Difference patterns are vital for the successful function of tracking radar employing monopulse techniques to estimate target direction. Traditional monopulse antenna pattern synthesis methods require the use of two independent distributions, e.g. Taylor and Bayliss distributions, for formation of sum and difference patterns for one antenna. Hence, these approaches require a feed network of considerable complexity. In this letter, a method for forming difference pattern in linear arrays using a very simple beamforming network and two additional elements is described. The sum pattern is determined by adding signals received by original radiating elements of the array whereas the difference pattern is determined by subtracting the output of the sum pattern from signals received from two external edge elements. The proposed method used to generate these two patterns offers significant hardware and software savings over current methods.

This paper deals with the design of small-sized bio-implanted spiral circular coils (pancake) with an operating frequency of 13.56 MHz. The external and internal coils' geometric dimensions are d_out = 56 mm, d_in = 10 mm and d_out = 11.6 mm, d_in = 5 mm, respectively, in which the electrical performance is verified through the commercial field solver High Frequency Structural Simulator (HFSS 13.0), which employs the finite-element method (FEM) technique. Mathematical models for the proposed coils are developed. The simulation is performed-based on the developmental model in the air and at depths 6\,mm in a human biological tissue of dry and wet-skin. The results demonstrate that the external and internal coils have maximum near-field gains of 54.15 dB and 53.30 dB in air. The maximum gains of the external coil contacted the wet and dry skin are 49.80 dB and 48.95 dB, respectively. The maximum gains of the internal coil at depths of 6 mm in the wet and dry tissue are 41.80 dB and 41.40 dB, respectively. However, the external coil radiation efficiencies on wet- and dry-skin are 92% and 90%, respectively, compared with that on air. The internal coil radiation efficiencies on wet- and dry-skin are 78.4% and 77.6%, respectively, compared with that on air. In this study, the specific absorption rate (SAR) and radiated power results of the internal coil are investigated using SEMCAD 16.4 software. The SAR and power loss studies show that the designed implanted coil has a negligible effect on the wet and dry skin and can be ignored.

A novel hybrid approach to the synthesis of non-uniformly spaced linear arrays of printed antennas is presented and thoroughly discussed in this paper. In order to account for parasitic mutual coupling between array elements, a dedicated optimization procedure in combination with a multiport network approach is adopted. Selected examples are included in order to assess the effectiveness and versatility of the proposed technique.

In high dynamic environment, due to the rapid relative movement between receiver and transmitter, the DOA (Direction of Arrival) of signals will change even between two consecutive snapshots. Thus, covariance-based DOA estimation algorithms are ineffective. Compressive sensing algorithms, as a kind of novel DOA estimation algorithms, are still effective with only one snapshot. At the same time, it is noted that the DOA changing is limited by relative moving speed and distance between receiver and transmitter. In this paper, a DOA tracking algorithm based on weighted L₁ minimization is proposed which utilizing the DOA changing scope between two consecutive snapshots as a prior to improve the tracking performance. Different from other multiple snapshots compressive sensing algorithms which assumed fixed DOA among multiple consecutive snapshots, the proposed algorithm takes into account the DOA changing among different snapshots. The simulation results demonstrate the advantages of the proposed algorithm.

At the ultra-high frequencies (UHF) common to portable radios, the mine tunnel acts as a dielectric waveguide, directing and absorbing energy as a radio signal propagates. Understanding radio propagation behavior in a dielectric waveguide is critical for designing reliable, optimized communication systems in an underground mine. One of the major parameters used to predict the power attenuation in lossy waveguides is the attenuation constant. In this paper, we theoretically and experimentally investigate the attenuation constants for a rectangular waveguide with dielectric walls. We provide a new derivation of the attenuation constant based on the classic Fresnel reflection coefficients. The new derivation takes advantage of ray representation of plane waves and provides more insight into understanding radio attenuation in tunnels. We also investigate the impact of different parameters on the attenuation constant, including the tunnel transverse dimensions, permittivity, conductivity, frequency, and polarization, with an aim to find their theoretical optimal values that result in the minimum power loss. Additionally, measurements of the attenuation constants of the dominant mode at different frequencies (455, 915, 2450, and 5800 MHz) for a straight concrete tunnel are presented and compared to theoretical predictions. It is shown that the analytical results match the measured results very well at all four frequencies.

For the chirp rate and its change rate estimation of cubic phase signal (CPS), conventional algorithms cannot achieve a trade-off between low computational cost and high performance. In this paper, by utilizing the numerical computational method (NCM), effects of Doppler frequency shift are quantified, and the relationships of the optimal signal length with the chirp rate and change rate of chirp rate are obtained. Then a fast parameter estimation algorithm (DMNUFFT), based on dechirp method (DM) and nonuniform fast Fourier transform (NUFFT), is proposed. Compared with existing algorithms, DMNUFFT can achieve high performance with relatively low computational cost. The performance analyses and an application to inverse synthetic aperture radar (ISAR) imaging are shown to validate the effectiveness of DMNUFFT.

In this paper, a novel technique for planar inverted-F antenna (PIFA) with broadband circular polarization and pattern diversity is proposed. A defeated ground structure (DGS) has achieved broadband circular polarized (CP) PIFA by using a square branch at the ground corner with arrow-shaped slot. The pattern diversity PIFA system consists of two CP PIFAs placed symmetrically on the diagonal of DGS. Furthermore, the DGS improves port-to-port isolation by using another smaller square branch at the opposite ground corner. Finally, a prototype is fabricated and measured. The measured results agree well with simulation, and show 10-dB matching bandwidth of 16.3% (825-986 MHz), 3-dB axial ratio (AR) bandwidth of 15.5% (830-982 MHz), and 25-dB isolation bandwidth of 12.4% (848-968 MHz), which shows suitability for radio-frequency-identification (RFID) application.

This paper proposes a method to design an even-order symmetric bandpass filter with Chebyshev response. The alternative J inverters and λ/4 short-ended resonators are used in the filter design. It is well known that a conventional even-order Chebyshev bandpass filter prototype can be designed by using J-inverters. However, to achieve the Chebyshev response, a problem is that the output admittance YL is unequal to the input admittance Y₀ since normalized g_n+1 is not equal to the g₀. But for the symmetrical structure, an additional impedance transform can be installed at the output port to solve this problem, thus the network of even-order symmetric bandpass filter with a Chebyshev response should be modified with new J-inverters. In this work, all J-inverters of the symmetric bandpass filter with Chebyshev response are extracted and described as curves to determine the circuit dimensions of the proposed structure. Two even-order Chebyshev bandpass filters with the second- and fourth-order are designed with the proposed method as its application examples. Finally, the fourth-order filter is fabricated and measured at center frequency of 2.5 GHz with the fractional bandwidth 25%. The measured result is in good agreement with the simulated one.

A compact wideband bandpass filter is proposed in this letter by means of short-circuited-stub loaded ring resonator and tapped feed lines. After the principle of an initial filter with wide operating bandwidth is described, a prototype filter with center frequency at 3.23 GHz and fractional bandwidth of 86.1% is designed and fabricated. Measured results well agree with the predicted ones, verifying the design principle.

In this work, we exploit photonic crystal heterostructures formed by the combination of periodic and Fibonacci structures to design promising optical devices acting in the visible and the near infrared domains. An hybrid structure of the type Bragg mirror-(Fibonacci)S is proposed to enhance the high reflection band through the one dimensional photonic crystal in the near infrared. The use of the configuration exhibits a large photonic band gap at any angle of incidence and for both polarizations. The proposed structure is a quarter wavelength omnidirectional mirror of 37 layers with a bandwidth larger than that of the periodic structure with an increasing ratio 3.7, and it covers all the optical telecommunication wavelengths 0.85, 1.3 and 1.55 μm. Then a second structure of the type Bragg mirror-(Fibonacci)^S-Bragg mirror with varied optical thicknesses permits to confine strongly the light giving a rise to a microcavity through the visible range with strong mode localisation. Since different physical phenomena have their own relevant physical scales, we exploit the physical properties of the proposed structures in different wavelength domains to obtain different optical devices. The transmission spectra are determined by using a theoretical model based on the Transfer Matrix Method (TMM).

In this paper, a compact tunable dual-stop-band filter is proposed. The proposed filter is based on the combination of double H-shaped defected ground structure (HDGS) and E-shaped defected microstrip structure (EDMS). The loaded HDGS/EDMS varactor diode is introduced to realize the tunable dual-stop-band filter. The equivalent-circuit models and theoretical analysis of the proposed structure are presented; also its performance evaluation is compared with traditional structure. The proposed filter has the characteristic of two independently adjustable stopbands and wide tuning range. EDMS also shows size reduction up to 38% compared with the T-shaped defected microstrip structure. The measured performance of the tunable dual-stop-band filter agrees well with the simulation results.

A compact tunable dual-band bandpass filter (BPF) based on substrate integrated waveguide (SIW) and defected ground structure (DGS) is investigated in this paper. The second passband can be flexibly controlled by changing the dimensions of the up-down DGSs whereas the first passband is fixed. The proposed filter exhibits improved selectivity due to the introduction of four left-right DGSs generating transmission zeroes. To verify the design method, a compact dual-band filter with the second center frequency switched among 5.4 GHz, 5.8 GHz, 6.4 GHz and 6.8 GHz and the first center frequency fixed at 4.78 GHz, is designed and fabricated. The simulated and measured results are in good agreement with each other.

Invisible cloak with its amazing functions has been turned into reality due to the advent of transformation optics during the past few years. However, the inhomogeneity and singularity of electromagnetic parameters in cloak are still the main bottlenecks for practical realization. In this paper, we propose a scheme of three-dimensional polyhedral invisible cloak to overcome these shortcomings by using a linear homogeneous transformation method. The constitutive parameters of the polyhedral cloak are homogeneous and anisotropic, which are relatively easy for realization. Numerical simulations demonstrate that good invisibility performance can be achieved for any polarization wave. Our work provides a novel approach to simplify three-dimensional cloak in practice.

Scattering characteristics of periodic dielectric gratings can be accurately and efficiently computed via a spectral volume integral equation combined with normal-vector fields defined on the grating geometry. We study the impact of the geometrical discretization on the convergence rate of the scattering characteristics for two-dimensional gratings in both TE and TM polarization and compare these with an independent semi-analytical reference for circular cylinders. We demonstrate that geometrically conforming normal vector fields lead to substantially faster convergence and shorter computation times, as opposed to the commonly applied staircasing or slicing.

This work presents an experimental study in W band about the behavior of a plane Fresnel reflector when the feeder changes its position on the surface of a sphere whose centre is the same of the Fresnel plate zones. For this purpose, an experimental system based on seven Fresnel plate zones and two different levels has been developed. The center frequency of the reflector is 96 GHz, the focal length is 100 mm and height between levels is 0.78 mm. Based on this Fresnel reflector, an experimental set up has been developed. The horn antenna feeder is fixed and situated in far field and the receiver is also a horn antenna located at the Fresnel focal distance. Both the reflector and the receiving antenna have some rotation capability to enable measurements from different angles. The experimental results show a good, stable behavior in gain versus the angular position of the feeder. This special property of Fresnel reflectors is impossible in parabolic reflectors and consequently, Fresnel reflectors could be used in new applications as radar imaging, increasing the radar field of view or improving the resolution by means of several squint feeders working simultaneously on the same lens or reflector. Therefore, the main objective of this paper is to analyze the behavior of this experimental set up for developing new Fresnel reflector-based applications.

In this paper, an efficient wireless power transfer (WPT) system integrating with highly sub-wavelength metamaterials is proposed. The negative refractive index (NRI) and negative permeability (MNG) metamaterials for operation at radio frequencies are designed and applied to WPT system for improvement of power transfer efficiency. A dual-layer design which consists of a planar spiral on one side and a meander line touching with narrow metallic strips on the other side produces the properties of effective negative permittivity and permeability simultaneously, i.e., negative refractive index. In addition, the structure of double spirals produces a negative permeability. The cell size of the NRI and MNG metamaterials is about 253 times smaller than the operation wavelength. By integrating one, two, three or four metamaterial slabs between the two coupling copper rings, the transfer efficiency is improved significantly. The measured results show that the contribution of high transfer efficiency is due to the property of negative permeability which can make the WPT system work in the mechanism of magnetic resonance.

An equivalent circuit based on propagating and evanescent accessible modes is discussed and the numerical values of its elements (susceptances and electrical length) are obtained starting from the knowledge of the generalized scattering matrix S of microwave discontinuities. A database of the circuit elements is then defined in the frequency and geometric ranges of the analyzed discontinuities and is used to evaluate the generalized scattering matrix for values not contained in the database, with very simple formulas for any number of circuit ports. The obtained S matrices can be used to analyze very complex structures such as iris-based filters and manifold filters.

A new structure of coupled-fed loop antenna connected with two branch radiators for eight-band LTE/WWAN (LTE700/GSM850/900/1800/1900/UMTS/LTE2300/2500) operation in the ultra-thin laptop computer is presented. The two branch strips of the antenna are efficient radiators and contributing multi-resonant modes to greatly enhance the bandwidth of the antenna. The proposed antenna on the top shielding metal wall of the laptop display, with a planar and compact size of 12.5×70×0.8 mm³, is suitable to be embedded inside the casing of the laptop computer. The proposed antenna is fabricated and tested, and good radiation performances are obtained. Compared with the existing published antennas, the volume of the planar antenna is quite small.

In multi-input multi-output (MIMO) radar transmit beampattern synthesis, most current literature formulates the problems in steradian space. However, since the beampattern and its parameters are both measured and defined in radian space, from the view point of physical meaning, it will be better to reformulate the problems in radian space rather than in steradian space. In this paper, we propose methods in the radian space to synthesize beampatterns based on minimizing sidelobe level for the two main designs in MIMO radar, i.e. minimum sidelobe beampattern design (MSBD) and beampattern matching design (BMD). For MSBD, the design criteria considering both peak sidelobe level and integrated sidelobe level is proposed. By this we can have a good tradeoff between the intensity and power distribution in beampattern synthesis. After a two-step converting, the formulation of the criteria is transformed into a convex programming, where a global optimal solution can be obtained. For BMD, instead of minimizing mean square error directly as in conventional methods, we propose a power-approximation-based method by minimizing integrated sidelobe level. Finally, numerical comparisons with classical methods demonstrate that the proposed MSBD maintains for all range of main lobe width and the proposed BMD has smoother main lobes with maximal power focused in.

Primary user (PU) signal detection is critical for cognitive radio networks as it allows a secondary user to find spectrum holes for opportunistic reuse. Eigenvalue based detection has many advantages, such as it does not require knowledge on primary user signal or noise power level. However, most of the work on eigenvalue based detection methods presented in the literature rely on multiple sensing nodes or receiving antennas so that they cannot be directly applied to single antenna systems. In this paper, an effective PU signal detection method based on eigenvalue is proposed for a cognitive user equipped with a single receiving antenna. The proposed method utilizes the temporal smoothing technique to form a virtual multi-antenna structure. The maximum and minimum eigenvalues of the covariance matrix obtained by the virtual multi-antenna structure are used to detect PU signal. Compared with the previous work, the presented method offers a number of advantages over other recently proposed algorithms. Firstly, the presented approach makes use of power method to calculate the maximum and minimum eigenvalues, it has lower computational complexity since the eigenvalue decomposition processing is avoided. Secondly, it can reduce system overhead since single antenna is used instead of multiple antennas or sensing nodes. Finally, simulation results show that performance of the proposed method is close to that of maximum-minimum eigenvalue detection using multiple antennas.