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A detailed analysis of ultrawide-band (UWB), dual-polarized, dielectric-loaded horn-fed bow-tie (Hill) antennas is carried out using the finite-difference time-domain (FDTD) method. The FDTD model includes realistic features of the antenna structure such as the feeding cables, wave launchers, dielectric loading, and resistive-film loading. Important antenna characteristics that are usually difficult to obtain via measurements can be obtained more directly from this FDTD model. Since the HFB antennas under consideration are intended for ground penetrating radar (GPR) applications, the effects of the half-space medium are also investigated. The simulated results serve to verify the performance of the HFB antenna design, and to optimize various antenna parameters. [ABSTRACT FROM AUTHOR]

Discrete methods of numerical analysis have been used successfully for decades for the solution of problems involving wave diffraction, etc. However, these methods, including the finite element and boundary element methods, can require a prohibitively large number of elements as the wavelength becomes progressively shorter. In this paper, a new type of interpolation for the wave field is described in which the usual conventional shape functions are modified by the inclusion of a set of plane waves propagating in multiple directions. Including such a plane wave basis in a boundary element formulation is found in this paper to be highly successful. Results are shown for a variety of scattering/radiating problems from convex and nonconvex obstacles on which are prescribed piecewise constant Robin conditions. Notable, results include a conclusion that, using this new formulation, only approximately three degrees of freedom per wavelength are required. [ABSTRACT FROM AUTHOR]

A new approximation of the Maliuzhinets function ψ Φ (α) by a single cosine function is proposed. This is the simplest approximation of the function available in the literature. The use of the cosine approximation permits representing fields scattered by nonmetallic wedges in terms of elementary functions only. The accuracy of the approximation, as applied to the case of an impedance wedge under normal illumination, is estimated both analytically and numerically. [ABSTRACT FROM AUTHOR]

The expressions for the impedance of a dipole—an element of an infinite phased array antenna placed over the planar boundary between the media (the upper being vacuum or air, and the lower a reflecting one) are derived. The characteristics of the lower medium are represented by the Fresnel coefficients for plane waves of two polarizations; the array elements are narrow flat dipoles having sinusoidal current distributions along their lengths. [ABSTRACT FROM AUTHOR]

An asymptotic solution based on the uniform geometrical theory of diffraction (UTD) is proposed for the canonical problem of surface field excitation on a circular cylinder with an impedance boundary condition (IBC). The radius of the cylinder and the length of the geodesic path between source and field points, both of which are located on the surface of the cylinder, are assumed to be large compared to a wavelength. Unlike the UTD based solution pertaining to a perfect electrically conducting (PEC) circular cylinder, some higher order terms and derivatives of Fock type integrals are found to be significantly important and included in the proposed solution. The solution is of practical interest in the prediction of electromagnetic compatibility (EMC) and electromagnetic interference (EM!) between conformal slot antennas on a PEC cylindrical structure with a thin material coating on which boundary conditions can be approximated by an IBC. The cylindrical structure could locally model a portion of the fuselage of an aircraft or a spacecraft, or a missile. Validity and accuracy of the numerical results obtained by this solution are demonstrated in comparison with those of an exact eigenfunction solution. [ABSTRACT FROM AUTHOR]

The impedance change that occurs when the user holds a mobile phone antenna is a well-known problem. This paper analyses the contributions to this change for a dual-band PIFA. In particular, the contribution of the slot is shown and a method of reducing it is proposed and analyzed. [ABSTRACT FROM AUTHOR]

Transmission line equations in air in the presence of corona are derived. The analysis shows that the corona caused by a voltage or a current pulse propagating along a transmission line can be represented by a series of corona current sources distributed along the line. Corona has two effects on the voltage or current pulses propagating along a transmission line. First, it will clamp down the pulse amplitude at the front of the pulse to the corona threshold. Second, it will cause the portion of the pulse whose amplitude is larger than the corona threshold to travel with a speed less than the speed of light. The effects of corona on the voltage or current pulses propagating along a transmission line can also be evaluated by introducing a time varying capacitance and a conductance into the transmission line. If the time varying capacitance is assumed to be proportional to the ratio between the corona charge and the applied voltage then one requires both this and the time varying conductance to represent the corona effects more accurately. Analysis of the return stroke as a current pulse propagating along a transmission line undergoing corona shows that the corona effects may explain the reason why the measured return stroke speeds are considerably less than the speed of light. Moreover, based on the effects of corona, a physical justification for the concepts used in the current generation type return stroke models is provided. [ABSTRACT FROM PUBLISHER]

Electromagnetic creeping waves on a 3-dimensional surface with an anisotropic impedance boundary condition are considered. The influence of the impedance is examined. The standard asymptotic formula for the creeping waves contains the factor 1/(tau-q2) where tau is the attenuation parameter and q is the Fock parameter which depends on the impedance matrix. Analysis of the equation for the attenuation parameter which describes its dependence on q shows that there exist such critical values of q when the factor 1/(tau-q2) diverges and the usual asymptotic formula gives infinite result. The equation for the critical values of the parameter q is derived and the 4 first critical values are found numerically. The new local asymptotics valid in domain of the size k-2/9 (where k is the wave number) is derived in the supposition that the divergence takes place on a curve crossed by creeping waves. This new asymptotic decomposition is carried out by powers of the small parameter k-1/9. The effect of creeping wave passing through the line where the usual asymptotics diverges is examined. [ABSTRACT FROM PUBLISHER]

Wavelength-scaled array architectures use scaled elements to achieve ultrawideband performance with significantly fewer overall radiators than traditional ultrawideband arrays based on a single element type. Compared to a conventional ultrawideband array with 8:1 bandwidth, a wavelength-scaled array that uses elements of three different sizes creates an aperture with fewer than 16% of the original element count, i.e., 6.4-times fewer elements, and by extension a comparable reduction in electronics required to feed the array. In this paper, a study of an asymmetric wavelength-scaled array architecture is presented for finite arrays of offset-centered dual-polarized flared-notch radiators. The unique element transitions within the finite array structure are modeled via a non-matching grid Domain Decomposition- Finite Element Method that allows for rigorous impedance and radiation pattern prediction of full-sized wavelength-scaled arrays. This design study shows that the wavelength-scaled array has comparable performance to traditional ultrawideband arrays in terms of VSWR, radiation patterns, array mismatch efficiency, and cross-polarization levels. [ABSTRACT FROM AUTHOR]

A novel wide-band printed antenna, named forked shape monopole antenna (FSMA), is proposed. The forked shape line structure is introduced to create a capacitive coupling effect to reduce the antenna size and enhance the impedance bandwidth. The proposed antenna is to be applied in a compact USB dongle. And it is designed for the reception of the digital video broadcasting (DVB) signal in the 470 \sim 860 MHz frequency band. In addition to the DVB application, the upper band limit of the FSMA even can be extended from 860 MHz to 1142 MHz, suitable for use in the Global System for Mobil Communication (GSM850/900). By employing the branched-line and forked shape configurations, an antenna with 86% impedance bandwidth (SWR<2.5) is provided. A prototype of the FSMA was constructed and demonstrated. Measured and simulated results for the FSMA alone and with real application on a laptop computer are obtained. [ABSTRACT FROM PUBLISHER]

An open stub feed planar patch antenna is proposed for UHF RFID tag mountable on metallic objects. Compared to conventional short stub feed patch antenna used for UHF RFID tag, the open stub feed patch antenna has planar structure which can decrease the manufacturing cost of the tags. Moreover, the open stub feed makes the impedance of the patch antenna be tuned in a large scale for conjugate impedance matching. Modeling and simulation results are presented which are in good agreement with the measurement data. Finally, differences between the open stub feed patch antenna and the short stub feed patch antenna for UHF RFID tag are discussed. [ABSTRACT FROM AUTHOR]

The design and analysis of a coplanar capacitive fed microstrip antenna suspended above the ground plane is presented. It is demonstrated that the proposed approach can be used for designing antennas with impedance bandwidth of about 50% and a good gain to operate in various microwave bands. The model of the antenna incorporates the capacitive feed strip which is fed by a coaxial probe using equivalent circuit approach, and matches simulation and experimental results. The capacitive feed strip used here is basically a rectangular microstrip capacitor formed from a truncated microstrip transmission line and all its open ends are represented by terminal or edge capacitances. The error analysis was carried out for validity of the model for different design parameters. The antenna configuration can be used where unidirectional radiation patterns are required over a wide bandwidth. [ABSTRACT FROM PUBLISHER]

The theory of monochromatic time-reversal mirrors (TRM) or equivalently phase conjugate mirrors is developed for electromagnetic waves. We start from the fundamental time-symmetry of the Maxwell's equations. From this symmetry, a differential expression similar to the Lorentz reciprocity theorem is deduced. The radiating conditions on TRM are expressed in terms of 6-dimension Green's functions. To predict the time reversal focusing on antenna arrays, a formalism that involves impedance matrix is developed. We show that antenna coupling can dramatically modify the focal spot. Especially, we observe, that in some circumstances, sub-wavelength focusing on a bi-dimensional array may arise. [ABSTRACT FROM PUBLISHER]

A wideband circularly-polarized printed antenna is proposed, which employs an asymmetrical dipole and a slit in the ground plane which are fed by an L-shaped microstrip feedline using a via. The proposed antenna geometry is arranged so that the orthogonal surface currents, which are generated in the dipole, feedline and ground plane, have the appropriate phase to provide circular polarization. A parametric study of the key parameters is made and the mechanism for circular polarization is described. The measured results show that the impedance bandwidth is approximately 1.34 GHz (2.45 GHz to 3.79 GHz) and the 3 dB axial ratio bandwidth is approximately 770 MHz (2.88 GHz to 3.65 GHz) which represent fractional bandwidths of approximately 41% and 23%, respectively, with respect to a centre frequency of 3.3 GHz. [ABSTRACT FROM PUBLISHER]

A composite cavity-backed antenna excited by crossed triangular bowtie dipoles is proposed and investigated for circular polarization (CP) applications. It is fed by a novel balun, i.e., a transition from a microstrip line to double slot lines, providing symmetrical electric field distributions at the feeding port. Measurements of an optimized antenna prototype show that it can achieve an impedance bandwidth of over 57.6% for SWR \leq 2, a 3-dB axial-ratio bandwidth of 39%, a broadside gain of 8-10.7~dBi, and symmetrical radiation patterns over the whole operating band. The operating principles of the proposed antenna are analyzed carefully and found quite different from crossed thin-wire dipoles with very weak coupling. Problems in the feeding balun, greatly deteriorating the CP performance at the resonance, are clearly addressed and solved. Detailed parametric studies are given in the final part of this paper. [ABSTRACT FROM PUBLISHER]

A compact ultrawideband antenna with hemispherical helical geometry is proposed and investigated both theoretically and experimentally. The radiating element of this antenna consists of a tapered metallic strip that assumes a non-conformal orientation relative to the hemispherical surface in order to yield maximum bandwidth. The number of turns is about four and half with a constant spacing between them. The antenna is fed by a coaxial cable with the inner conductor connected to the radiating strip through a matching section, and its outer conductor connected to a ground plane. Radiation properties of the proposed hemispherical helical antenna, including far-field patterns, axial ratio, directivity, input impedance and voltage standing wave ratio (VSWR), are studied numerically and evaluated experimentally. Simulation and measured results are in good agreement. This design provides a maximum directivity of 9\pm1 dB, a VSWR\leq 2 (relative to a 50 \Omega reference impedance), and nearly equal E- and H-plane far-field patterns with high degree of axial symmetry over a bandwidth of more than 50%. Also, over a bandwidth of about 24% the axial ratio remains below 3 dB. The compact size and ultrawideband performance of this antenna make it advantageous for high speed wireless communication systems and avionics. [ABSTRACT FROM PUBLISHER]

We have developed a method for controlling electromagnetic surface wave propagation and radiation from complex metallic shapes. The object is covered with an artificial impedance surface that is implemented as an array of sub-wavelength metallic patches on a grounded dielectric substrate. We pattern the effective impedance over the surface by varying the size of the metallic patches. Using a holographic technique, we design the surface to scatter a known input wave into a desired output wave. Furthermore, by varying the shape of the patches we can create anisotropic surfaces with tensor impedance properties that provide control over polarization. As an example, we demonstrate a tensor impedance surface that produces circularly polarized radiation from a linearly polarized source. [ABSTRACT FROM PUBLISHER]

Impedance matching for coupled antenna arrays has received considerable attention in the research community. Because optimal matching requires implementation of a coupled network, leading to high complexity and often narrow operation bandwidth, new research has focused on the development of uncoupled matching networks with good performance. This paper explores traditional uncoupled impedance matching techniques for coupled arrays, specifically matching to the array active and passive impedances, within the context of multiple-input multiple-output communication. The concept of matching to the array active impedance is extended to the case where the propagating field is specified stochastically, and the performance of this solution is compared to that of traditional solutions using simulations. While emphasis is placed on matching for maximum power transfer, the paper concludes with a discussion on matching for minimum amplifier noise figure. [ABSTRACT FROM PUBLISHER]

Biogeography based optimization (BBO) is a new inclusive vigor based on the science of biogeography. Biogeography is the schoolwork of geographical allotment of biological organisms. BBO employs migration operator to share information between the problem solutions. The problem solutions are identified as habitat and sharing of features is called migration. In this communication, BBO algorithm is matured to optimize the element length and spacing for Yagi-Uda antenna. The gain of Yagi-Uda is a multimodal function and is hard to optimize because of its reliance on changes in lengths and spacings. To confirm the capabilities of BBO, Yagi-Uda antenna is optimized for three different design objectives which include gain, input impedance and side lobe level (SLL). During optimization, NEC2—a method of moment's code evaluates the performance of each design generated by BBO algorithm. The results obtained by BBO are compared with the genetic algorithm (GA), evolutionary programming (EP), comprehensive learning particle swarm optimization (CLPSO), simulated annealing (SA) and computational intelligence (CI). [ABSTRACT FROM PUBLISHER]

A physical limitation on the power transfer efficiency between two electrically small antennas in the near-field range is presented. By using a Z-parameter which describes the interaction between two antennas for TE10/TM10 spherical modes in connection with antenna parameters, the maximum power transfer efficiency and the optimum load impedance are shown as functions of the distance between two antennas, the radiation efficiency and the input impedance of the isolated antenna. The theory is verified by a simulation with a small helical antenna, which generates TE10 and TM10 modes, simultaneously. [ABSTRACT FROM PUBLISHER]

A novel compact wideband antenna, which adopts a multi-state switchable matching circuit, is proposed in this paper. The design of the switchable matching is systematically studied and a two-step iterative procedure is used to obtain optimized values for both the bandwidth division between the various stages and the matching circuit components in each stage. Without loss of generality, a meander line monopole, with dimensions of 80\,\times\,10\,\times\,10 mm^3, is used as the radiator in the proposed wideband antenna design. To validate the proposed new design, a four-state matching circuit controlled by four PIN diodes is fabricated and measured. The reflection coefficient of the prototype antenna is found to be better than -7.3 ~dB, and the peak gain is higher than -3 ~dBi across the 470–770 MHz bandwidth for the Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) application. [ABSTRACT FROM PUBLISHER]

A simple printed-loop antenna with wideband characteristics is presented for laptop computer applications. The proposed rectangular loop pattern generates four resonant modes below 4 GHz. The excited modes, two unbalance and two balance, are analyzed and discussed. A novel U-shaped tuning element printed on the back side of the circuit board adjusts the resonant modes to cover GSM850/GSM900/DCS/PCS/UMTS/WLAN and WiMAX bands. Using HFSS software, this study conducted a simulation to optimize the antenna design and fabricate a practical structure to investigate its performance and characteristics. This study also measures various antenna parameters to validate the proposed antenna. [ABSTRACT FROM PUBLISHER]

A quasi-planar conical antenna with broad bandwidth (more than 10:1) and omnidirectional radiation pattern has been designed. In order to achieve mechanical stability, conical radiator is realized through thin metallic coating over a cavity etched into a dielectric slab. Due to dielectric loading, the conical antenna in this paper is quasi-planar, low-cost, light, mechanically robust, and easy to fabricate, integrate, and re-configure. A simulation tool based on the Method of Moments is developed to analyze the quasi-planar antenna. With simulation results as the guideline, the dielectric loading's material and geometries are optimized to attain wideband input impedance and omnidirectional radiation pattern. Three antenna prototypes are fabricated by using high density polyurethane foam as the dielectric loading material. Simulation and measurement data show excellent agreements. Input impedance bandwidths of all the three prototypes are greater than 10:1; and all of them show omnidirectional patterns in the azimuth plane. These quasi-planar conical antennas can be readily applied to ultrawideband communication and radar applications. In this paper, an ultrawideband radar sensor network testbed is constructed using one of the three prototype designs. Target localization is successfully demonstrated by five radar nodes with the aid of grid based location estimation algorithm. [ABSTRACT FROM PUBLISHER]

A backfire-to-broadside beam-scanning antenna with a periodic structure is described. The proposed antenna consists of the offset double-sided parallel-strip lines (DSPSL). The characteristics of the offset DSPSL have been studied, and then a series of offset DSPSL have been applied as the periodic antenna with a backfire-to-broadside symmetrical beam scanning capability. Both in y\-z plane and x\-z plane, main beams symmetrically steer from the backfire direction to the broadside direction with the increase of the operating frequency. The fractional impedance bandwidth has been broadened to nearly 50% by using the offset DSPSL structure. [ABSTRACT FROM PUBLISHER]

The design and performance is discussed of equiangular four-arm modulated arm width (MAW) spiral antennas. Effects of expansion factor, modulation ratio, termination and feedpoint region on cross-polarization, pattern symmetry, gain, and impedance for both forward and reverse mode 1 operation are studied. The bandstop region is characterized using the transmission line theory and full-wave modeling. Three composite ideal beamforming measurements are developed and are shown to be in excellent agreement with theoretical predictions. Obtained results indicate that the expansion factor should be less than 1.5 and the modulation ratio needs to be higher than 4 to achieve high purity radiation patterns in both modes of operation. Beamformer design, effects of the dielectric loading, and non-ideal cavity-backing are not discussed. [ABSTRACT FROM PUBLISHER]

A numerical method using synthetic basis functions (SBF) is introduced to analyze the scattering problem of the large-scale non-periodic arrays. With the employment of the auxiliary sources, the scattering solution space of a specific scatterer can be determined, and its scattering characteristics will be effectively extracted from the solution space in the form of SBF, which is a high order basis function. The scattering field of an array can be calculated by using the scattering characteristics of every array unit. The number of the unknowns of the SBF linear system is reduced significantly compared with the conventional method of moments (MoM), leading to higher efficiency and lower cost of computation. The SBF approach is universal regardless of the geometrical differences. There are also specific mechanisms to control the precision and efficiency. The SBF method costs less computational time than the characteristic basis function (CBF) method. Moreover, by utilizing the SBF approach together with the generalized transition matrix (GTM) and the generalized surface integral equations (GSIE), the scattering analysis of dielectric bodies and chiral bodies and their arrays will be much more efficient with decreased unknowns and scale-reduced linear system. To deal with large-scale and complex scattering problems, the SBF method will be of great help to improve the performance. [ABSTRACT FROM PUBLISHER]

An impedance surface is presented that reduces the dispersion experienced upon propagation of broadband pulses within rectangular waveguides. The surface impedance is selected so that, within a frequency range, the transverse resonance condition is satisfied for longitudinal wavenumber that varies linearly with frequency. A synthesis procedure for practical surface topologies consisting of periodic dipole arrays is described. An example involving a finite structure is employed to illustrate the reduced dispersion. Numerical simulation results obtained from in-house mode-matching method as well as HFSS are presented. A prototype is fabricated and tested experimentally validating the theoretical predictions. [ABSTRACT FROM PUBLISHER]

Design procedures of Jaumann and circuit analog absorbers are mostly formulated for normal angle of incidence. Only a few design methods considering oblique angle of incidence are published. The published methods are restricted to single resistive layer circuit analog absorbers or multilayered Jaumann absorbers with low permittivity spacers. General design procedures are developed in this paper for multilayered Jaumann and capacitive circuit absorbers. By expanding the scan and frequency compensation techniques to multilayered structures, Jaumann absorbers with outstanding performances are designed. A capacitive circuit absorber is presented with a stable frequency response up to 45^\circ for both polarizations, having an ultrawide bandwidth of 26 GHz. [ABSTRACT FROM PUBLISHER]

A small size, low profile spiral antenna (2^\prime\prime thick and 14^\prime\prime in aperture) is described that is backed by a thin ferrite ground plane. The antenna is intended to cover Satellite (UHF TacSat earth to LEO: 243–318 MHz) and enhance Radio (PRC-148, PRC-117F, PSC-5D: 243–512 MHz) communications. We carry out a step-by-step design to achieve the challenging goals of RHCP gain >0 dBi at \theta=0^\circ; Bandwidth 220–500 MHz; VSWR <2:1; Low LHCP (cross-pol) gain (<-10 dBi); Low profile (2^\prime\prime) and desired aperture size 14^\prime\prime. Ground plane ferrite treatment, inductive loading, resistive termination and impedance matching techniques are adopted to raise the gain between 200–300 MHz and achieve the stated goals. As the aperture size is about \lambda/3.5 at 220 MHz and only \lambda/30 in height, achieving 0 dBi (from 220–500 MHZ) is quite challenging. Measured and simulated data are presented in the last section leading to impressive conclusions about the capability of the magnetic material coating to overcome the perennial issue associated with the ground plane's negative effects. [ABSTRACT FROM PUBLISHER]

A passive RFID tag antenna in a recessed cavity in metallic objects such as vehicles, air craft, and metallic containers, and an impedance matching technique for this configuration are proposed. The proposed tag antenna can be considered as a cavity-backed antenna and it is comprised of a bowtie antenna installed in a recessed cubic volume in a large metallic object, and the input impedance of the bowtie antenna can be easily matched by adjusting the coupling effect between the tag antenna and the cavity. Main parameters affecting the coupling are the dimensions of the cavity and the location of the bowtie antenna inside the cavity. For more practical analysis of the impedance matching process, an equivalent-circuit model consisting of several lumped elements is introduced and each element is optimized using a genetic algorithm (GA). Experimental results show that the maximum reading distance of the proposed tag antenna with optimized dimensions can be improved about 3.2 times compared to a commercial RFID tag. [ABSTRACT FROM AUTHOR]

An investigation of the performance of printed dipole antenna with finite periodically corrugated-reflectors for low-profile ultrawideband (UWB) applications is presented. According to the relative orientations of the linear-polarized source antenna and the corrugated-reflectors, the problem could be divided into H and E cases. Their fundamental differences and design guidelines are discussed in detail. A novel composite corrugated-reflector based on the combinations of H-type and E-type ones is proposed for the first time, and fully characterized with a UWB source dipole which was optimized in the free space for better comparisons. An example antenna with only 21 mm core profile, composed of a printed UWB dipole, a microstrip balun and a composite corrugated-reflector, has an impedance bandwidth (VSWR<2.0) of 2.75 GHz to 8.35 GHz. Both numerical and experimental results show that the example antenna also has stable uni-directional radiation patterns and a realized gain of over 6 dB within the frequency band from 2.3 to 6.0 GHz, which could be a decent alternative to the EBG antennas due to significant improvements on pattern and gain bandwidth. [ABSTRACT FROM AUTHOR]

A planar inverted F antenna (PIFA) is integrated with a software-defined match control (SDMC) circuit in order to create an impedance match for a diverse set of frequency standards. The individual PIFA achieves better than -10 dB reflection coefficient for a 13.6% bandwidth at 2.45 GHz. The SDMC is a microstrip circuit with tuning diodes that are adjusted to tune the input impedance of the antenna and to select a targeted frequency range of operation. It is both an antenna impedance tuner and tunable filter. This paper discusses the design approach for the system and presents simulated and measured results. The results show that the combined antenna and SDMC can be tuned to operate at many worldwide frequency ranges even when the individual PIFA is not specifically matched to the frequencies. [ABSTRACT FROM AUTHOR]

The surface integral equation method is applied to solve electromagnetic scattering by arbitrarily shaped three- dimensional imperfectly conducting objects modeled with the Leontovitch impedance boundary condition (IBC). Dual basis functions are used to remove previously reported difficulties associated with the solution of the combined field integral equation for objects with the IBC. By expanding the magnetic current with quasi-curl conforming dual functions and the electric current with divergence conforming primary (RWG) functions leads to a stable discretization procedure for the rotation (\mbi n \times ) operator required by the IBC. [ABSTRACT FROM AUTHOR]

The reflection coefficient phase is investigated for several different artificial magnetic conductors (AMCs) having canonical FSS-type shapes. Three of them are selected, each representing a different class, and fine tuned to exhibit identical resonant frequency. Polarization and angular dependence as well as the effects of losses on these structures are studied. Next, a low-profile inverted L-shape monopole antenna (ILSMA) is placed horizontally above the ground plane. Vertical monopole antenna (VMA) is also placed above them. It is shown that using some of the aforementioned AMCs, the input impedance of both ILSMA and VMA can not only be matched, but also the input impedance bandwidth enhancement as wide as 27% and 35% are obtained, respectively. The VMA study on AMC ground planes which reveals a counter-intuitive phenomenon has not been explored in the literature, previously. It is revealed that the broadband characteristics can also be achieved for smaller size of the AMC ground planes, which enables the antenna to be designed in compact size. It is also illustrated that reflection characteristics of the AMC is not sufficient to evaluate AMC performance when it is used as an antenna ground plane. This is illustrated through extensive simulation and measurement results. [ABSTRACT FROM AUTHOR]

Miniaturization of slow wave antennas exploiting the slow wave enhancement factor is presented. The printed antennas are periodically loaded with shunt capacitors to slow down the guided wave in the structures. In this paper, the loaded unit cell of the equivalent transmission line model is utilized to extract the slow wave enhancement factor, the ratio of the loaded to the unloaded propagation constants of the wave in the antennas. From this model, the slow wave enhancement factor of a loaded antenna agrees very well with the miniaturization factor, and therefore load parameters in the circuit model can be readily obtained when a specific size reduction is attempted. This claim was substantiated by demonstrating two small radiators, a high-frequency (HF) slot-loop antenna and a planar inverted F antenna (PIFA), to achieve the desired size reductions. Experimental results show that both of the antennas demonstrate greater than ten-times size reduction from their unloaded counterparts at the expense of the degraded gains and impedance bandwidths. Specifically, the loaded slot loop presents the predicted gain and measured bandwidth on the order of -34.9 ~dBi and 0.38% for VSWR \le 2, respectively. Therefore, a matching network derived from filter design techniques is proposed to increase the antenna bandwidth so that a measured fractional bandwidth of 1.78% is achieved. The slot loop combined with the impedance matching circuit occupies a footprint size of 0.031\lambda 0 \times 0.017 \lambda0 at the operating frequency. On the other hand, the measured radiation gain and bandwidth of the loaded PIFA are reduced to -22.6 ~dBi and 0.15% for VSWR \le 2, respectively, with a footprint of 0.013 \lambda 0 \times 0.018 \lambda0 at the operating frequency. [ABSTRACT FROM AUTHOR]

We propose a topology optimization strategy for the systematic design of a three-dimensional (3D), conductor-based sub-wavelength antenna. The post-processed finite-element (FE) models of the optimized structure are shown to be self-resonant, efficient and exhibit distorted omnidirectional, elliptically polarized far-field radiation patterns. The computed approximate Q value for this antenna is QZ(\omega0) \approx 7.74 for \omega0=2\pi\times 350.8~MHz and it is 1.64 times larger than the theoretical lower bound value. [ABSTRACT FROM AUTHOR]

Today's advanced reflectometry methods provide an efficient solution for the diagnosis of electric transmission line hard faults (open and short circuits), but they are much less efficient for soft faults, in particular, for faults resulting in spatially smooth variations of characteristic impedance. This paper attempts to fill an important gap for the application of the inverse scattering transform to reflectometry-based soft fault diagnosis: it clarifies the relationship between the reflection coefficient measured with reflectometry instruments and the mathematical object of the same name defined in the inverse scattering theory, by reconciling finite length transmission lines with the inverse scattering transform defined on the infinite interval. The feasibility of this approach is then demonstrated by numerical simulation of lossless transmission lines affected by soft faults, and by the solution of the inverse scattering problem effectively retrieving smoothly varying characteristic impedance profiles from reflection coefficients. [ABSTRACT FROM AUTHOR]

A rigorous treatment on the computation of the various integrals that arise in the method of moments (MoM) formulation of the straight-wire electric field integral equation is provided. For triangle basis functions along with delta function, pulse or triangle weights, particular attention is given to integrals whose integrands are weakly singular. A singularity extraction technique is employed that splits the integral under question into two parts: one that is numerically integrable and one that is analytically integrable. Closed-form approximations based on Taylor series techniques are also provided for the former. These approximations are very robust resulting in errors less than 0.1% when 2a/\delta<1; here \delta is the length of a single MoM segment and a is the wire radius. Results are compared with data from the literature to demonstrate the robustness of the presented approach for fat wires. [ABSTRACT FROM AUTHOR]

For horizontally and vertically polarized line sources in HF-VHF band, a detailed analysis of the propagation over one-dimensional highly-conducting smooth and rough sea surfaces is addressed from an efficient rigorous numerical method: the method of moments combined with the BMIA-CAG approach and with the impedance boundary condition (Leontovitch approximation). This method can treat a huge problem, typically, ranging from 200 000 to 300 000 for the number of unknowns on the surface, which allows us to show, for the TM polarization, the ground wave propagation over a long distance. The contribution of the surface wave is then exhibited for a smooth sea surface and compared with the Collin asymptotic formulation deduced from the Sommerfeld integral. The surface roughness effect on the propagation is also investigated. [ABSTRACT FROM AUTHOR]

The wideband circularly polarized (CP) dielectric resonator antenna (DRA) is investigated with an underlaid quadrature coupler. The idea is used to realize a CP hollow rectangular DRA. Since the coupler is located beneath the DRA, it does not increase the footprint of the antenna, making the system very compact. The underlaid coupler is placed entirely inside the hollow region of the DRA and, thus, it can be designed easily as if there is no overlaid DRA. Two configurations are considered in this paper. In the first configuration, an external 50-\Omega load is used for the matching port of the coupler. For the second one, a strip loaded by the DRA is used to provide a load and, thus, no lumped elements are required in this configuration. In this paper, a network model is also given to aid engineers in designing the proposed integrated DRA. The reflection coefficient, axial ratio, antenna gain, and radiation pattern for each configuration are studied. It was found that wide impedance and axial-ratio bandwidths can be obtained with the proposed CP DRAs. Measurements were carried out to verify the simulations, and reasonable agreement between them was obtained. [ABSTRACT FROM AUTHOR]

The shape representation of planar wire dipole antennas of fixed radius is discussed. Away from the feed point the shape can be always represented as a arc-length parametrizable Lipschitz-continuously differentiable curve. The representation is applied to classical directivity optimization design problems as well as impedance optimization problems. By such an optimization, it is shown that directive wire dipoles of length (3/2)\lambda can be tuned by shape optimization and that the real part of input impedance of \lambda /2 dipole is bounded from above if the imaginary part must be small. [ABSTRACT FROM AUTHOR]

This paper introduces a new analytical method suitable for the design of the single-feed circular polarization (CP) microstrip patch antenna. Specifically, the proposed method imposes simultaneously the circular polarization condition as well as an arbitrary input impedance matching condition. The two conditions are enforced by an analytical method derived from an equivalent circuit model of a quasi-symmetrical patch antenna, and manipulated to control the modal detuning. This method can be used as an aid to speed up the design procedure for CP antennas even working with a numeric CAD tool. The validation of this approach is proven designing and fabricating a prototype implementing an original design, which consists of a circular disc slotted by a concentric elliptical cut with coaxial feed and operating at the center frequency of 2.45 GHz. The fabricated prototype exhibited a return loss of about 19 dB, within an impedance bandwidth of 130 MHz, a measured gain of 3.85 dB and a 0 dB axial ratio at 2.45 GHz with a corresponding modal phase difference of 87 degrees. [ABSTRACT FROM AUTHOR]

In array antennas with residual coupling, the signal-to-noise ratio (SNR) can suffer from noise coupling from the inputs of the low noise amplifiers (LNAs) to the other channels. As shown recently, the lost SNR can be retrieved by changing the individual matching circuits. This paper explains the theory behind array noise matching and generalizes it for non-equal LNAs and non-reciprocal sources. It also shows an analogy between noise matching of an array and power matching into the complex conjugate of the optimum noise impedances of the individual LNAs. This turns out to be useful for practical noise matching of mutually coupled arrays. In some cases it becomes impossible to reach the theoretical optimum matching with passive matching networks. Therefore an additional boundary condition will be introduced and investigated. [ABSTRACT FROM AUTHOR]

The multiscale compressed block decomposition algorithm (MS-CBD) is presented for highly accelerated direct (non iterative) solution of electromagnetic scattering and radiation problems with the method of moments (MoM). The algorithm is demonstrated to exhibit N^2 computational complexity and storage requirements scaling with N^1.5, for electrically large objects. Several numerical examples illustrate the efficiency of the method, in particular for problems with multiple excitation vectors. The largest problem presented in this paper is the monostatic RCS of the NASA almond at 50 GHz, for one thousand incidence angles, discretized using 442,089 RWG basis functions. Being entirely algebraic, MS-CBD is independent of the Greens function of the problem. [ABSTRACT FROM PUBLISHER]

An extension of the discrete mode matching (DMM) method is used to analyze microstrip antennas conformed to cylinders. The structure needs to be discretized only along the azimuthal and the axial directions by sampling the fields and current densities in the interfaces between dielectric layers. The solution in the radial direction is obtained analytically with the dyadic Green's function, which is calculated using the full-wave equivalent circuit (FWEC) in the spectral domain. The structure is truncated in the axial direction by employing absorbing boundary conditions (ABCs). The mathematical formulation is given in this paper, followed by its application to compute the main properties of conformal microstrip antennas, such as resonant frequency, radiation patterns and input impedance. Validation with the commercial software HFSS has been done and good agreement between the results has been observed. [ABSTRACT FROM AUTHOR]

A planar dual-band antenna array for point-to-point communication is presented. The proposed compact antenna array operates at 2.4–2.5 GHz (4%) and 5.1–5.5 GHz (7.5%) with \vert S11\vert<-10\ dB, and corresponding gains of 11–13.3 dBi and 10.6–14.5 dBi respectively. It consists of four branches suspended at a height of 5 mm above a 250 \times 250 mm ground plane. Each quarter consists of a series-fed meandered step-impedance filter connected to a L-shaped radiator, and they transverse horizontally away from the epicenter. The insertion loss method with non-redundant synthesis for filter design is applied to control the radiation aperture of the antenna array, allowing both the 2.4 GHz and 5.5 GHz radiators to co-exist and operate independently. A stable broadside radiation pattern and gain are achieved. [ABSTRACT FROM AUTHOR]

The close displacement of UHF RFID tags can be considered as an electromagnetic interconnected system having specific properties. The so denoted RFID Grid includes single-chip tags in close mutual proximity or a single tag with a multiplicity of embedded microchips. A multi-port scattering framework is used to derive the macroscopic parameters governing the system response which could be optimized for the specific application. Moreover, unique features are introduced, such as the possibility to improve the power scavenging and the generation of analog identifiers and fingerprint. The last ones are electromagnetic responses independent on the position and orientation of the reader and on the nearby environment, with great relevance for Sensing and Security. [ABSTRACT FROM AUTHOR]

This paper studies the electrical separation and fundamental resonance of differentially-driven microstrip antennas with dual-probe feeds on both electrically thick substrate of high permittivity and electrically thin substrate of low permittivity. The electrical separation is defined as the ratio of the distance \xi of the dual-probe feeds to the free-space wavelength \lambdao. It is found that the occurrence of resonance of the fundamental mode is related with the electrical separation of the dual-probe feeds. When the electrical separation \xi/\lambdao\geq\psi is satisfied, the resonance occurs. Otherwise, the resonance does not occur. It is shown that the empirical factor \psi is smaller for the electrically thicker substrate of higher permittivity than that for electrically thinner substrate of low permittivity and is smaller for the circular patch than that for the rectangular patch. To validate the relationship of the occurrence of fundamental resonance with the electrical separation, several differentially-driven microstrip antennas were fabricated on the electrically thin substrate of the low permittivity and measured. It is observed that the simulated and measured results are in acceptable agreement for these differentially-driven microstrip antennas. Thus, the electrical separation condition derived in this paper should be very useful in guiding the design of differentially-driven microstrip antennas. [ABSTRACT FROM AUTHOR]

We describe the physical phenomena that contribute to the behavior of an electrically small TM10 antenna using characteristic mode theory. The application of characteristic modes to antenna tuning and bandwidth enhancement serves as demonstration of the broad utility of the modal technique. A modal analysis of the TM10 antenna's impedance match yields several interesting observations as to the nature of resonances and antiresonances, which has implications for the impedance matching of small antennas in general. Furthermore, to overcome the bandwidth limitations inherent in small antennas, we determine that multiple resonances must be combined and use a conductance ratio as a figure of merit for design. We then investigate the TM10 antenna's potential for multiresonant operation by examining different candidate modes. Using the appropriate characteristic modes to form multiple resonances, we show how the bandwidth of the TM10 antenna can be designed to be nearly double that expected from the physical limit for a single resonance. [ABSTRACT FROM AUTHOR]

Connected arrays constitute the only family of antenna arrays for which the spectral Green's Functions have been derived analytically. This formalism is extended here to the receiving case. When the arrays are assumed to be infinitely extended and periodically excited, rigorous equivalent networks can be derived to represent the electromagnetic field distribution in transmission and/or reception. These equivalent networks are based on Green's functions, thus each components can be associated with a specific physical wave mechanism. Moreover, all components are evaluated analytically. The total power transmitted, received and/or scattered by a connected array with and without backing reflector is discussed. The full efficiency of an array with backing reflector is demonstrated and explained. Finally, measurements from a dual-band connected array prototype validate the equivalent circuit representation. [ABSTRACT FROM AUTHOR]