This paper presents a comprehensive method to analyze negative group delay (NGD) phenomenon at microwave frequency. This method is based on a coupling matrix with finite unloaded quality factor resonators. Unlike conventional NGD circuit topologies that use a lumped resistor R along with bandstop resonators, the proposed topology does not require any R for generating NGD and therefore, provides fully distributed circuit realization. The proposed topology has both source to load and inter-resonator coupling structures. Analytical design equations are provided to obtain predefined NGD with matched input/output ports; the proposed structure therefore does not require any extra matching networks. From analytical analysis, it is also found that the NGD bandwidth as well as magnitude flatness can be controlled by inter-resonator couplings. The proposed design theory is proven through fabrications of NGD circuit at a center frequency of 2.14 GHz. The measurement results are in good agreement with simulations and predicted theoretical results.

In this work, an anisotropic zero index material is designed for use in Vivaldi antennas. The metasurface structures are placed within the aperture of a Vivaldi antenna to improve the directivity and gain of the emitted radiation. The range of operation is in the ultrahigh frequency (UHF) range, between 300 MHz and 3 GHz. Two approaches are presented: a type of resonant metallic metamaterial that belongs to the larger class of anisotropic zero index metamaterials and a non-resonant material. A technique for lowering the dimensions of the resonant metamaterial unit cell is presented and applied. The work presented consists of simulation results obtained with HFSS modelling software from ANSYS.

In this paper, the slot-shape and slot-size introduced on the radiating surface of a microstrip antenna as well as the inserted air-gap between the substrate sheet and ground plane are predicted, simultaneously. This synthesizing-prediction is carried out using knowledge based neural network (KBNN) model as this approach requires very less amount of training patterns. The suggested approach is validated by fabricating and characterizing three prototypes. A very good agreement is attained in measured, simulated and predicted results.

An extended transmission-line model is presented for an inset-fed rectangular microstrip patch antenna. The transmission-line model agrees to the cos4 impedance variation for inset-fed microstrip antennas with an addition of a corrective extended feed length upto the inner radiating edge. Verification of the model's complex reflection coefficient is concluded with good agreements with measured results. Further extension of the transmission-line model with for or more thin shorting post connected to multiple varactor diodes have been conducted. Fourty two test cases across five independent antenna designs have been worked upon. Results obtained using the transmission-line model are compared with those obtained with a 3D full-wave solver and measurements. In 69% of the test cases, the transmission-line models have less than 3% deviation to the measured or simulated results. 41% of them have less than 1% deviation. For the first two antennas, both simulated and measured results were compared with the transmission-line model. For the rest of three, results from the transmission-line model were compared to the simulated ones.

In this paper, we present an efficient meshing scheme for physical optics calculation of electromagnetic scattering from bodies of revolution. Piecewise linear approximation is used to represent the generatrix and circular perimeter of the body's cross section. This results in quadrilateral meshes and enables the application of multilevel search algorithms for efficient determination of the illuminated portion of the surface. Besides, the physical optics surface integral is reduced to a closed form expression using the Gordon's method. Simulation results conrm the proper accuracy and efficiency of the presented algorithm.

An active reconfigurable Radar absorbing structure (RAS) with the pin diode was proposed to reduce the radar cross section (RCS) of antenna. The operating states of the RAS reflector can be switched by using the pin diode. For ON-state and OFF-state diodes, the reflection coefficients of the RAS reflector were less than -25 dB and more than -0.8 dB around 8.3 GHz, respectively. The RAS reflector with ON-state diodes can be used as a dipole antenna reflector and has the same radiation performance as a dipole with a metal reflector, while the RAS reflector with OFF-state diodes can be used as a radar absorber for RCS reduction. Meanwhile, chessboard-like geometry RAS reflector was proposed to achieve wideband RCS reduction. The RCS reduction band covers the working band and is extended to 5-18 GHz. The results show that the reconfigurable RAS reflector can contribute to the antenna RCS reduction at working frequency without loss of radiation performance of dipole antenna.

A low-profile wideband bowtie antenna backed by artificial magnetic conductor (AMC) ground is presented for gain enhancement.The proposed bowtie antenna, loaded with an open stub in the upper layer, has broadband property. By using an AMC reflector,consisting of 6×9 metallic patches, the bidirectional radiation of the bowtie antenna is changed to unidirectional radiation. The distance between the bowtie antenna and the AMC surface is onlyλ/10 at 3.75 GHz. Both the bowtie antenna and the AMC surface are fabricated and measured. The measured results demonstrate good and stable performances, including maximum gain of 8.27 dBi, and flat gain response with variation of 0.6 dB in the wide impedance matching (S11 < -10 dB) band from 3.05 GHz to 4.35 GHz(35.1%). Furthermore, the maximum cross-polarization level is -17 dB for both E and H planes, and the measured front-to-back ratios are more than 18 dB.Good agreement between the simulated and measured results validates the proposed design approach.

In this paper, a combination of the Jerusalem cross (JC) as a fractal load and fractal Minkowski slot antenna for dual-band application is investigated. The prototype slot antenna has a Minkowski fractal formation with four Jerusalem cross (JC) loads to achieve dual-band application with compact size to improve the bandwidth. A T-shaped feed line is implemented in the final modeled antenna. The fabricated antenna has a bi-directional pattern with sufficient bandwidth at 2.4-3.1 GHz and 5.1-5.9 GHz with VSWR<2 for Wi-Fi, WiMAX, Bluetooth application as well as an IEEE WLAN protocol with a gain of 5-6 dBi, respectively. The size of the prototype patch antenna is 40×40 mm², and the antenna is designed and fabricated on an FR-4 low cost substrate with ε_r=4.4 and thickness of 1.6 mm. It is simulated by HFSS full wave software. In addition, the VSWR, pattern and axial ratio of experimental results are presented and compared with simulation models. As a result, improvements of the Jerusalem cross compared with conventional cross have been achieved with some parameter tuning to improve the band width.

We present the numerical parametric study of a reconfigurable plasma antenna array (PAA) composed of a metallic half-wavelength dipole and a set of cylindrical plasma discharges arranged in a planar square lattice. Our results, obtained with the linear embedding via Green's operators (LEGO) method, indicate that beam-forming and beam-steering functionality can be achieved and controlled by appropriately choosing the number and position of the active plasma discharges around the dipole. Furthermore, we show that an external static magnetic field and the plasma density have a noticeable effect on the radiation pattern of the antenna.

In this article, a method of broadband flat gain enhancement for a planar double-dipole quasi-Yagi antenna using multiple directors is proposed. The proposed antenna consists of two dipole drivers with different lengths, a truncated ground plane, and three parasitic strip directors. First, the length ratio of the two dipoles is adjusted to increase the gain in the low-frequency region. Next, three parasitic strip directors are employed to increase the impedance bandwidth and improve the gain of the antenna in the middle- and high-frequency regions. A detailed design procedure for the proposed antenna, covering a frequency band of 1.70-2.70 GHz with a gain > 8 dBi, is explained, along with a step-by-step analysis of the effects of placing each director on input impedance, voltage standing wave ratio (VSWR), and gain characteristics. Experiment results show that the proposed antenna has the desired impedance characteristics with a frequency band of 1.66-2.88 GHz (53.7%) for a VSWR < 2, and a stable flat gain of 8.0-8.4 dBi in the 1.70-2.70 GHz frequency range. Moreover, a measured front-to-back ratio > 11 dB within the band is achieved.

Sparse signal recovery algorithms can be used to improve radar imaging quality by using the sparse property of strong scatterers. Traditional sparse inverse synthetic aperture radar (ISAR) imaging algorithms mainly consider the recovery of sparse scatterers. However, the scatterers of an ISAR target usually exhibit block or group sparse structure. By utilizing the inherent block sparse structure of ISAR target images, an iterative reweighted l_p(0 < p ≤ 1) block sparse signal recovery algorithm is proposed to enhance imaging quality in this paper. Firstly, an ISAR imaging signal model is established with the aid of sparse basis, and the imaging is mathematically converted into block reweighted cost function optimization problem. Then, an iterative algorithm is used to solve the reweighted function minimization problem. In each iteration, the weights are updated based on the closed form solution of the previous iteration. The proposed method is effective to exploit the underlying block sparse structures which does not need the prior knowledge of the number of the blocks. Real data ISAR imaging results are provided to verify that the proposed algorithm in this paper can achieve better images than the images obtained by several popular sparse signal recovery algorithms.

Miniaturization and metal-housing environment are the two most critical problems in the design of antennas, because they can highly deteriorate the performances of antenna, which not only affects the antenna efficiency, but also influences the bandwidth. In this paper, a compact size antenna with full metal housing for GPS/WLAN applications is studied. The proposed antenna can excite triple-band operation that covers the GPS (1.575 GHz), WLAN 2.45 GHz and WLAN 5.2/5.8 GHz bands, and its corresponding measured average efficiencies over these three desired bands were 40%, 41%, and 70%, respectively. The proposed antenna has a volume of 20.5×5×4 mm³, which is probably the smallest antenna in the industry for full metal housing applications.

Orbital angular momentum (OAM) with a huge potential application in multiplexing and coding has become the subject of intense research in recent years. This paper presents a method to generate radio beams carrying OAM based on a circular patch antenna. A 3 dB quadrature hybrid is employed in the design to enable the circular patch to reconfigure opposite OAM states of a radiated field. The results of numerical simulations are presented to show that the circular patch radiates two OAM modes with opposite rotation directions simultaneously. The proposed circular patch is believed to be significant to the wireless communication applications due to its simple geometry, low cost, and OAM mode reconfiguration.

This paper presents the design and implementation of a novel fully planar modified, extended composite right/left-handed transmission lines (E-CRLH-TLs) utilizing a complementary split ring resonator (CSRR) loaded on the ground plane. The multiband behavior of the proposed layout is demonstrated by an equivalent circuit which in this case is distinct from the standard form of the equivalent circuits presented for an E-CTLH-TL; therefore, in order to design a quad band E-CRLH unit-cell, the design procedure is investigated. The main advantages of the proposed layout over other topologies are size reduction and fabrication simplicity which are proved by the design and fabrication of a quad-band Y power divider.

A dual band printed inverted-F antenna with a nested structure is proposed. In this antenna, matching can be controlled for both frequency bands by changing element lengths. The measured and calculated frequency characteristics of the antenna's reflection coefficient match very well, if the measurement cable connector is considered in the simulation. The measured -10 dB relative bandwidths of the reflection coefficient are 4.7% at 2.45 GHz (2.5 GHz to 2.62 GHz), and 9% at 5.5 GHz (5.28 GHz to 5.78 GHz). The calculated radiation efficiencies are 92% and 88%, at 2.45 GHz and 5.5 GHz, respectively, with calculated peak realized gains of 1.07 dBi and 3.36 dBi, respectively.

The transcranial magnetic stimulation (TMS) technology development becomes a painless, noninvasive, green treatment and detection method in recent years. However, because of the difference in efficiency of the stimulation system, the technology is not widely used. The focality of the magnetic field is one of the key issues that affect the efficiency of magnetic stimulation. If the focusing problem cannot be solved, the development of TMS technology will be restricted. Therefore, research of focusing has become a hot spot in recent years. In this paper, we mainly carry out three meaningful works. First, a hybrid algorithm is proposed based on a simplified particle swarm optimization algorithm (sPSO) and simulated annealing (SA) algorithm. The convergence of those algorithms is tested. The current through the coils is optimized and solved. Second, the influence of discharge circuit parameters on the magnetic field distribution in the head model is analyzed. Finally, five array coils are established, and the related parameters are configured by using the results of above research. The simulation results show that the hybrid algorithm can improve focality performance. The hybrid algorithm is made up of sPSO and SA. The proposed optimization algorithm and the study to the parameters of the discharge circuit are useful to enhance the focality of the TMS technology in the further development.

Classification of land cover types is one important application of polarimetric synthetic aperture radar (PolSAR) remote sensing. There are numerous features that can be extracted from PolSAR images. Among them, eigenvalues λ_i, entropy H, alpha angle α, and anisotropy A are effective and popular tools for the analysis and quantitative estimation of the physical parameters. Nevertheless, the speckle noise appearing in PolSAR images reduces the accuracy of image classification. Consequently, it should be filtered correctly. Generally, filtering PolSAR data generate biased estimates of λ_i/H/A/α parameters. In this paper, we studied the effects of bias compensation on supervised and unsupervised PolSAR image classification. We applied the asymptotic quasi maximum likelihood estimator AQ-MLE and Yahia/Aguili's bias compensation methods. To improve the classification accuracies, we demonstrated that bias compensation must be associated with speckle reduction. The combination of the span with biased parameters reduced the effects of bias but did not eliminate it totally. Simulated and real data were used for validation.

A novel double-folded quarter mode substrate integrated waveguide (DFQMSIW) filter is designed in low temperature co-fired ceramic (LTCC). This filter consists of multi-layer substrate integrated waveguide. More than 93.75% of the area of the filter is effectively decreased compared with original substrate integrated waveguide (SIW) filter for the technologies of Half-Mode and Folded are applied. Meanwhile, the dimensions are further obtained reduction because of the technologies of LTCC and the vertical dimensional cavities configuration. The fabricated insertion loss and return loss are 1.9 dB and 13.5 dB, respectively.

In this paper, we focus on the 3D SC model reconstruction from data with wide azimuthal aperture at a single elevation. Since the existing method is difficult to implement for high-frequency signal or large-size target, we propose a modified RANSAC method for the extraction. In our approach, the 3D positions of the SCs are estimated from the 1D SCs via a modified RANSAC method. Then the scattering coefficients are refined via a linear least squares algorithm. The approach is robust with noise because the RANSAC method is able to tolerate a tremendous fraction of outliers. Moreover, it does not suffer from limited accuracy caused by the discretization of the parameter space in [13]. Experiments demonstrate the effectiveness of the proposed approach.

In this paper, a compact asymmetric coplanar strip (ACS)-fed mirrored L-shaped monopole antenna is presented. The proposed design consists of three mirrored L-shaped branches and a split ring resonator (SRR) loaded beneath the substrate, which are responsible for achieving multiband characteristics, compactness and good impedance matching. The proposed antenna with a compact dimension of 22 × 16.08 × 1.6 mm³ fabricated and tested. The experiment result indicates that the proposed design, having -10 dB impedance bandwidth of 200, 670 and 530 MHz for 2.44, 5.3 and 8.2 GHz, respectively, covers 2.4/5.2/5.8 GHz WLAN, 5.5 GHz WiMAX and 8.2 GHz ITU band. It has good radiation characteristics for both E-plane and H-plane in all the desired frequency bands, very compact and produces good performances compared to the existing literature. The loaded SRR structure performance is validated through various parametric studies.