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A quasi microstrip leaky-wave antenna (QMLWA) with a two-dimensional (2-D) beam-scanning capability is presented in this paper. QMLWA consists of two half-width microstrip leaky-wave antennas with a phase-shifter. This new type of microstrip leaky-wave antenna has the advantages of reducing size, 2-D beam-scanning and suppressing back lobes. The main lobe scanning in H-plane (y-z plane) is achieved through varying the operating frequency. When the operating frequency increases from 4.4-6 GHz, the main lobe scans from 84[degrees] to 26[degrees] in H-plane continuously. The main lobe steers in quasi-E-plane with varying the phase difference between two haft-width microstrip leaky-wave antennas. The lobe scans from 78[degrees] to 103[degrees] in quasi-E-plane at 5.4 GHz. The experimental results show this short QMLWA (about 2 wavelengths) leaks power effectively. The back lobe in H-plane of QMLWA is suppressed 13 dB as compared with the conventional whole width MLWA at 5.4 GHz as example. The H-plane radiation characteristics of QMLWA are mainly determined by the width 8 of haft-width MLWA and the distance D between two half-width MLWA together. This size-reduced QMLWA is useful in the automotive radar system and air traffic control. Index Terms--Half-width, microstrip leaky-wave antenna, suppressing back lobe, two-dimensional (2-D) beam-scanning.

A method is proposed to characterize the reflection coefficient of an insulated coaxial antenna with consideration of the feed gap effect. This method models the insulated antenna as a transmission line and extends the method proposed by Su and Wu [1]. The original model assumed the gap to be small and considered a simple capacitance which is ignored in the calculation. Our improvement to this model is a component added to account for the gap impedance. When using insulated dipoles in medical treatment (hyperthermia, ablation, etc.), optimized antennas may require larger gaps, which this 'improved' model accounts for. Excellent agreement is observed between theoretical and numerically simulated [S.sub.11] data, which indicates that this is an appropriate model for designing an efficient applicator for interstitial microwave hyperthermia. Index Terms--Antenna gap model, coaxial antenna, microwave ablation, microwave hyperthermia, reflection coefficient.

New design of a single feed reconfigurable circular patch antenna with polarization pattern diversity is presented. The radiating circular patch, printed on a thin substrate, is supported by non conductive posts on a conducting ground plane and is excited with a capacitively coupled feed. Wideband impedance matching is achieved. The pattern diversity is generated using two switch parasitic pins between the circular patch and the ground plane. By optimizing the position of the switching elements, two orthogonal patterns with the right-hand circular polarization (RHCP) and the left-hand circular polarization (LHCP) are generated. These patterns are used for diversity reception in indoor fading channel for wireless LAN applications. Detailed design of the antenna is described. Experimental results for return loss, radiation patterns, radiation efficiency, antenna gain, axial ratio and the diversity performance in Rayleigh fading channel are presented. Index Terms--Antenna diversity, circular polarization, microstrip antennas, parasitic antennas, reconfigurable antennas.

The control of the radiation of a telecommunication multispot satellite antenna is discussed in which the power can be flexibly routed towards the various spots so as to contrast at best the time variant atmospheric attenuation while keeping at minimum the number of users which do not receive enough signal level. The antenna control is exerted by applying a particular strategy based on the knowledge of the distribution of the weather conditions all over the served region as derived by standard sources like the European Centre for Medium Range Weather Forecasting (ECMWF) or the Meteosat. The paper describes the principles of two types of optimizations: a fixed one based on climatological long term data, which acts as reference, and an adaptive one based on quasi instantaneous pictures that represent the weather situation across the served region. After an explanation of the basic principles of the optimizations, the paper gives an example of application that, despite the brevity of the trial period we are now able to present, indicates an excellent effectiveness of the system which, in the period observed, allows a reduction of the number of non-served users of the two orders of magnitude in the case of low-margin systems (approximately 2-3 dBs over free space) and a still conspicuous advantage (a reduction of a factor about 5) for greater available margins (approximately 6-7 dBs). Index Terms--Attenuation, rainfall, satellite broadcasting, satellite communication.

We demonstrate a new class of low-profile frequency selective surfaces (FSS) with an overall thickness of [lambda]/24 and a third-order bandpass frequency response. The proposed FSS is a three layer structure composed of three metal layers, separated by two electrically thin dielectric substrates. Each layer is a two-dimensional periodic structure with sub-wavelength unit cell dimensions and periodicity. The unit cell of the proposed FSS is composed of a combination of resonant and non-resonant elements. It is shown that this arrangement acts as a spatial bandpass filter with a third-order bandpass response. However, unlike traditional third-order bandpass FSSs, which are usually obtained by cascading three identical first-order bandpass FSSs a quarter wavelength apart from one another and have thicknesses in the order of [lambda]/2, the proposed structure has an extremely low profile and an overall thickness of about [lambda]/24, making it an attractive choice for conformal FSS applications. As a result of the miniaturized unit cells and the extremely small overall thickness of the structure, the proposed FSS has a reduced sensitivity to the angle of incidence of the EM wave compared to traditional third-order frequency selective surfaces. The principles of operation along with guidelines for the design of the proposed FSS are presented in this paper. A prototype of the proposed third-order bandpass FSS is also fabricated and tested using a free space measurement system at C band. Index Terms--Frequency selective surfaces (FSSs), radomes, spatial filters.

A full coarse-grid based finite-difference time-domain (FDTD) model is proposed for an efficient analysis of resistively loaded cylindrical antennas driven by coaxial feed lines. In the case of the electrically thin resistive antenna, the thin-wire approximation is applied to the near fields around the antenna. The resistive antenna is equivalently represented by a series connection of piecewisely lumped resistors along the antenna axis. And the coaxial line is replaced by an equivalent source over the feed aperture of the line. Then the corresponding FDTD update equations make it possible to implement the full coarse-grid model without additional grid refinements for the antenna and the feed line. The transient reflected feed voltage and the input impedance of resistive antennas are calculated numerically and compared with those of a full fine-grid. Index Terms--Coaxial transmission lines, cylindrical antennas, finite-difference time-domain (FDTD) methods, loaded antennas.

In this study, a new method for designing low profile frequency selective surfaces (FSS) with second-order bandpass responses is presented. The FSSs designed using this technique utilize non-resonant subwavelength constituting unit cells with unit cell dimensions and periodicities in the order of 0.15[[lambda].sub.0]. It is demonstrated that using the proposed technique, second-order FSSs with an overall thickness of [[lambda].sub.0]/30 can be designed. This is considerably smaller than the thickness of second-order FSSs designed using traditional techniques and could be particularly useful at lower frequencies with long wavelengths. To facilitate the design of this structure, an equivalent circuit based synthesis method is also presented in this paper. Two bandpass FSS prototypes operating at X-band are designed, fabricated, and tested. A free space measurement setup is used to thoroughly characterize the frequency responses of these prototypes for both the TE and TM polarizations and various angles of incidence. The frequency responses of these structures are shown to have a relatively low sensitivity to the angle of incidence. Principles of operation, detailed design and synthesis procedure, and measurement results of two fabricated prototypes are presented and discussed in this paper. Index Terms--Antenna arrays, frequency selective surfaces (FSSs), periodic structures, radomes, spatial filters.

In this paper, the electromagnetic field of electrically large circular loop antennas, with different radii and different nonuniform current values, around a dielectric coated conducting sphere is considered. One or more loop antennas are located on the outer surface of a spherical dielectric shell covering a conducting sphere. Eigenfunction series solutions for the field are assumed in two regions. The current distribution on the wire loop, driven by a voltage source, is determined by Fourier series expansion and all necessary harmonics are taken into account. Exact analytical field expressions in closed forms are derived and field patterns are plotted. The antenna model and formulation presented in this paper offer exact analytical solutions to several loop antenna problems. Index Terms--Conducting bodies, dielectric bodies, loop antennas, nonuniform current, radiation.

An innovative approach is introduced to form virtual transmitting and receiving radar antenna beams simultaneously by transmitting orthogonal coding waveforms from the antenna elements and digitally processing of their echoes at the receiver. Multiple virtual transmitting-receiving beams can be formed simultaneously by employing an equal number of beamforming filters without increasing transmitting power or antenna gain or resolution loss. The virtually formed antenna beams can provide equivalent antenna gains and spatial resolutions as the conventional phased arrays of the same size. Because the actual antenna radiation pattern can be made almost isotropic, the new system has low probability of intercept (LPI) property. With both transmitting and receiving beams virtually implemented through digital filtering, costly radiation phase shift used in phased arrays is not needed for beam scanning in the proposed system. Index Terms--Active antenna array, digital beamforming, orthogonal waveforms, phased arrays, virtual antenna beams.

A novel type of microstrip antenna is proposed for compact wideband wireless applications. The antenna is composed of six unit cells of left-handed metamaterial (LHM) and a dipole dement. The dipole is directly connected to three of six LHM unit cells, which are arranged in a 2 x 3 antenna array form. In this aspect, the proposed antenna is regarded as LHM loaded dipole antenna. The antenna is matched with a stepped impedance transformer and rectangular slot in the truncated ground plane. The coupled LH resonances and simultaneous excitation of different sections of unit cells and dipole result into broad bandwidth. The proposed antenna has a maximum gain of -1 dBi at 2.5 GHz. The measured return loss indicates 63% bandwidth for |S11| < -10 dB over the band of 1.3-2.5 GHz. The overall size of LHM loaded antenna is [[lambda].sub.0] / 2.87 x [[lambda].sub.0] / 11.27 x [[lambda].sub.0]/315.80 at the center frequency. The radiation of the electrically small LHM unit cells is also demonstrated by the simulated radiation pattern, which is an important concept for the antenna miniaturization. Index Terms--Dipole antennas, left-handed metamaterials (LHMs), loaded antennas, periodic structure.

The capability of non-planar adaptive antennas for suppressing interference in GPS applications is addressed. An optimization problem based on power minimization of incident signals is formulated, resulting in a solution that maximizes the signal-to-interference-plus-noise ratio at the output of the antenna array and subsequently the carrier-to-noise-ratio of a GPS satellite signal. Spherical adaptations of a planar seven element controlled reception pattern antenna array are presented for various low-horizon interference environments. Index Terms--Adaptive antenna arrays, controlled reception pattern antenna (CRPA), GPS, non-planar, power minimization.

A focal plane array (FPA) feeding a reflector can be used to achieve a large field of view (FOV) with overlapping simultaneous beams. In order to provide a continuous FOV over more than an octave bandwidth, the inter-element spacing in the FPA has to be electrically small over large parts of the band. This will inevitably result in strong mutual coupling effects between the array elements. On transmit, the total lost power due to mutual coupling can be quantified by the decoupling efficiency, a term recently introduced for antenna arrays. This paper presents measured decoupling efficiencies of a Vivaldi element FPA operating between 2.3 and 7 GHz. The radiation patterns of the FPA are calculated for two beam excitations by using measured embedded dement patterns, and the corresponding decoupling efficiencies are evaluated by using measured S-parameters between all element ports. The FPA is assumed to illuminate a deep reflector with F/D = 0.35, and the overall reflector aperture efficiencies are computed. The decoupling efficiencies are also determined through the measurements of the total radiation efficiencies in a reverberation chamber, which includes material absorption losses. Index Terms--Antenna array feed, aperture efficiency, decoupling efficiency, mutual coupling, radiation efficiency, tapered slot antenna array.

Based on 2D meander line methods, 3 x 3 x 3 point, symmetrical, 31) dipole antennas structure was investigated exhaustively to identify those configurations with the highest efficiency and lowest resonant frequency for a fixed length. Maximum efficiency and input impedance occurs when adjacent high current segments in the antenna are oppositely directed. The optimal antenna structure was compressed and the effect of a nearby conducting plane was investigated. The meander line structure greatly improves antenna performance with a resonant frequency of 498 MHz and an efficiency of 62% when located on a conducting ground plane. Index Terms--Antenna efficiency, Hilbert curve, Numerical Electromagnetic Code (NEC), Radio Frequency Identification (RFID), wire antenna.

A linear array of surface-wave launchers (SWLs) is presented for surface-wave (SW) beam steering on a grounded dielectric slab (GDS). Specifically, a two-element array of directive Yagi-Uda like SWL antennas (operating between 21-24 GHz) is investigated. By varying the relative phase difference between SWL elements, the excited SW field distribution can be steered. In addition, a six-element linear array of SWLs with nonuniform weighting is also presented. If metallic gratings are placed on top of the GDS, the bound SW mode can be transformed into a radiated leaky-wave (LW) mode and directive beam patterns can be generated in the far field. A specific elliptical grating configuration is also investigated demonstrating single frequency cylindrical-sector LW beam steering. These slotted SWL arrays can be useful for novel SW beam scanning designs, millimeter wave power distribution systems and new LW antennas. Index Terms--Leaky-wave (LW), leaky-wave antenna (LWA), surface-wave (SW), surface-wave launcher (SWL).

With the increasing interest in the applications of large deployable reflector antennas operating at high frequencies, the requirement on the reflector surface accuracy becomes more demanding. Thermal effects inevitably cause certain reflector surface distortions, thus degrading the overall antenna performance. This paper introduces a novel reflector surface distortion compensation technique using a subreflectarray and presents detailed discussions. A microstrip reflectarray is used as a subreflector, illuminated by a primary feed. By properly adjusting the additional phase shift provided by the subreflectarray, the aperture phase errors caused by the main reflector surface distortions are compensated, resulting in a considerably improved antenna performance. As an example, a distorted 20-m offset parabolic reflector antenna operating at X-band is successfully compensated by a subreflectarray, and the simulation results are compared with those obtained by array feed and shaped subreflector compensation techniques. The microstrip subreflectarray is low-profile, lightweight, and cost-effective. Only one primary feed is required, and a reconfigurable design can be achieved if electronically reconfigurable reflectarray elements are adopted. Index Terms--Distortion compensation, reflectarray, reflector antenna.

A multiband printed monopole slot antenna promising for operating as an internal antenna in the thin-profile laptop computer for wireless wide area network (WWAN) operation is presented. The proposed antenna is formed by three monopole slots operated at their quarter-wavelength modes and arranged in a compact planar configuration. A step-shaped microstrip feedline is applied to excite the three monopole slots at their respective optimal feeding position, and two wide operating bands at about 900 and 1900 MHz are obtained for the antenna to cover all the five operating bands of GSM850/900/1800/1900/UMTS for WWAN operation. The antenna is easily printed on a small-size FR4 substrate and shows a length of 60 mm only and a height of 12 mm when mounted at the top edge of the system ground plane or supporting metal frame of the laptop display. Details of the proposed antenna are presented and studied. Index Terms--Internal laptop computer antennas, monopole slot antennas, multiband antennas, quarter-wavelength slot antennas, WWAN antennas.

A new single-feed reconfigurable antenna for pattern diversity is presented in this paper. The proposed structure is based on a metallic cubic cavity which radiates through rectangular slots. The pattern reconfiguration is achieved with PIN diode switches by short-circuiting slots in their center. The designed antenna can switch between three different radiation patterns which radiate in a 4[pi] steradian range and can receive any incident field polarizations. A prototype of the antenna, including PIN diodes and operating in the 5 GHz band, has been built to demonstrate the feasibility of the concept. Measurements have been conducted and three-dimensional radiation patterns are provided. Diversity performances are evaluated by calculating the envelope correlation coefficient. Index Terms--Envelope correlation coefficient, pattern diversity, PIN diode switches, reconfigurable antennas, slots and cavity.

A broadband planar antenna capable of dual linear polarization is designed to be used for the detection of breast tumors. This antenna along with a breast model is simulated using the Ansoft HFSS v10.1. The S-parameters are used as input to Artificial Neural Networks for data processing to investigate the effect of polarization on the statistical detection. Using a dual polarized antenna allowed the calculation of all vertical and horizontal co- and cross-polarizations. The numerical results show that using polarizations parallel to the tumor's major axis increases the likelihood of tumor detection. Index Terms--Artificial neural network, breast cancer detection, broadband antenna.

This paper investigates information-theoretic characterization, via Shannon's information capacity and number of degrees of freedom, of wave radiation (antenna) and wireless propagation systems. Specifically, the paper derives, from the fundamental physical point of view of Maxwell's equations describing electromagnetic fields, the Shannon information capacity of space-time wireless channels formed by electromagnetic sources and receivers in a known background medium. The theory is developed first for the case of sources working at a fixed frequency (time-harmonic case) and is expanded later to the more general case of temporally bandlimited systems (time-domain fields). In the bandlimited case we consider separately the two cases of time-limited and essentially bandlimited systems and of purely bandlimited systems. The developments take into account the physical radiated power constraint in addition to a constraint in the source [L.sup.2] norm which acts to avoid antenna superdirectivity. Based on such radiated power and current [L.sup.2] norm constraints we derive the Shannon information capacity of canonical wireless and antenna systems in free space, for a given additive Gaussian noise level, as well as an associated number of degrees of freedom resulting from such capacity calculations. The derived results also illustrate, from a new information-theoretic point of view, the transition from near to far fields. Index Terms--Antenna, degrees of freedom, electromagnetic information, information capacity, wave information, wireless.

A probe-fed rectangular dielectric resonator antenna (RDRA) placed on a finite ground plane is numerically investigated using method of moments (MoM). The whole structure of the antenna is exactly modeled in our simulation. The feed probe, coaxial cable and ground plane are modeled as surface electric currents, while the dielectric resonator (DR) and the internal dielectric of coaxial cable is modeled as volume polarization currents. Each of the objects is treated as a set of combined field integral equations. The associated couplings are then formulated with sets of integral equations. The coupled integral equations are solved using MoM in spatial domain. The effects of ground plane size, air gap between dielectric resonator and ground plane, probe length, and position on the radiation performance of the antenna including resonant frequency, input impedance, radiation patterns, and bandwidth are investigated. The results obtained for the antenna parameters based on the MoM investigation shows that there is a close agreement with those obtained by measurement. Moreover it is shown that the MoM results are more accurate than other simulation results using software package such as High Frequency Structure Simulator (HFSS). Index Terms--Dielectric resonator antenna (DRA), input impedance, method of moments (MoM).

A lossy transmission-line model that incorporates the radiation resistance of small loop antennas is introduced to characterize the input impedance of a spiral antenna over a large frequency range. This model also leads to a concept of controlling the current velocity and impedance on a spiral antenna by introducing distributed reactive loading along the spiral arms. An important application of the concept is to reduce antenna size and this is demonstrated by numerical simulation examples. Index Terms--Broadband antenna, miniaturization, spiral, transmission lines.

Circularly polarized single-layer U-slot microstrip patch antenna has been proposed. The suggested asymmetrical U-slot can generate the two orthogonal modes for circular polarization without chamfering any corner of the probe-fed square patch microstrip antenna. A parametric study has been carried out to investigate the effects caused by different arm lengths of the U-slot. The thickness of the foam substrate is about 8.5% of the wavelength at the operating frequency. The 3 dB axial ratio bandwidth of the antenna is 4%. Both experimental and theoretical results of the antenna have been presented and discussed. Index Terms--Circular polarization, printed antennas, U-slot.

The design and performance of a folded shorted patch antenna for dual-band operation is presented in this paper. The proposed antenna consists of a folded shorted patch that is excited by a coaxial probe. By cutting two slots in different parts of the patch, good input impedance matching has been achieved over two frequency bands. The lower band possesses an impedance bandwidth (SWR < 2) of 21.1% (0.790 to 0.976 GHz) and a peak gain of 7.3 dBi, while the upper band has an impedance bandwidth (SWR < 2) of 32.2% (1.698 to 2.350 GHz) and a peak gain of 7.5 dBi. Other than the wideband and high gain features, this antenna also has a low profile of 0.05[[LAMBDA].sub.L] and a small size of 0.25[[LAMBDA].sub.L] x 0.36[[LAMBDA].sub.L], where hE is the free space wavelength at the center frequency of the lower band. Therefore, this antenna is very suitable for the indoor base station that is required to service several wireless communication systems, included CDMA800, GSM900, PCS, UMTS, BLUETOOTH and WLAN, by a single compact antenna. Index Terms--Dual-band, folded antenna, shorted antenna, wideband.