In this paper, a novel scheme for inverse synthetic aperture radar (ISAR) imaging under low signal-to-noise ratio (SNR) condition is proposed. The method is a preprocess of the high-resolution range profiles and relies on the oversampling in the azimuth direction. It divides the entire coherent processing interval into segments according to the down sampling factor. In each segment, original low SNR echoes are noncoherently integrated to obtain a new high SNR echo. With the new high SNR echoes, conventional methods for ISAR imaging can perform much better and obtain a better focused ISAR image. The presented algorithm has the advantage of effectiveness under low SNR condition and computational efficiency. Experimental results based on both the simulated and real radar data of an airplane verify the superiority of the proposed strategy.

In this paper we present the full process of designing anisotropic metamaterial (MM) wide angle impedance matching (WAIM) layers. These layers are used to reduce the scan losses that occur in active phased arrays for large scanning angles. Numerical results are provided to show the improvement in performances that such layers can ensure. The proposed anisotropic MM-WAIM layers achieve an improvement of about 1 dB of more radiated power at θ = 70˚ from broadside, in a 13% of fractional bandwidth on the azimuthal plane Φ = 90˚. Weaker improvements are obtained in the other azimuthal planes, however keeping the active reflection coefficient below -9 dB for all azimuthal planes up to θ = 60˚ and up to θ = 70˚ off-axis for planes Φ = 45˚; 90˚ in the whole operational band.

This paper presents the design, simulation, fabrication and measurement of a wideband bandpass filter with wide stopband performance operating at 3.5 GHz. The proposed filter consists of two parallel coupled lines (T-PCL) centered by T-inverted shape. The location of transmission zeros can be adjusted by varying the physical lengths of T-inverted shape to improve the filter selectivity. The wide bandwidth is achieved through enhanced coupling between the input and the parallel coupled lines. Due to the transmission zeros in the lower and upper stopbands, the filter exhibits good performance including an extremely wide stopband and sharp attenuations near the passband together with low insertion and good return losses in the passband. The filter performance is investigated numerically by using CST-MWS. Finally, the microstrip wideband BPF with minimum insertion losses 0.3 dB, centered at 3.5 GHz with a 3-dB fraction bandwidth of 70 % and four transmission zeros is implemented and verified experimentally. In addition, good agreement between the simulated and measured results is achieved.

In this letter, a design for broadband circularly slotted patch antenna fabricated on a single-layered printed circuit board (PCB) with unidirectional radiation is described. It is fed by a simple center-fed feeding scheme. Unlike conventional slotted patch antennas, the proposed antenna is loaded with four short pins such that both matching and axial-ratio (AR) bandwidths are improved. In addition to realize the CP radiation at higher frequencies, a cross slot with different arm lengths, which is cut from the center-fed patch is used. Then, the proposed antenna operates as magnetic dipoles to generate circular polarization at low frequencies after four short pins are added to connect the patch to the ground. Thus, by combining the two modes, bandwidths of 30.2% (1.83-2.48 GHz) for |S₁₁| < -10 dB and 21.2% (1.98-2.45 GHz) for axial ratio (AR) < 3 dB around the center frequency of 2.15 GHz are measured. Moreover, stable gain of over 8 dBic and good radiation patterns are also observed across the operating band.

In this paper, based on cross-dipoles, a right-handed circularly polarized (RHCP) antenna with both broad impedance and axial-ratio (AR) bandwidth is proposed. Moreover, metal vias and branches are loaded to the cross-dipoles aiming to decrease the size of the antenna. The measured results show that a broadband impedance matching bandwidth ranging from 1.83 to 3.42 GHz for S₁₁ less than -10 dB is obtained. Meanwhile, the 3-dB axial-ratio bandwidth is more than 29.0%, and the peak gain of the antenna is about 7.3 dBi with small variation at 2.5 GHz. A prototype is fabricated and measured for validating the design. The good agreements between the simulated and measured results show that the proposed antenna maybe a good candidate in circularly polarized (CP) antenna family.

In this paper the authors perform a comparison among three different stator structures for a Tubular Permanent Magnet Linear Machine. Each structure is characterized by its own lamination which is expected to contribute to the overall performance of the machine. A detailed analysis of the main figures of merit of the three configurations has been carried out in order to identify the configuration with the best characteristics. Significant data such as flux distribution, rated voltage and current, force on the moved and power losses have been compared. The results show that the choice of a mixed stator lamination allows to improve the performance of these machines.

In this paper a recent new quasi-TEM surface impedance approach has been applied to fast characterization of multiconductor microstrip lines in terms of inductance and resistance matrices and conductor losses. Application examples for frequency-dependent parameters of interconnect circuits with up to five conductors (three-, four-, and five-strips) have been reported. The propagation characteristics and attenuation of multimode symmetrical multiconductor system are obtained. The effectiveness of the applied approach is confirmed by comparison of the computed numerical results with those obtained by full-wave simulators. They are found to be in good agreement.

This paper presents a mathematical analysis of the magnetic field homogeneity for an Equilateral Triangular Helmholtz (ETH) coil. The magnetic field analysis is based on the Biot-Savart law in which a Taylor series approximation is performed to obtain the analytical distance that complies with the Helmholtz condition between the pair of coils. This is done to compare the magnetic field distributions of the ETH and the Circular Helmholtz (CH) coils for the parameters side length (2a, 3a) and radius (a) respectively. Furthermore, an approximate expression of the magnetic field homogeneity with regard to the side length parameter is obtained and finally a computational model of the ETH coil using COMSOL® is performed in order to validate the calculated and experimental results. The results show that the ETH coils have a lower magnetic field homogeneity than the CH coils for the described parameters, and the implementation of either one basically depends on the application specifications.

In this paper, a dual-band multiple-input multiple-output dielectric resonator antenna (DRA) with pattern diversity is presented. L-shape of the DRA produces patterns diversity at the lower band whereas, at the upper band, it is caused by exciting TE^x₁₂₁/TE^y₂₁₁ mode in the DRA. Two copper strips are pasted at the corner of the dielectric radiator to improve matching at both the bands. A cylindrical air-gap introduced in the radiator improves isolation up to 25 dB and 20 dB at lower and upper bands, respectively. The MIMO system possesses pattern diversity and isolation without applying any special decoupling technique. The design covers the WiMAX and WLAN bands at 3.6 and 5.2 GHz, respectively. Simulated and measured reflection coefficients and envelope correlation are in good agreement.

Many traditional sparse signal recovery based ISAR imaging methods did not utilize the block scatterers information of targets. Some block Bayesian learning based ISAR imaging algorithms are computational expensive. In this paper, a 2D block l₁l₀ norms homotopy sparse signal recovery algorithm (the BL1L0 algorithm) is proposed and utilized to form the ISAR image. Compared with Bayesian-based algorithms, this algorithm can obtain ISAR images with similar image quality, but the computation speed is faster. Real data experiments verify the merits of our algorithm.

A wide-beamwidth circularly polarized (CP) asymmetric microstrip antenna is proposed by etching four novel unequal fan-shaped notches at the vertexes of the square radiator. A bandwidth of 1.5% and beamwidth of 156° are well achieved for an axial-ratio ≤ 3 dB (3-dB AR) at the central frequency of 1.575 GHz. To widen the bandwidth, the asymmetric microstrip antenna is further expanded with the construction of a 2×2 antenna array using sequentially rotated feeding technique. Moreover, by properly optimizing the distance between each two neighboring elements and the radii of the fan-shaped notches, the 3-dB AR bandwidth of the sequential-rotation array (SRA) is approximately extended to 7.8% with a wide-beamwidth at the central frequency of 1.6 GHz. In addition, the gain variation within the bandwidth is less than 1 dB. Finally, a laboratory model of the SRA has been fabricated, and acceptable agreement of the simulated and measured results makes it a good candidate for applications where wide-bandwidth, wide-beamwidth and small gain variation are needed.

A compact printed ultra-wideband (UWB) multiple-input-multiple-output (MIMO) antenna with embedded structure is proposed. The proposed MIMO antenna consists of two coplanar waveguide-fed (CPW) antenna elements, and the element with smaller size is built in the radiator of the other element. Thus, the compact size, 30 × 22 mm², is completely determined by the larger one, which makes the MIMO antenna have similar size to the conventional single UWB antennas. The antenna elements are fed perpendicularly to achieve superior isolation. A T-shaped parasitic stub, a pair of C shaped slots, an extended Z-shaped stub and a rectangular slot are employed and each two structures filter the interference for one antenna element. The achieved rejection bands remain relatively stable over the stop-band frequencies and decline rapidly at cut-off frequencies. Thus the proposed MIMO antenna obtains superior filtering performance. The proposed antenna is fabricated and measured. The measured results indicate that two antenna elements can operate in the impedance bandwidth larger than 3.1-10.6 GHz with notched bands of 5-6 GHz, and the mutual coupling is below -15 dB over the entire UWB frequency spectrum. The proposed MIMO antenna is a competitive candidate for UWB MIMO communication systems.

The objective of this paper is to propose an improved approach based on a novel non-intrusive method for easily assessing the high frequency CW EM radiated susceptibility of an electronic system by characterizing its nonlinear electromagnetic effects. For this purpose, we have developed a specific harmonic frequency detection system coupled with a mode stirrer reverberating chamber. We describe the principles of the method and study a generic device board which is representative of a real electronic system. We evaluate the EM susceptibility of a micro controller in full functional mode and the data exchanges with two types of external 8 Mb SRAM memories. We observe the EM radiated susceptibility of this device by a functional EMC analysis method; then we measure the harmonic frequency content and make a correlation with the EM susceptibility results. We obtain significant differences between the two memory devices, as a consequence of their different managements of internal voltage over stress. We are well aware that this method is currently not validated in industrial environments EMC. In this paper, we only want to show that the appearance of the highest harmonic level occurs only when DUT has the highest functional failure.

We apply the Gabor frame as a projection method to numerically solve a 2D transverse electric-polarized domain-integral equation for a homogeneous medium. Since the Gabor frame is spatially as well as spectrally very well convergent, it is convenient to use for solving a domain integral equation. The mixed spatial and spectral nature of the Gabor frame creates a natural and fast way to Fourier transform a function. In the spectral domain we employ a coordinate scaling to smoothen the branchcut found in the Green function. We have developed algorithms to perform multiplication and convolution efficiently, scaling as O(NlogN) on the number of Gabor coefficients, yielding an overall algorithm that also scales as O(NlogN).

A miniaturized microstrip rat-race coupler with harmonics suppression is proposed by using shorted trans-directional (TRD) coupled lines. The shorted TRD coupled lines consist of a set of capacitor-loaded λ/4 coupled microstrip lines with two shorts, which are used to replace the 3λ/4 uniform transmission-line section (UTLS) in the traditional 3λ/2 ring coupler for miniaturization. To attain perfect matching for any coupling factor of the TRD coupled lines, shorted TRD coupled lines are synthesized and the design equations are derived. To further reduce the ring size, T-type transmission-line equivalent circuits are also adopted to replace the λ/4 UTLS and associated with a transmission zero for harmonic attenuation. Using the proposed method, a microstrip ring coupler with 26.7% circuit size of a traditional one is fabricated and tested. The measured results show that the bandwidth for the return loss of better than 10 dB is 43.9% and that for isolation of better than 20 dB is 18.7% with a maximum isolation of 40.6 dB. There is no spurious passband up to the sixth harmonic of the design center frequency with more than 20 dB suppression from the third to fifth harmonics.

In this article, a dual-polarized hybrid cylindrical dielectric resonator antenna (CDRA) is studied. A ring-shaped patch along with an inverted L-strip is used to excite two different hybrid modes (HE_11δ and HE_12δ-like mode) in CDRA. HE_12δ mode in CDRA is very advantageous in terms of gain and radiation characteristics. The proposed antenna design shows triple-band characteristics i.e. 1.88-2.06 GHz, 2.29-2.58 GHz and 2.9-3.93 GHz with the fractional bandwidth of 9.13%, 11.9% and 30.16%, respectively. Due to inverted L-strip, it shows circular polarization characteristics within the frequency range 3.3-3.55 GHz (AR<3 dB). The simulated results are practically verified by using archetype of proposed antenna structure. The proposed antenna design is applicable to WLAN (2.4 GHz) and WiMAX (2.5/3.5 GHz) applications.

Based on the theory of ionospheric heating, with the self-consistent model in the low ionosphere, the Extremely-Low-Frequency (ELF) and Very-Low-Frequency (VLF) waves generated by modulated beat-wave ionospheric heating are analyzed theoretically. In the consideration of the stratified ionosphere, the magnetic fields generated by the equivalent ELF/VLF dipole source above thesea surfaceare studied by using the quasi-longitudinal approximation method.Taking the high latitude regions as an example, the variations of the electron temperature, the increments of Pedersen and Hall conductivities and the changing of the oscillating current densitywith the modulation frequency in beat-wave heating are numerically discussed. The distribution of the magnetic fields ispresented. It turns out that in high latitude regions, the efficiency of rectangular wave modulated heating ingenerating ELF/VLF wave is higher than that of modulated beat-wave heating, and the order of magnitude of the magnetic fields received above the sea surface is 10^-7 in beat-wave modulation.

In this paper, a compact wideband planar balun is studied and investigated. The proposed balun comprises a broadband Wilkinson divider followed by non-coupled lines to attain wideband 180° phase shift. Due to the inherent broadband characteristics of the proposed structure, good performance is accomplished in terms of phase and amplitude balance. The balun is optimally designed and validated by experiments. Both measured and computed results have shown a return loss better than -10 dB, an insertion loss around of -3.15 dB with a maximum absolute phase and amplitude imbalance around 2.5° and 0.2 dB over frequency range from 700 to 3200 MHz. Practical and computed results of the present balun are in good agreement.

This paper presents a novel compact Koch snowflake fractal ring based Dielectric Resonator Antenna (DRA) for ultra wideband application. Firstly, Koch snowflake fractal geometry is implemented on the conventional Cylindrical Dielectric Resonator Antenna (CDRA). Further, the performance of the DRA is enhanced by fractal ring created on the snowflake geometry. With the application of the fractal and the fractal ring geometry, the Q-factor of DRA is reduced, thus the bandwidth of DRA is increased. The proposed antenna offers a wide impedance bandwidth of 90% ranging from 4.7 GHz-12.4 GHz. The effect of the fractal geometry enhances the gain of DRA. The proposed antenna achieves radiation efficiency more than 78%, throughout the bandwidth. Interestingly, the proposed configuration reduces the DRA volume by 76.63% with reduced volume of 7.91 cm³. The experimental verification of the proposed structure shows good agreement between simulated and measured results.

We present a revised Cauchy method to accurately extract the high quality factor of dielectric resonators from measurements. Since the losses displace all the zeros and poles of the transfer function horizontally to the left in the complex plane, the accurate evaluation of the unloaded quality factor of microwave resonators can be achieved based on the complex frequency transformation. The results show that if the three-point method is employed, the accuracy of the quality factor values deteriorates when the input/output coupling is strong. Nevertheless, a nearly constant factor value can be obtained by our proposed technique whether the input/output couplings are weak or strong. This algorithm provides an alternative method to measure the unloaded quality factor when the signal-to-noise ratio is high.