlation coefficients at 95% confidence level are also given.
Non-contact vital sign detection using radar is relevant for many applications. In search and rescue missions in disaster-stricken areas, this technology can be used to non-invasively detect live survivors on the ground. However, in a very large disaster area, a fast and effective detection approach is required. This need has suggested radar mounted on a flying platform such as a drone as the most feasible approach. This task is challenging, since human respiration is weak, and the signal recorded is easily affected by disturbances such as noise and movement of the platform. Therefore, in this study, we propose a signal processing technique to deal with this problem. Human respiration signals modulate a hyperbolic pattern recorded by moving radar because of distance projection, leading us to applying sequential image processing steps and hyperbolic pattern reconstruction to extract respiration signals. A Fourier transform is then applied to seek the respiration frequency component. The results of laboratory experiments show that the proposed method can detect human respiration. As an important note, the flying speed of the platform should be determined carefully to cope with slow human respiration.
The performance of direction-of-arrival (DOA) estimation algorithms degrades when a partly calibrated array is adopted due to the existing unknown gain-phase uncertainties. In addition, the spatial discretized searching grid also limits the performance improvement and effectiveness of subspace-based DOA estimation algorithms, especially when the true angles do not lie on the grid points which is referred to the off-grid problem alike. In this paper, a self-calibration DOA estimation algorithm is proposed which solves the array calibration and off-grid problems simultaneously. Firstly, the signal model for a partly calibrated array with gain-phase uncertainties is established. To suppress the off-grid errors, an optimization problem for joint parameters estimation is constructed by substituting the approximation of the steering vector into a newly constructed objective function. The alternative minimization (AM) algorithm is employed to calculate the joint DOA and gain-phase uncertainty estimations. Within each iteration step of the optimization problem, a closed-form solution is derived that guarantees the convergence of the proposed algorithm. Furthermore, the Cramer-Rao bound (CRB) for the partly calibrated arrays with unknown gain-phase uncertainties is also derived and analyzed in the paper. Simulation results demonstrate the effectiveness of the proposed algorithm.