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A novel method for generating a reconfigurable reflectarray based on the creation of photoinduced plasma inside the semiconductor substrate of a reflectarray is presented in this paper. A reflectarray antenna prototype is fabricated and measured to demonstrate the validity of the design methodology. Measurements were carried out to characterize the bandwidth and the gain of the antenna in both the OFF and ON states of an optical flash lamp that was used to induce photoconductivity inside the silicon substrate. [ABSTRACT FROM AUTHOR]

In this paper, we present a new coplanar wave-guide-fed tapered ring slot antenna for ultrawideband (UWB) applications. This antenna consists of a 50 Ω coplanar waveguide feeding line, wideband coplanar waveguide-to-slotline transition, and a pair of curved radiating slots. The impedance bandwidth with VSWR< 2 is from 3.1 GHz to more than 12 GHz. The actual operating bandwidth is, however, limited by the distortion of radiation patterns. Such pattern distortion can be attributed to the antenna mode transition and is investigated in this paper with the help of the radiation patterns in the traditional sense as well as a dimensionless normalized antenna transfer function. By suitably allocating such mode-transition phenomenon to the notched band in a UWB radio, we demonstrate that antennas with desirable radiation characteristics in both UWB low and high bands can be readily achieved. The system responses of a transceiving antenna system in free space are addressed as well. [ABSTRACT FROM AUTHOR]

This paper proposes a new design methodology of arbitrarily shaped integrated lens antennas (ILAs). First, we describe the design principles and numerical techniques of the optimization iterative loop. The starting lens shape, deduced from a general synthesis method based on geometrical optics principles, is optimized so that the radiation pattern of the ILA complies with an arbitrary amplitude-shaped template. The optimization procedure is local and is based on a multidimensional conjugate gradient method. Then, the capabilities and potentialities of this approach are demonstrated numerically using two examples. In the first one, an ILA radiating a secant-squared beam in one principal plane and a flat-top beam in the orthogonal plane is designed for indoor communications in V-band. In the second example (Gaussian/flat-top beam), we show for the first time that the joint optimization of the feed together with the lens shape is a very promising design methodology. Finally, our design tools are validated experimentally in Q-band through the synthesis and optimization of a peculiar small shaped ILA (28 mm × 28 mm × 15 mm) fed by an aperture-coupled microstrip patch antenna. [ABSTRACT FROM AUTHOR]

This paper describes a design strategy to achieve the maximum bandwidth and efficiency for a printed slot antenna surrounded by EBGs. First the dielectric constant and the thickness of the dielectric slab that guarantees an acceptable front to back radiation ratio is identified. Then electromagnetic bandgap (EBG) structures are designed to achieve the optimal bandwidth (BW) while obtaining high surface wave efficiency in the radiating half space. To achieve this goal the wave interaction between the slot and the EBG structures is investigated in depth and clearly described. For the case of Planar circularly symmetric (PCS) EBGs the maximum of radiation BW is shown to occur when the distance between the central antenna and the EBG is approximately half wavelength of the first surface wave, λsw, /2. [ABSTRACT FROM AUTHOR]

We investigate several sampling strategies in performing model-based parameter estimation based on the rational function model. They are: i) uniform-like sampling; ii) random sampling; and iii) sampling based on the Fisher information theory. The sampling strategies are implemented for the calculation of the input impedance of an antenna over a broad frequency range. The model parameters of the rational functions are found by the Cauchy method. The results are evaluated against the optimal sampling solution obtained by a genetic algorithm search. [ABSTRACT FROM AUTHOR]

Results are presented from an experimental investigation to characterize path loss in urban terrain within the military ultrahigh frequency band (225 to 450 MHz) between ground-based communicators. Testing was conducted over a two-day period in downtown Philadelphia. Profiles of received power versus distance were generated along a variety of straight and L-shaped paths for different radio frequencies and antenna heights. The results were analyzed to determine the dependency of the received power on street width, frequency, antenna height, and corner distance (the distance between the transmitter and the corner on an L-shaped path). The results showed well-defined dependencies on all of the parameters except antenna height, which did not appear to affect the results over the range of values considered in this study. An empirical model for predicting received power was developed by fitting regression curves to the data. The model was found to be accurate to within approximately ±10 dB when compared against the data collected in this investigation. This study addresses an important but heretofore largely overlooked propagation environment. [ABSTRACT FROM AUTHOR]

Wide-band, focused aperture antennas like impulse radiating antennas (IRAs) can see improved aperture efficiency through control of the aperture shape and aperture field distribution. The shape of the aperture fields is largely dictated by the transverse electromagnetic mode distribution on the nondispersive feed of the IRA. In this paper we consider reshaping the aperture field by controlling the polarization of the fields reflected or transmitted by the focusing optic. In reflector IRAs, this polarization control is obtained by making the reflector out of uniaxially conducting filaments or wires. In lens IRAs, the polarization control is obtained by using a spatially varying wire grid polarizer. Optimum configurations are derived, and the expression for the far-zone radiated fields are given and compared with experimental measurements. For common antenna configurations and impedances, the aperture height can be increased by amounts ranging from 7% to 67%. An experimental prototype of a 200 Ω reflector IRA demonstrated a 23% improvement in aperture height, and agreed almost exactly with the numerical predictions. [ABSTRACT FROM AUTHOR]

A wide-band probe fed patch antenna with low cross polarization and symmetrical broadside radiation pattern is proposed and studied. By employing a novel meandering probe feed and locating a patch about 0.1⅄0 above a ground plane, a patch antenna with 30% impedance bandwidth (SWR < 2) and 9 dBi gain is designed. The far field radiation pattern of the antenna is stable across the operating bandwidth. Parametric studies and design guidelines of the proposed feeding structure are provided. [ABSTRACT FROM AUTHOR]

A realistic model is presented for the prolate-spheroidal monopole antenna fed through an image plane by a coaxial line. In the model, a magnetic current frill replaces the coaxial aperture in the image plane, and this current is approximated using the electric field for the transverse electromagnetic mode in the coaxial line. An analytical expression is derived for the input admittance of the antenna in the form of an infinite series involving special functions, and an efficient method for summing this series is presented. This method applies for spheroids that have height-to-width ratios covering a large range. An experimental model for the antenna is described, and admittances measured using this model are compared with the theoretical results. The two are found to be in very good agreement. [ABSTRACT FROM AUTHOR]

A method based on the volume-surface-wire integral equation and precorrected-fast Fourier transform (FFT) method is presented for accurate analysis of probe-fed microstrip antennas on arbitrarily-shaped, finite-sized ground plane and substrate. The method of moments (MoM) is used to solve the integral equation. Three triangular-type basis functions are used to represent the unknown currents in the substrates, on the conducting surfaces, and probes respectively. The connection of a vertical probe feed to the patch is rigorously modeled by an attachment mode at the junction. The precorrected-FFT method is applied to reduce the memory requirement and computational cost of the traditional MoM to facilitate analysis of large antenna arrays. [ABSTRACT FROM AUTHOR]

A boundary value problem is solved for the current induced by any source, in particular for an incident plane wave, on any number of identical rings that are arranged to form an axisymmetric, linear array. The net aperture distribution along the finite array is approximated by a combination of three physically-identifiable components: a term from the related infinite array plus contributions from two independent edge-effect waves that originate from each of the two ends. However, if individually resonant loops are closer than a critical spacing, the entire structure exhibits a wild resonance and the simple approximation becomes invalid. [ABSTRACT FROM AUTHOR]

In this paper the ultrawide-band properties of a long slot array are described. The study provides the rigorously derived Green's function (GF) for an infinite long slot array. From these GF active impedances and radiation patterns for various cases were obtained. The ultrawide bandwidths and the array performances of such apertures are highlighted, providing physical insights to understand the mathematical formulation throughout the paper. [ABSTRACT FROM AUTHOR]

This paper describes a spatial beamformer which by using a rectangular array antenna steers a beam in azimuth over a wide frequency band without frequency filters or tap-delay net- works. The weighting coefficients are real numbers which can be realized by attenuators or amplifiers. A prototype including a 4 × 4 array of square planar monopoles and a feeding network composed of attenuators, power divider/combiners and a rat-race hybrid is developed to test the validity of this wide-band beamforming concept. The experimental results prove the validity of this wide-band spatial beamformer for small size arrays. [ABSTRACT FROM AUTHOR]

This paper presents a radiating and balanced mode analysis of planar inverted F antennas (PIFAs) mounted on wave- length-scale printed circuit boards (PCBs). The radiating mode shows, in a direct way, the extent to which the PCB influences both radiation and bandwidth. The balanced mode shows that the shorting pin acts as an impedance transformer in combination with a parallel reactance. Importantly, it can also be used to show that slots in the PIFA top plate nominally provide series reactance when the slot opening is close to the feed. Understanding of this behavior allows bandwidth improvements to be realized. It also allows dual-band antennas to be designed in a straightforward manner. [ABSTRACT FROM AUTHOR]

A novel internal dual-band patch antenna with a small thickness of 3 mm for application in Global System for Mobile Communication/Digital Communication System (GSMIDCS) mobile terminals such as the mobile phone or Personal Digital Assistant (PDA) phone is presented. The patch antenna occupies an area of 15 × 60 mm², with its top patch embedded with a simple T-shaped slit, which separates the top patch into two resonant paths to generate two resonant modes for 900/1800 MHz operation. Then, by extending a small portion of the top patch beyond the top edge of the system ground plane of the mobile terminal, the antenna can provide two wide bandwidths covering the GSMIDCS bands. The proposed antenna is experimentally studied, and effects of the extended top-patch portion and the ground-plane length on the obtained bandwidths are discussed. [ABSTRACT FROM AUTHOR]

In this study, we use the matrix pencil (MP) method to compute the direction of arrival (DOA) of the signals using a very efficient computational procedure in which the complexity of the computation can be reduced significantly by using a unitary matrix transformation. This method applies the technique directly to the data without forming a covariance matrix. Simulation results show that the variance of the estimate approaches to the Cramer-Rao lower bound. Using real computations through the unitary transformation for the MP method leads to a very efficient computational methodology for real time implementation on a digital signal processor chip. A unitary transform can convert the complex matrix to a real matrix along with their eigenvectors and thereby reducing the computational cost at least by a factor of four without sacrificing accuracy. This reduction in the number of computations is achieved by using a transformation, which maps centro-hermitian matrices to real matrices. This transformation is based on Lee's work on centro-hermitian matrices. [ABSTRACT FROM AUTHOR]

The slot-fed hemispherical dielectric resonator antenna (DRA) with a parasitic rectangular slot in the ground plane is studied in this paper. It is found that the length and position of the parasitic slot can be used to widen the DRA bandwidth or to tune the DRA frequency. The exact DRA Green's function is used in the formulation, and the unknown slot currents are solved using the method of moments. Measurements were carried out to verify the calculations, and good agreement between theory and experiment is obtained. [ABSTRACT FROM AUTHOR]

We propose a ferrite-loaded groovy antenna that consists of a ferrite-loaded ground plane with multiple rectangular grooves. The ferrite-loaded groovy antenna is designed, implemented, and tested at 10.08 GHz. The bandwidth of the fabricated antenna is narrow (0.5 % for S11 ≤ -10 dB at 10.08 GHz). A beam-scanning range of 21° is achieved when the dc magnetic field increases from 0 to 1100 oe. Measured radiation patterns favorably agree with theoretical results. [ABSTRACT FROM AUTHOR]

It is demonstrated that the bandwidth of circular polarization (CP) can be significantly increased when one more parasitic loop is added inside the original loop. A single-loop antenna has only one minimum axial ratio (AR) point while the two-loop antenna can create two minimum AR points. An appropriate combination of the two minimum AR points results in a significant enhancement for the CP bandwidth. A comprehensive study of the new type of broad-band circularly polarized antennas is presented. Several loop configurations, including a circular loop, a rhombic loop, and a dual rhombic loop with a series feed and a parallel feed, are investigated. The AR (≤2 dB) bandwidth of the circular-loop antenna with a parasitic circular loop is found to be 20%, more than three times the AR bandwidth of a single loop. For the rhombic-loop antenna with a parasitic rhombic loop, an AR bandwidth (AR ≤ 2 dB) of more than 40% can be achieved by changing the rhombus vertex angle. The AR (≤2 dB) bandwidths of the series-fed and parallel-fed dual rhombic-loop antennas with a parasitic element are 30% and 50%, respectively. A broad-band balun is incorporated into the series-fed dual rhombic-loop antenna for impedance matching. The broad-band CP performance of the loop antennas is verified by experimental results. [ABSTRACT FROM AUTHOR]

To reduce numerical dispersion in finite-difference time-domain (FDTD) methods, large computational stencils are often used. This paper proposes an optimized two-dimensional method by weighting the (2,4) stencil and the "neighborhood" stencil. After obtaining the amplification factor and the numerical dispersion relation, the optimal value of the weight parameter is obtained to minimize the numerical dispersion at a designated frequency. The anisotropy, dispersion error and the accumulated phase errors are greatly reduced over a broad bandwidth. Both the maximum anisotropy and the maximum dispersion error are 8.9 × 10-5, and the accumulated phase error is 0.002367° per cell, respectively, for a broad band of frequencies if optimized at 10 cells per wavelength. Numerical experiments are performed which show very good agreement with theoretical analysis. The time step size bound is the same as for Yee's FDTD. [ABSTRACT FROM AUTHOR]

This paper presents an analysis of a new class of magnetic photonic crystals (MPCs) constructed from periodic arrangements of available (possibly anisotropic) homogeneous material layers. Earlier, analytical studies of semi-infinite versions of these crystals demonstrated that they exhibit the phenomena of minimal reflection at their interface, large amplitude growth of the harmonic wave within the crystal, and concurrent group velocity slow-down. These characteristics are associated with the so called frozen mode and occur at a specific frequency associated with a stationary inflection point within the Bloch diagram. In this paper, we present a characterization of these phenomena for a practical, finite thickness crystal slab and propose a realizable combination of materials consisting of available ferrite and dielectric media. The existence of significant wave amplitude growth and slow down are verified for materials with realistic losses. In addition, we identify and characterize the bandwidth of the magnetic photonic crystals and examine its relationship to the amplitude growth. [ABSTRACT FROM AUTHOR]

A suspended plate antenna with a novel offset feeding is presented. Broadband operation is achieved by loading 50 ohm transmission line feed with microstrip loop. An impedance bandwidth of 30% for VSWR < 2 is obtained over 1.45 to 2.0 GHz. Finite element simulations have been used in designing and optimizing the antenna. [ABSTRACT FROM AUTHOR]

This paper describes two modifications to the real frequency technique of broadband impedance matching. A fundamental limitation of all broadband matching techniques is that the real and imaginary parts of the equalizer impedance must be related by the Hilbert transform. It is shown that additional degrees of freedom in the Hubert transform, and thus a better match, can be obtained by employing equalizer impedances with nonminimum reactance and by optimizing the equalizer resistance outside the stated bandwidth. Examples will be presented for a thin wire dipole antenna. [ABSTRACT FROM AUTHOR]

This paper presents three approaches to improving the efficiency of an array aperture by interleaving two arrays in the same aperture area. The interleaved arrays have aperiodic spacings that are integer multiples of a set minimum spacing and are optimized to reduce the maximum sidelobe level. Fully and partially interleaved sum arrays operating at the same frequencies are demonstrated as well as interleaved sum and difference arrays for a monopulse system. A genetic algorithm is used to optimize arrays of isotropic point sources as well as arrays of dipoles modeled using the method of moments. Narrow beamwidths are possible while avoiding high sidelobes. The available aperture area is efficiently used. [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 problem of coupling through an open-ended coaxial line array in a conducting plane is solved using the integral transform, superposition principle, and eigenfunction expansions. Computation is performed to illustrate the coupling behavior of coaxial line array. Our theory agrees well with other simulation results. [ABSTRACT FROM AUTHOR]

Slot apertures in metallic ground planes are very attractive candidates for conformal antenna applications. However, a low-profile unidirectional antenna requires a backreflector close to the slot aperture and the resulting stripline feed causes strong excitation of parallel-plate modes. In this paper, we investigate unidirectional reflector-backed slot configurations with parallel-plate mode suppression by shorting pins. Starting from a parametric study with respect to shorting pin location and backreflector distance, we present a stripline-fed rectangular slot element with radiation efficiency of more than 80%, a bandwidth of about 5% at a center frequency of 10 GHz. For increased bandwidth applications, a bow-tie slot element with about 8% bandwidth and radiation efficiency of close to 80% is proposed. [ABSTRACT FROM AUTHOR]

A novel square cylindrical monopole antenna easily fabricated from bending a cross-shaped metal plate for wide-band omnidirectional operation is presented. With a cross-sectional area of 8 × 8 mm² and a length of 23 mm, the antenna can provide a wide operating bandwidth of about 4.7 GHz (about 1.9∼6.6 GHz), making it very promising for WMAN operation with the 802.16e standard in the 2 to 6 GHz band. In addition, over the operating bandwidth, the antenna shows very good omnidirectional radiation (gain variations less than 1.5 dB in the azimuthal plane over the bandwidth). Experimental and simulation results of the proposed antenna are presented and discussed. [ABSTRACT FROM AUTHOR]

The U-slot patch antenna, a single-layer single-patch antenna on a relatively thick substrate (∼0.08λ0), is a wide-band antenna with an impedance bandwidth in a range of 20%-30%, which is about an order of magnitude larger than that of the reg- War patch antenna. Recently, it was shown that a half U-slot patch antenna, with half of one of the dimensions of the U-slot patch, maintains similar wide-band behavior. The half-structure was also successfully applied to the U-slot with shorting wall. In this paper, new results on half structures, not previously published, are presented. First, the shorting pin technique is used to reduce the size of the half U-slot patch antenna. By both simulation and experimental studies, it is concluded that the impedance bandwidths, radiation patterns, radiation efficiencies and gains of the haifa structures are comparable to the corresponding full structures. Bandwidth of 28.6% and radiation efficiencies exceeding 90% are obtained for the half U-slot patch with shorting pin. Radiation patterns are stable across the matching band. Second, it is shown that a structure obtained from baking the E-shaped patch, which is a derivative of the U-slot patch, also maintains its wide-band behavior. [ABSTRACT FROM AUTHOR]

This paper presents the design and testing of a dual-band circularly polarized patch antenna. The portion of the antenna for upper band operation consists of two stacked circular patches which are supported by the plastic posts in the air. The patches are excited by four L-probes which are located sequentially under the circumference of the lower patch. The distance between the upper patch and the ground plane is about 0.1 free space wavelength at 2.492 GHz. This upper band portion possesses an impedance bandwidth of 54.9% (SWR ≤ 2) and a 3-dB axial ratio bandwidth of 47.7%. The antenna peak gain is 8 dBi. For dual-band operation, a larger circular patch for lower band operation is added below the upper band patches. The lower band patch is also coupled by four L-probes sequentially. The integrated antenna has an impedance bandwidth of 43.9% (SWR ≤ 2) and a 3-dB axial ratio bandwidth of 33% for the lower band, while an impedance bandwidth of 55.2% (SWR ≤ 2) and a 3-dR axial ratio bandwidth of 44.7% can be obtained for the upper band. Both bands have a peak gain of around 7.8 dBi. [ABSTRACT FROM AUTHOR]

In this paper, we present a numerical analysis of an infinite periodic array of planar spiral elements with octave bandwidth. For off-broadside scan the array is found to exhibit very narrow resonances, which are independent of scan angle. They occur when the spiral arms are multiples of half a wavelength, in which case the current forms a high amplitude standing wave along the spiral arms. The resonances are conveniently suppressed by making the arms unequally long. We also discuss the equivalent 3-port for this nonsymmetrical array element and evaluate the element polarization performance. [ABSTRACT FROM AUTHOR]

In this paper, we introduce a design for a compact low cost dual polarized adaptive planar phased array for Wire- less Local Area Networks (WLAN). The uniqueness of the design is that a wide variety of beam shapes can be electronically selected from a large set of possible patterns while maintaining compact size low cost with reasonable efficiency and having independent dual diversity ports. An important application of the array is in WLAN systems where compact and low cost adaptable antennas promise increases in capacity and coverage. The array design makes use of key innovations in the antenna element and feed network, along with improved low cost phase shifting circuits with low loss performance. The compact antenna element we propose exhibits low mutual coupling between elements with good bandwidth and dual polarized diversity feeds. The feed network is designed to minimize coupling between ports while the phase shifters are low cost but allow flexible control of the antenna patterns. The design is verified both through numerical simulations and also measurements of an experimental prototype and these confirm the good performance of the antenna system and suitability for use in WLAN systems. [ABSTRACT FROM AUTHOR]

A new resistively loaded vee dipole (RVD) is designed and implemented for ultrawide-band short-pulse ground-penetrating radar (GPR) applications. The new RVD is improved in terms of voltage standing wave ratio, gain, and front-to-back ratio while maintaining many advantages of the typical RVD, such as the ability to radiate a short-pulse into a small spot on the ground, a low radar cross section, applicability in an array, etc. The improvements are achieved by curving the arms and modifying the Wu-King loading profile. The curve and the loading profile are designed to decrease the reflection at the drive point of the antenna while increasing the forward gain. The new RVD is manufactured by printing the curved arms on a thin Kapton film and loading them with chip resistors, which approximate the continuous loading profile. The number of resistors is chosen such that the resonant frequency due to the resistor spacing occurs at a frequency higher than the operation bandwidth. The antenna and balun are made in a module by sandwiching them between two blocks of polystyrene foam, attaching a plastic support, and encasing the foam blocks in heat-sealable plastic. The antenna module is mechanically reliable without significant performance degradation. The use of the new RVD module in a GPR system is also demonstrated with an experiment. [ABSTRACT FROM AUTHOR]

A microstrip line on very thin membrane substrate (0.05 mm) is slot-coupled through a thick air substrate (12.7 mm) to excite an L-band radiating patch element. A breadboard unit achieved a relatively wide bandwidth of 100 MHz (8%). Very narrow coupling slot is used with aspect ratio of 160 (conventional slot aspect ratio is between 10 to 30). This new feed design makes the thin-membrane or inflatable array antenna technology more practical. It reduces the number of thin-membrane layers required for a dual-pol wide-band inflatable array from three layers in a previous design to two layers. In addition, this technology allows the thin-membrane array antenna elements to be readily integrated with thin-membrane-mounted transmit/receive (T/R) modules. Calculation shows that, without the patch, the micros trip line and slot combination can also radiate as a paper-thin (0.05 mm) radiator at the L-band frequencies. [ABSTRACT FROM AUTHOR]

The next generation of large telescopes for radio astronomy at frequencies below 100 MHz will consist of tens of thousands of wide-band dipole-like antennas, each individually instrumented with a receiver and combined using digital signal processing. At these frequencies, the sensitivity of a telescope is limited by Galactic noise, with the result that even simple dipoles can deliver extraordinary useable bandwidth. In this paper the necessary characteristics for these antennas are explained, some bounds on performance are developed, and a few candidate designs are analyzed. It is shown that antenna systems consisting of simple wire dipoles, a 360 K active balun, and a long coaxial feedline can achieve Galactic noise-limited performance over large portions of the range 10-100 MHz. It is further shown that when these antennas are used as elements in a compact array, their Galactic noise-limited characteristics are not significantly affected. [ABSTRACT FROM AUTHOR]

Local to multipoint distribution systems (LMDS) operate in the frequency range of 26 and 43 GHz with a large bandwidth of 0.5-2 GHz but a very limited area coverage of a few kilometers only (1-5 km) due to line of sight constraints. As suggested in the Cellular Radio Access for Broadband Services (CRABS) report, the maximal spectral efficiency can be obtained with a dual frequency and polarization reuse plan. This frequency and polarization reuse leads to interference. In this paper we report the interference calculations for urban scenarios at 28 GHz. In a real life scenario, the interference is governed by some or all of the following reasons: half-power beamwidth in azimuth and in elevation as well as blocking due to tall buildings. These three effects have been further investigated with regards to the influence of the half-power beamwidth, the cell size and the effect of varying the base-station heights. In this context interference calculations have been done using the urban three-dimensional database of Munich in order to show the cochannel interference situation in a real urban scenario. It has been found that the spatial-filtering effect of the elevation pattern has a significant impact only for small cells. In a dense city like Munich, coverage is a major problem with a maximum coverage of 70% in a 1 km cell even for a very high base station. Further, to reduce interference, all base stations should be located at nearly the same height. If this is the case, about 3%-7% of the covered customers would not be able to use higher modulation schemes due to cochannel interference. To reduce these values, the receiving antennas should be located as low as possible, but with a line of sight to the desired base station. [ABSTRACT FROM AUTHOR]

An inverted FL antenna (InvFLA) is analyzed to obtain dual-frequency operation at 2.45 and 5.2 GHz (wireless LAN system frequencies). The InvFLA is composed of inverted FL elements, a parasitic element, and a ground plate, where these lie in the same plane, i.e., the structure is a card-type structure having a co-planar ground plate. The antenna height above the ground plate is very small: 5.5 mm = 0.045 wavelength at 2.45 GHz. The analysis shows that the InvFLA has a 4.1 % bandwidth around 2.45 GHz and a 31.8% bandwidth around 5.2 GHz, both for a VSWR = 2 criterion. The gain is calculated to be 0.9 dBi at 2.45 GHz and 1.7 dBi at 5.2 GHz, with a small gain variation in each of the VSWR bands. [ABSTRACT FROM AUTHOR]

In this communication, the alternating-direction implicit finite-difference time-domain (ADI-FDTD) method is extended to analyze periodic structures. In the implicit updates of the ADI-FDTD method, the periodic boundary condition leads to a cyclic matrix. Instead of inverting the cyclic matrix directly, the problem is converted into two auxiliary linear systems that can be solved using the tridiagonal matrix solver. Consequently, only 7n arithmetic operations are required for each implicit update and the efficiency of the ADI-FDTD method is retained. Numerical examples further demonstrate the effectiveness of this periodic ADI-FDTD method. [ABSTRACT FROM AUTHOR]

The performance of three preconditioners, namely, diagonal, block-diagonal, and incomplete lower-upper preconditioners, are investigated in this communication for increasing the efficiency of iterative solvers in adaptive integral method implementation. These preconditioners are implemented to accelerate the solution of electromagnetic scattering problems in a comparative study. Numerical results have shown some improvements in convergence rate, when these preconditioners are employed. [ABSTRACT FROM AUTHOR]

Based on the generalized sidelobe canceller (GSC), we propose a novel adaptive broadband beamformer, where the quiescent vector and blocking matrix are replaced by a series of two-dimensional frequency invariant filters(FIFs). As opposed to standard beamspace techniques, the number of FIFs is flexible and they are not required to posses a very low sidelobe level. Compared with a standard GSC, a faster convergence speed and a lower computational complexity of its adaptive part are achieved. [ABSTRACT FROM AUTHOR]

The effects of individual antenna element bandwidth on the performance of adaptive arrays is studied when the incident interfering signals are wide-band signals. For these signal scenarios, as expected, the antenna element response over the bandwidth of the system plays an important role. If the individual antenna elements are dispersive, then the mutual coupling between array elements leads to dissimilar and dispersive channels in an adaptive array. This in turn leads to degradation in the performance of the adaptive array in that the adaptive array has to use more degrees of freedom to null a strong wide-band interfering signal. It is shown that by using wide-band antenna elements, one can significantly improve the performance of an adaptive array. [ABSTRACT FROM AUTHOR]

A novel approach to realize uniform amplitude time modulated linear arrays with both suppressed sidelobes and sidebands is proposed. The approach utilizes the direct optimization of the "switch-on" lime sequence of each array element via the simple genetic algorithm (SCA). As compared to previous time modulated linear arrays, the new array has the advantages of having low sidelobe level (SLL), low sideband level (SBL) and uniform excitations simultaneously. Experimental results on an experimental prototype of a 16-element printed dipole linear array with the SGA optimized time sequences verified the approach. [ABSTRACT FROM AUTHOR]

A dual tapered meander slot antenna (TMSA) is designed for use as an element in a phased array antenna system for RF and microwave sensors and radar applications. The novel design of dual TMSA element, excited by a coplanar waveguide (CPW) feed, provides a wide band and high efficiency characteristics with 50 Ω input impedance at 10 GHz operating frequency. [ABSTRACT FROM AUTHOR]

The Wheeler cap method for measuring the efficiency of microstrip antennas is revisited. First, we show that the parallel RLC model is a more appropriate model to use than a series one for the microstrip antennas under consideration. Our results are verified by numerical simulation using ENSEMBLE. Second, the role of the interior cavity modes is investigated and a reduced-height Wheeler cap is proposed. Finally, the Wheeler cap method is applied to measure the efficiency of miniaturized microstrip antennas designed using genetic algorithms. [ABSTRACT FROM AUTHOR]

A new excitation technique is developed to improve the impedance bandwidth and to lower the manufacturing cost of a short backfire antenna (SBA). The new excitation structure consists of a planar monopole and a microstrip feed line, both of which are printed on the same dielectric substrate. By splitting the printed monopole with a slot, a wide-band performance can be achieved. The new split-monopole-excited SBA achieves an impedance bandwidth of about 15% [voltage standing wave ratio (VSWR < 2)] while maintaining good radiation performance. As an example, an SBA configuration with the new excitation topology was designed and measured at the 5 Gift UNII band, and good agreement was observed between the simulation and experiment. The effects of the geometric parameters of the excitation structure on the impedance performance are investigated and the operating mechanism of the split-monopole-excited SBA is discussed. Being a low-cost, high-gain, and wide-band directional antenna, the new SBA can find applications in various wireless systems, such as LMDS, WLAN, and the emerging WiMAX networks. [ABSTRACT FROM AUTHOR]

A novel compact and ultrawide-band (UWB) antenna is presented in this paper. The basis for achieving such an UWB operation is through proper magnetic coupling of two adjacent sectorial loop antennas in a symmetrical arrangement. A large number of coupled sectorial loop antennas (CSLA) with different geometrical parameters are fabricated and their measured responses are used to experimentally optimize the geometrical parameters of the antenna for achieving the maximum bandwidth. Through this optimization process an antenna with a VSWR of lower than 2.2 (S11 < -8.5 dB) across an 8.5:1 frequency range is designed. The maximum dimension of this antenna is smaller than 0.37λ0 at the lowest frequency of operation and provides an excellent polarization purity. Furthermore, the antenna exhibits a relatively consistent radiation pattern. Modified versions of the CSLA are also designed to reduce the overall metallic surface and weight of the antenna while maintaining its wide-band characteristics. This allows modifying its dimensions to design low frequency light-weight UWB antennas. [ABSTRACT FROM AUTHOR]

This paper presents and studies three frequency-domain models for optimizing source pulses and detection templates in ultrawide-band (UWB) radio systems. The optimization aims mainly at maximizing the equivalent isotropically radiated power band efficiency in the free space and the output of correlation detection at a receiver. These models are based on the differential evolution, an improved version of the genetic algorithm, and carried out on a set of UWB signals with given mathematical forms. As examples, these models are used to optimize the UWB signals for both narrow-band thin-wire and wide-band planar antenna systems. In addition, the optimized results are validated by nonoptimization simulation. [ABSTRACT FROM AUTHOR]

A downlink transmission scheme in a switched-beam antenna system for providing diversity and beamforming simultaneously based on the space-time block coding technique is proposed. The scheme involves selecting multiple beams based on the uplink reception of the switched-beam antenna system, and determining multiple orthogonal beams from the selected beams. With the orthogonal transmit beams, the proposed switched-beam antenna system outperforms conventional switched-beam systems and approaches the optimal performance achievable by a much complex adaptive antenna system. [ABSTRACT FROM AUTHOR]

The miniaturization of a broadband square slot spiral antenna is proposed using high-contrast (ϵr > 30) dielectric materials. A previously developed circular slot spiral antenna is the starting point of the design. This spiral is first modified in terms of its feed and termination to facilitate the dielectric loading of the antenna by means of a superstrate. The subsequently placed superstrate is also modified by tapering its thickness to improve impedance matching. Several measurements were carried out using square spiral apertures having diameters of 2" and 6", respectively. It is demonstrated that the ceramic loading of a 2" spiral with an ϵr. = 90 shifts the initial operating frequency by nearly 320 MHz down to 564 MHz representing a size reduction of 36%. Additionally, a 6" aperture loaded with a superstrate having ϵr = 30 is shown to operate down to 270 MHz corresponding to a size reduction of 18 %. [ABSTRACT FROM AUTHOR]

In this paper, the use of a smart antenna system for the estimation of the directions of arrival (DOAs) of multiple waves is considered. An efficient method based on the support vector regression is proposed, in which the mapping among the outputs of the array and the DOAs of unknown plane waves is approximated by means of a family of support vector machines. Several numerical results are provided for the validation of the proposed approach, considering multiple impinging waves both in noiseless and noisy environments. [ABSTRACT FROM AUTHOR]