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A reconfigurable miniaturized-element frequency-selective surface (FSS) is presented in this paper. A standard wave-guide measurement setup is used to evaluate the performance of the design. The proposed FSS consists of two periodic arrays of metallic loops, with the same periodicity, on either side of a very thin dielectric substrate. The tuning of the reconfigurable surface is shown numerically to be possible by incorporating tuning varactors into the structure. Using varactors on both layers, a reconfigurable frequency response is achieved, which has two modes of operation: bandstop and bandpass. In addition to two completely different modes of operation, the center frequency, as well as the bandwidth of the response can be tuned independently. Frequency tunability with a constant bandwidth over 3-3.5 GHz is shown. A bandwidth tuning at a fixed center frequency is also demonstrated. Simulation results are verified experimentally by fabricating prototypes of the design at S-band loaded with lumped capacitors. To demonstrate the tunability, different pairs of fixed-valued capacitors, as opposed to varactors, are used in a waveguide measurement setup to avoid difficulties associated with biasing varactors in the waveguide. Index Terms--Electronic tuning, miniaturized-element frequency-selective surface (FSS), reconfigurable.

A full 3D hybrid method for analyzing electromagnetic wave propagation in buildings with realistic wall types and structures is presented. This hybrid method combines full-wave techniques which are used for the analysis of one unit cell of inhomogeneous periodic walls with a ray-tracing-like field calculation for the interaction between the walls. Consequently, the method can overcome the inadequacy of traditional ray-tracing in the presence of inhomogeneous periodic wall structures like cinderblock, rebar structure, etc. but is still computationally tractable. Because of this, both indoor wireless channel characterizations and signal fading statistics are shown to be more accurate with this method. Numerical results of the hybrid method are presented for two simple scenarios and validated with theory and measurement. Furthermore, numerical results for a realistic indoor scene are given and compared to a scene where the periodic walls are replaced with effective homogeneous walls. For both setups, the propagation channel is characterized, fast fading signal statistics are extracted and the inadequacies of the homogeneous wall implementation are pointed out. Index Terms--Indoor propagation, ray-tracing, wireless channel.

An algorithm for imaging of targets behind walls is proposed to reduce the wall reflection and enhance the signal-to-clutter ratio. The image formation is based on differential synthetic-aperture-radar image formation employing a continuous-wave radar system. In this approach, instead of using individual backscattered signals, the image is formed by employing the difference signals obtained by subtracting two successive signals along the track. This way, specular reflections are totally eliminated without the need for the knowledge of the wall parameters. This also affects the point target response which is corrected by an integration process. By using backscattered fields from small trihedral corner reflectors behind a poured concrete wall, measured over a frequency band of 1-2.5 GHz, the proposed method is demonstrated. Index Terms--Synthetic aperture imaging, through-wall imaging.

In this paper, the applicability of a procedure for retrieval of vegetation parameters using a coherent scattering model that considers the botanical properties of Sahelian grassland and a stochastic optimization algorithm is studied. This African vegetation is mainly composed of shrubs and grass. Since the coherent scattering model is computationally time-consuming, a simplified empirical model is constructed by fitting of simulation results obtained by the scattering model. Inputs to the empirical model are the sensitive parameters that, for the studied class of vegetation, are the soil moisture content, grass density, and grass moisture content. The model outputs are the polarimetric backscattering coefficients as a function of the incidence angle. Employing the empirical model and a genetic algorithm, a search routine is implemented to estimate the biophysical parameters of the African vegetation from a data set of backscattering coefficients. The estimation of Sahelian-grassland parameters using the set of C-band HH-polarized measured data shows that this procedure achieves good agreement with the ground-truth data. Index Terms--Inversion algorithm, radar remote sensing, Sahelian grassland.

A low-profile, electrically small antenna with omnidirectional vertically polarized radiation similar to a short monopole antenna is presented. The antenna features less than [lambda]/40 dimension in height and [lambda]/10 or smaller in lateral dimension. The antenna is matched to a 50 [OMEGA] coaxial line without the need for external matching. The geometry of the antenna is derived from a quarter-wave transmission fine resonator fed at an appropriate location to maximize current through the short-circuited end. To improve radiation from the vertical short-circuited pin, the geometry is further modified through superposition of additional resonators placed in a parallel arrangement. The lateral dimension of the antenna is miniaturized by meandering and turning the microstrip fines into form of a multi-arm spiral. The meandering between the short-circuited end and the feed point also facilitates the impedance matching. Through this technique, spurious horizontally polarized radiation is also minimized and a radiation pattern similar to a short dipole is achieved. The antenna is designed, fabricated and measured. Parametric studies are performed to explore further size reduction and performance improvements. Based on the studies, a dual-band antenna with enhanced gain is realized. The measurements verify that the proposed fabricated antennas feature excellent impedance match, omnidirectional radiation in the horizontal plane and low levels of cross-polarization. Index Terms--Dual-band antennas, electrically small antennas, monopole antennas, omnidirectional antennas.

This paper presents a novel multipole miniaturized-element frequency-selective surface (FSS) having a very low thickness and a desired multipole frequency response. For this design, new miniaturized elements for the FSS are developed to achieve low thickness solution and improved functionality. The proposed FSS enables implementation of higher order spatial filters over low-profile conformal antenna arrays. First, design of a very thin-layer modified miniaturized-element FSS producing a single-pole bandpass response in addition to a transmission zero is presented. The modified design is just a single-sided circuit board with a particular unit cell consisting of a loop centered within a wire grid. Next, using a similar metallic pattern on the other side of a very thin substrate, a dual-bandpass frequency response is produced. This response is achieved by choosing proper dimensions for the loops and wire of each layer and by appropriately positioning the layers with respect to each other. To establish a benchmark, dual-pole FSSs using cascaded layers of a previously designed miniaturized-element FSS are considered. In comparison, the modified dual-bandpass design has only two metal layers, instead of four, and a single substrate, instead of three. The proposed multipole FSS has a thickness of [lambda]/300 which is six times thinner than the benchmark structures. Moreover, the frequency response of the new FSS shows higher out-of-band rejection values. Performance of the multipole screens is tested by fabricating FSSs with maximally flat and dual-bandpass responses and measuring their frequency responses using a standard measurement setup in a freespace environment. Index Terms--Dual-band, miniaturized-element frequency-selective surface (FSS), multipole.

A probe station based setup for on-wafer antenna measurements is presented. The setup allows for measurement of return loss and radiation patterns of an on-wafer antenna--henceforth referred to as the antenna under test (AUT), radiating at broadside and fed through a coplanar waveguide (CPW). It eliminates the need for wafer dicing and custom-built test fixtures with coaxial connectors or waveguide flanges by contacting the AUT with a coplanar microwave probe. In addition, the AUT is probed exactly where it will be connected to a transceiver IC later on, obviating de-embedding of the measured data. Sources of measurement errors are related to calibration, insufficient dynamic range, misalignment, forward scattering from nearby objects, and vibrations. The performance of the setup is demonstrated from 2 to 40 GHz through measurement of an on-wafer electrically small slot antenna ([[lambda].sub.o]/35 x [[lambda].sub.o]/35, 3.5 x 3.5 [mm.sup.2]) radiating at 2.45 GHz and an aperture coupled microstrip antenna (2.4 x 1.7 [mm.sup.2]) radiating at 38 GHz. Index Terms--Aperture coupled microstrip antenna, electrically short antenna, on-wafer antenna measurements, probe station.

A novel method of analyzing wave propagation in a building consisting of a heterogeneous mixture of homogeneous and periodic walls is presented. In this method walls are discretized into finite-size building blocks and a finite-difference time-domain (FDTD) approach is used to compute their electromagnetic response in a periodic arrangement as well as in corner and terminal locations. These results in conjunction with an iterative field/current calculation algorithm are used to compute the interactions among the walls. The theory and the implementation of this method are described for the 2-D case. The algorithm will henceforth be referred to as 'brick-tracing' algorithm. This field calculation leads to more accurate indoor field predictions, as it accounts for multiple scattering from periodic (cinderblocks, rebar structures, etc.) and nonperiodic wall structures and yet is computationally tractable. The FDTD computation of the walls is fast because of the finite and relatively small size of the building blocks and the iterative current computation only uses simple radiation integral calculations. Consequently computational time is much less than any full-wave simulation. At 1.0 GHz, the 2-D approach of this hybrid method is validated with a full-wave FDTD code and its capability to analyze complex indoor environments is shown. Index Terms--Indoor propagation, ray-tracing.

Scattering from reinforced concrete walls, such as vertical rebar and crossed rebar walls, is analyzed by using an approximate analytical approach. The solution is obtained for a plane-wave incidence with an arbitrary angle of incidence and polarization under the thin-wire approximation. The formulation is based on derivation of the Green's function of a 1-D priodic sources inside a dielectric slab. Since the metal thickness is assume to be small compared to the wavelength, the induced currents are predominantly axial and the transverse components of the induced currents are ignored. This drastically simplifies the solution for this problem which is obtained using a straightforward point-matching technique. The solution for two-dimensional periodic crossed rebar concrete walls is also cast in terms of the periodic Green's function obtained for the 1-D periodic structure of the vertical rebar walls. Using the finite-difference time-domain (FDTD) simulation results, it is shown that the induced currents on thin crossed rebars (diameter > [[lambda].sub.0]/5) have primarily a progressive phase equal to that of the incident field in both directions. This observation also points to a simple approximate solution where the problem of scattering from a 2-D periodic crossed rebar concrete wall is decomposed into two coupled 1-D periodic structures of vertical and horizontal rebar walls. The Bragg mode scattered fields of the reinforced angles for an operation frequency range of 0.5-2.0 GHz and validated by FDTD simulation results. Index Terms--Electromagnetic scattering, reinforced concrete, remote sensing.

We model electromagnetic scattering from two-dimensional objects under seismic or acoustic illumination. The vibration of the object gives rise to a frequency modulated, or Doppler, component in the scattered electromagnetic field. The Doppler component is primarily produced by two mechanisms, the boundary perturbation on the surface of the object and the density modulation within the object. The contributions from the boundary perturbation, which are analyzed in a previous publication, are reviewed and expanded upon herein; the contributions from density modulation are also presented. The wave interaction is analyzed with a unique, two-step approach. First, a finite element acoustic simulation is used to obtain the displacements within the acoustically excited object. This information is then input to a specialized formulation of the finite-difference time-domain method to simulate electromagnetic scattering. Analytical solutions for a cylinder are used to verify the computational modeling approach. Results for a more complex geometry are also presented. Index Terms--Acoustics, electromagnetic scattering, finite-difference time-domain (FDTD) methods.

This paper describes an analytic design procedure for microstrip tunable filters. Step-impedance resonators are employed and loaded with varactors for achieving agility in the filter response. Fixed lumped capacitors are utilized as admittance inverters in order to minimize the number of varactors in the filter. An analytic approach for filter design makes it possible to achieve a tunable filter showing the same frequency response when the center frequency is adjusted. A two-pole microstrip tunable filter whose center frequency can be adjusted from 1.1 to 1.5 GHz is designed to demonstrate the validity of design theory. A prototype tunable filter operating from 2.1 to 2.7 GHz is also designed and measured. A good agreement between the measured and simulated results is shown. Finally, three- and four-pole tunable filters are designed to show straightforward application of the presented design method to higher order tunable filter design. Index Terms--Microwave filters, tunable filters, varactor.

A novel miniaturized cavity-backed composite slot loop antenna (CBCSLA) with low profile and omnidirectional radiation similar to a short dipole is presented. The antenna displays vertical polarization while having less than [lambda]/100 dimension in height. The geometry of the antenna is inspired from a small magnetic loop which is realized using a slot configuration on metallic ground. Since a small slot loop antenna cannot be impedance matched, a composite geometry is devised that acts like as a small slot loop and can easily be impedance matched using this configuration. The diameter of the CBCSLA can be made as small as [lambda]/10 or smaller. The feeding mechanism is designed in a manner to create a circumferentially-constant current distribution around the composite loop while effectively matching to a 50 [OMEGA] coaxial feed from the antenna center. Additional methods are discussed to further enhance the bandwidth and achieve a dual-band response. The antennas are designed, fabricated, and measured. It is shown that despite their small lateral dimension and extremely small height, the miniaturized cavity-backed slot loop features extremely well input impedance match, uniform radiation pattern, low cross-polarization levels and expected gain. Index Terms--Cavity-backed antennas (CBAs), dipole antennas, dual-band antennas, omnidirectional antennas, slot antennas.

A self-aligned fabrication process for capacitive fixed-fixed beam RF microelectromechanical system (MEMS) components is disclosed. It enables the scaling of the critical dimensions and reduces the number of processing steps by 40% compared with a conventional RF MEMS fabrication process. RF MEMS varactors with beam lengths of 30 [micro]m are demonstrated by using the self-aligned fabrication process, and the performance of a four-by-four RF MEMS varactor bank is discussed as well. At 20 GHz, the measured capacitance values range between 180.5 and 199.2 fF. The measured capacitance ratio is 1.15 when a driving voltage of 35 V is applied, and the measured loaded Q-factor ranges between 14.5 and 10.8. The measured cold-switched power handling is 200 mW. [2008-0018] Index Terms--RF microelectromechanical system (MEMS), scaling, self-aligned sacrificial spacer and dielectric.

A new miniaturized-element frequency-selective surface is presented in this paper. This frequency-selective surface is made up of a 2-D periodic array of metallic loop and a wire grid of the same period printed on either side of a very thin substrate. Unique features of the new design include localized frequency-selective properties, high-order frequency response achieved by a single substrate, lack of passband harmonics in the frequency response, and very low frequency response sensitivity to the incidence angle. High-order frequency response is accomplished through the application of a thin substrate that allows considerable couplings between the elements on the two sides of the substrate. The layers' couplings in conjunction with each layer characteristics are designed to produce a high-Q bandpass frequency response, in addition to a transmission zero. It is shown that by inserting variable capacitors in the gap between the metallic loops, the center frequency of the passband can be tuned over nearly an octave. In addition, using a cluster of loops as the unit cell and modifying the parameters of the loops within the cluster, a dual-band characteristic from a single-layer miniaturized-element frequency-selective surface can be achieved. A prototype sample of the miniaturized-element frequency-selective surface, whose unit cell can be as small as [[lambda].sub.o]/12, is fabricated to verify the design performance through a standard free-space measurement setup. The transmission characteristic of the structure is measured and compared with numerical simulation results. Index Terms--Bandpass, bandstop, dual band, miniaturized-element frequency-selective surface, single band.

An investigation into the simulation and performance of low-profile, electrically-small antennas for unattended ground sensor (UGS) networks is featured in this study. Because UGS transceivers are intended for operation near the ground, ground proximity effects become extremely important for considerations related to antenna efficiency, input matching, radiation pattern, and the overall path loss between the transmitter and receiver nodes. While vertical wire antennas are shown to be less susceptible to path loss, a low-profile alternative is desired for UGS applications and is often required in the fabrication of low-cost, monolithic on-chip, miniaturized systems. In this work, the performances of different transceiver systems utilizing different types of near-ground antenna structures including the dipole, loop, ordinary circular slot, and cavity-backed circular slot are analyzed using a full-wave hybrid approach consisting of the moment method in conjunction with a near-ground asymptotic field propagation model. The figure of merit for comparison among the various configurations is identified as the efficiency factor calculated from the ratio of the input power at the transmitting antenna terminal to the received power at the receiving antenna terminal. The unique features of ground proximity effects as pertaining to near-ground operation are discussed and an optimal radiator is identified from the set of structures analyzed. Index Terms--Ground proximity effects, hybrid full-wave analysis, low-profile antennas, near-earth radiowave propagation modeling and simulation, unattended ground sensor (UGS) networks.

This paper introduces a synthesis method for triplepassband microwave filters. A frequency transformation is developed for finding the locations of poles and zeros of the triple-passband filter. The poles and zeros obtained as such are optimized to achieve a transfer function with a reduced number of transmission zeros in order to reduce the number of cross-couplings. Six and 12-pole triple-passband filters are synthesized for validation of this proposed method. A 12-pole triple-passband filter is fabricated with a microstripline structure and shown to provide good agreement between synthesis and measurement results. Finally, the frequency transformation for asymmetric triple-passband filters is briefly discussed. Index Terms--Circuit synthesis, microwave filters, triple-passband filters.

Folded and self-complementary structures are considered as two effective approaches to increase the bandwidth of miniaturized antennas. A folded realization of a previously reported miniaturized slot antenna is devised and shown to provide more than 100% increase in the bandwidth as compared with that of the miniaturized slot antenna with the same size and efficiency. The complementary pair of the miniaturized folded slot, namely, the folded printed wire is also discussed in this paper. It is shown that the folded slot has a much higher radiation efficiency when compared with its complementary folded wire antenna. Another approach for bandwidth improvement is the implementation of the serf-complementary structure of the same miniaturized topology to moderate the frequency dependence of the antenna input impedance. To examine this approach, a folded self-complementary miniature antenna is studied, where further increase in bandwidth is observed. A miniaturized folded slot, its complementary miniaturized folded printed wire, as well as their self-complementary realization, were fabricated and tested. These antennas can fit into a very small rectangular area with dimensions as small as 0.065[[lamnda].sub.0] x 0.065 [[lamnda].sub.0]. While the folded slot ranks the highest in the efficiency/gain, the self-complementary structure falls between the slot and printed wire since it consists of equal proportions of the both slots and strips. A self-complementary H-shape antenna is also introduced to demonstrate that by relaxing the miniaturization to a moderate value, a significant improvement in bandwidth can be accomplished. With yet small dimensions of 0.13[[lamnda].sub.0] x 0.24[[lamnda].sub.0], a very wide bandwidth of (2.3:1) is obtained. For the case of no dielectric substrate, even wider bandwidth of (3:1) is achievable. Index Terms--Folded dipole, folded slot, miniaturized antenna, self-complementary antenna, slot antenna.

Acousto-electromagnetic wave interaction occurs when an electromagnetic wave scatters from an object under seismic or acoustic illumination. The vibration of the object under acoustic excitation gives rise to a frequency modulated scattered field which depends on both the object and electromagnetic and acoustic source parameters. The objective of this study is to accurately calculate the Doppler spectrum of the bistatic scattered field which is usually orders of magnitude smaller than the fundamental component of the scattered field (the stationary target response). In this analysis the recently developed sheet boundary conditions are used to set up a duplicate problem of a stationary object having time varying sheet impedance and admittance accounting for the object vibration. The problem is formulated using the two-dimensional finite-difference time-domain (FDTD) method in an iterative scheme in order to find the broadband response of the scattered field Doppler component. Two-dimensional analytical solutions for a canonical geometry are used to verify the FDTD simulation results. Index Terms--Acoustic, Doppler spectra, electromagnetic scattering, finite-difference time-domain (FDTD).

This paper describes a synthesis method for symmetric dual-passband microwave filters. The proposed method employs frequency transformation techniques for finding the locations of poles and zeros of a desired filter. This method can be used to design dual-passband filters with prescribed passbands and attenuation at stopbands directly without the need for any optimization processes. To validate the procedure a dual-passband stripline filter is designed and fabricated. The stripline dual-passband filter is designed with passbands at 3.90-3.95 and 4.05-4.10 GHz, and 30-dB attenuation at the stopband. This measured results show a good agreement with the theoretical ones. The frequency transformation for symmetric dual-passband filters is also extended to include asymmetric dual-passband responses. This flexible frequency transformation preserves the attenuation characteristics of the low-pass filter prototype. Examples are shown to discuss the flexibility of this transformation. Index Terms--Circuit synthesis, dual-passband filters, microwave filters.

We demonstrate a new class of bandpass frequency selective surface (FSS), the building block of which, unlike the traditional FSSs, makes use of resonant dipole and slot structures that have dimensions much smaller than the operating wavelength. This design allows localization of bandpass characteristics to within a small area on the surface which in turn facilitates flexible spatial filtering for an arbitrary wave phasefront. The proposed FSS is made up of periodic array of metallic patches separated by thin air-gaps backed by a wire mesh having the same periodicity (<< [lambda]). The array of metallic patches constitute a capacitive surface and the wire mesh a coupled inductive surface, which together act as a resonant structure in the path of an incident plane wave. Like traditional FSSs, the capacitive and inductive surfaces of the proposed FSS can easily be fabricated using printed circuit technology on both sides of microwave substrates. It is shown that by cascading such bandpass surfaces in a proper fashion, any arbitrary multipole filter or non-commensurate multiband response can be obtained. The frequency response of the proposed miniaturized-element frequency selective surface (MEFSS) is demonstrated for various incident angles and it is shown that one-pole designs are less sensitive than two-pole designs to the angle of incidence. Dual band designs are also possible based on two-pole designs, but are more sensitive to incident angle than single band designs because of their larger (in terms of wavelengths) spacing. Prototypes of single-pole and dual-pole MEFSSs are fabricated and tested in a waveguide environment at X-band frequencies and excellent agreements between the measured and simulated results are demonstrated. Index Terms--Bandpass radome, frequency selective surfaces (FSSs), spatial filter.

Metamaterial EM insulators are shown to suppress mutual coupling between densely packed array elements. This technique allows for array element design in isolation, without consideration of adjacent elements and mutual coupling effects. Suppressing mutual coupling allows for denser packing and enhanced directivity in antenna arrays approaching the superdirective theoretical maximums. Metamaterial isolation walls 0.05[[lambda].sub.0] thick exhibit 20 dB peak isolations with 10 dB isolation bandwidths of 2-6% and very low losses. These insulators achieve lower than -30 dB coupling levels for 0.2[[lambda].sub.0] periodicity arrays. A simulated 1.18[[lambda].sub.0] five element broadside array exhibits a superdirective 65 degree first-null beamwidth with -9.5 dB side-lobes and when physically fabricated, experiment validates theory with a squinted 75 degree first-null beamwidth and only slight degradation of sidelobe levels. The main beam of the metamaterial insulated array is steerable over a full +/- 90 degree horizon with little scan loss and no instances of scan blindness, a null can be placed to any angle including broadside, and as narrow as 25 degree peak-to-null separation is possible. Index Terms--Antenna array, antenna array mutual coupling, electromagnetic bandgap materials (EBG), insulators, metamaterial.

A novel 3-arm spiral antenna structure is presented in this paper. This antenna similar to traditional two-arm or four-arm spiral antennas exhibits wideband radiation characteristic and circular polarization. Advantages offered by the new design are two fold. Unlike the traditional spiral antennas the three-arm spiral can be fed by an unbalanced transmission line, such as a coaxial line or coplanar waveguide, and therefore an external balun is not needed at the feed point. Also by proper choice of arms' dimensions the antenna can be directly matched to any practical transmission line characteristic impedance and therefore external matching networks are not required. This is accomplished by feeding the antenna at the outer radius by a coplanar waveguide (CPW) transmission line and tapering it towards the center. The antenna can also be fed from the center using a coaxial or CPW line perpendicular to the plane of the spiral antenna. A full-wave numerical simulation tool is used to optimize the geometry of the proposed 3-arm spiral to achieve a compact size, wide bandwidth operation, and low axial ratio. The antenna is also designed over a ground plane to achieve a unidirectional radiation and center loading is examined that improves the axial ratio. Simulated results like return loss, radiation pattern, gain, and axial ratio are compared with those obtained from measurements and good agreements are shown. Because of its unique feed structure and compact size, application of the proposed 3-arm spiral antenna for wideband array applications is demonstrated. Index Terms--Circular polarization, unbalanced spiral, wideband antenna.

In this paper the problem of uplink array calibration for deep-space communication is considered. A phased array of many modest-size reflectors antennas is used to drastically improve the uplink effective isotropic radiated power of a ground station. A radar calibration procedure for the array phase distribution is presented using a number of in-orbit targets. Design of optimal orbit and the number of calibration targets is investigated for providing frequent calibration opportunities needed for compensating array elements phase center movements as the array tracks a spacecraft. Array far-field focusing based on the near-filed in-orbit (low Earth orbit (LEO)) calibration targets is also presented and array gain degradation analysis based on the position error of the array elements and in-orbit targets has been carried out. It is shown that errors in the in-orbit targets positions significantly degrade the far-field array gain while the errors in array elements positions are not very important. Analysis of phase errors caused by thermal noise, system instability, and atmospheric effects show insignificant array gain degradation by these factors.

The design and modeling of an embedded-circuit metamaterial with [epsilon]-[mu] constitutive parameters as the substrate for patch antennas is presented. The magneto-dielectric metasubstrate is constructed of periodic resonant loop circuits embedded in a low dielectric host medium, and is capable of providing both permittivity and permeability material parameters at any frequency of interest. The embedded-circuit building blocks are very small in size (< [lambda]/20) and constitute artificial material molecules. Geometric control of the embedded-circuits allows [epsilon] and [mu] to be tailored to the application. A transmission line circuit model analogy is developed to theoretically investigate the behavior of designed embedded-circuit metamaterial and predict its physical parameters. In addition, a full wave analysis based on finite difference time domain (FDTD) technique is applied to comprehensively characterize the complex periodic structure. The potential advantage of magneto-dielectric metasubstrate for the design of small antennas having relatively wide bandwidth is investigated. Index Terms--Antenna miniaturization, embedded-circuits, finite difference time domain (FDTD), metamaterial, periodic structures.

In this paper, a hybrid target--foliage model based on existing electromagnetic techniques is developed to investigate the scattering behavior of hard targets embedded inside a forest canopy at high frequencies. The proposed model is composed of two basic scattering models, one for foliage and the other for the hard targets. The connection between these two models, which accounts for the interaction between the foliage scatterers and the target and vice versa, is accomplished through the application of the reciprocity theorem. Wave penetration through the forest canopy and near-field and far-field scattering from the canopy's constituents is calculated using a coherent discrete scattering model that makes use of realistic tree structures. Calculation of scattering from a hard target illuminated by the reduced incident field and the scattered field of nearby vegetation is carried out using an iterative physical optics (PO) method formulated for fast computation of foliage--target interaction. To reduce the number of iterations, geometrical optics (GO) approximation is initially used for determining the shadowed areas over the hard target when illuminated by individual foliage scatterers. Furthermore, using a scaled measurement system at millimeter-wave frequency, the accuracy of the iterative PO model is demonstrated, employing a complex target that occupies a volume as big as 86[lambda] x 33[lambda] x 20[lambda]. Index Terms--Electromagnetic (EM) scattering, hybrid solution methods.

A hybrid target-foliage model is developed to investigate the scattering behavior of hard targets embedded inside a forest canopy. The proposed model is composed of two existing electromagnetic-scattering models, one for the foliage and the other for the hard targets that are coupled in a computationally efficient manner. The connection between these two models, which accounts for the interaction between the foliage scatterers and the target, is accomplished through the application of Huygens' principle. Wave penetration through the forest canopy and near-field and far-field scattering from its constituents is calculated using a coherent single-scattering theory, which makes use of realistic tree structures. Defining a Huygens' surface enclosing the hard target and calculating the illuminating field (the scattered fields from the nearby vegetation scatterers and reduced incident field), the interaction between the foliage and the hard target is accounted for. Computing the scattered field from target on the Huygens' surface and using a reciprocity theorem target-foliage interaction is captured very efficiently. Calculation of scattering from a hard target is carried out using a finite-difference time-domain (FDTD) technique. For a typical vehicle dimensions, the required time and memory for the FDTD computation and exact field calculation inside the foliage limits the simulation frequency to upper very high frequency (VHF) band. Index Terms--Electromagnetic (EM) scattering, finite-difference time-domain (FDTD) method, hybrid solution methods.

Calculation of electromagnetic (EM) scattering from vibrating penetrable cylinders of arbitrary cross-section is presented using a general class of time-varying sheet boundary conditions (SBCs) in conjunction with the method of moments (MoM). Sheet impedance and admittance expressions are first derived from the exact scattering solution for a penetrable circular cylinder with perturbed radius. Then, using the SBCs, integral equations are derived and solved numerically so that vibrating cylinders with arbitrary cross-section can be treated. Cylinder vibrations are assumed to be non-relativistic, allowing a simplified calculation of the scattered Doppler spectrum. A critical factor in the calculation of the potentially small Doppler components is that the time-varying nature of the cylinder boundary, contained within the sheet impedance and admittance expressions, can be isolated from the unperturbed terms in the scattered field. Comparison with exact and analytical perturbation solutions are presented to demonstrate the accuracy of the numerical solution. Index Terms--Doppler spectrum, electromagnetic scattering, impedance boundary conditions, method of moments (MoM), RCS, sheet boundary conditions.

A technique for reducing the dimensions of the cavity of a cavity-backed slot antenna (CBSA) is presented, which facilitates proper fabrication and integration of miniaturized slot antennas on nmltilayer substrates. This is accomplished by replacing the solid metal around the traditional slot antennas with a specific metallic pattern that can be viewed as a series of parallel strip lines placed around the slot antenna. This metallic pattern is then modified by designing the parallel strips in a compact fashion to reduce the overall antenna dimensions and obtain a reduced-size CBSA. It is shown that, for a simple straight slot antenna, the overall occupied volume of the modified cavity backing the slot antenna can be reduced by more than 65% without affecting the high radiation efficiency of the antenna. A number of traditional cavity backed slot antennas and the proposed modified CBSAs are designed, fabricated, and measured. The reduced-size CBSAs show a very low input VSWR, low cross-polarized radiation levels, and high radiation efficiency. Despite their small ground plane size, the proposed cavity backed slot antennas have front-to-back ratio (FTBR) values in the range of 6-7 dB. Index Terms--Aperture antennas, cavity-backed antennas, microstrip antennas, slot antennas.

In this paper, a method for reducing the size of coplanar-waveguide (CPW) resonators, used in the design of compact bandpass filters, is presented. This is accomplished by incorporating a distributed array of capacitive loads along a resonant CPW line. Design of a miniaturized bandpass filter using capacitively loaded CPW resonators, which are inductively coupled, is presented. Inductively coupled bandpass filters with the proposed capacitively loaded CPW line resonators are designed and a size reduction of 25%, compared with filters using a standard CPW line resonator, is demonstrated. It is shown that, through this size reduction, the insertion loss is minimally increased ( Index Terms--Bandpass, capacitively loaded, coplanar waveguide (CPW), filter, millimeter wave, miniaturized, W-band.

A new technique for designing dual-band reconfigurable slot antennas is presented. Dual-frequency operation is achieved by loading a slot antenna with two lumped variable capacitors (varactors) placed in proper locations along the slot. Loading the slot antenna with lumped capacitors shifts down the resonant frequencies of the first and second resonances of the antenna. However, these frequency shifts depend not only on the values of the capacitors, but also on their locations along the slot antenna. Here, it is shown that by choosing the locations of the varactors appropriately, it is possible to obtain a dual-band antenna whose first and second resonant frequencies can be controlled individually. In other words, the frequency of either the first or the second band can be fixed, while the other one is electronically tuned. Using such a design, an electronically tunable dual-band antenna is designed and fabricated using two identical varactors having a capacitance range of 0.5-2.25 pF. The antenna is shown to have a frequency ratio ([f.sub.R] = [f.sub.2]/[f.sub.1]) ranging from 1.3 to 2.67. An important feature of this antenna is its consistent radiation pattern, polarization, and polarization purity at both bands and across its entire tunable frequency range. Index Terms--Multifrequency antennas, multiple band antennas, reconfigurable antennas, slot antennas.

A new technique for designing dual-band reconfigurable slot antennas is introduced. The technique is based on loading a slot antenna with a lumped capacitor (or varactor) at a certain location along the slot. Given a fixed capacitor location along the slot, decreasing the capacitance results in increasing the first and second resonant frequencies of the slot antenna. However, the changes in the resonant frequencies are significantly different for the first and second resonances and, hence, a dual-band antenna with considerable frequency ratio tuning range can be obtained. Based on this technique, an electronically tunable dual-band antenna with a frequency ratio in the range of 1.2-1.65 is designed and fabricated using a single varactor with a capacitance range of 0.5-2.2 pF. The antenna has similar radiation patterns with low cross-polarization levels at both bands and across the entire tunable frequency range. Index Terms--Multifrequency antennas, multiple band antennas, reconfigurable antennas, slot antennas.

Magnetic properties were imparted to a naturally nonmagnetic material by metallic inclusions. A patch antenna tested the performance of the magnetic metamaterial as a substrate and validated that a single substrate can achieve a range of miniaturization values. The effective medium metamaterial substrate employed electromagnetically small embedded circuits (ECs) to achieve permeability and permittivity greater than that of the host dielectric. Geometric control of the ECs allowed [mu], and [epsilon] to be tailored to the application. The magnetic metamaterial exhibited enhanced [mu] and [epsilon] with acceptable loss-factor levels. Models for predicting [mu] and [epsilon] are presented, the benefits of employing metamaterial substrates are discussed, and the results in this antenna experiment are presented. The metamaterial exhibits performance characteristics not achievable from natural materials. Of particular significance is that with the permeability varying strongly and predictably with frequency, the miniaturization factor may be selected by tuning the operating frequency. Simulations indicate that such performance can be extended to several gigahertz with current technology. Relative permeability values in the [[mu].sub.r] = 1-5 range are achievable for moderately low-loss applications. Representative antenna miniaturization factors on the order of 4-7 over a moderate (approximately 10%) transmission bandwidth and efficiencies in a moderate range (20%-35%) are demonstrated with the possibility of higher efficiencies indicated. Index Terms--Magnetic materials, metamaterials, microstrip antennas.

The problem of near-earth wave propagation in the presence of a dielectric layer such as a vegetation or snow covering is considered in this paper by modeling the propagation environment as a homogeneous two-layer medium (air/dielectric/ground). A number of studies have demonstrated the relevancy of the lateral wave for the case when both the transmitter and receiver are located within a simple half-space dielectric medium [1]-[6]. Unfortunately, for the generalized two-layer model, for configurations in which the transmitter or receiver (or both) is located above the dielectric layer, far-field analytical expressions that include all propagation features do not exist. In this paper, in order to arrive at a computational efficient solution for the two-layer model, a second-order asymptotic evaluation for the electric fields of an arbitrarily oriented, infinitesimal electric dipole--for source and observation points located in the vicinity of the air/dielectric interface--is carried out through the method of steepest descents. The formulations are valid in the far field, with the limitation that the exponentially decaying pole and branch cut contributions have been ignored. It is observed that the Norton wave, though it is highly localized near the air/dielectric interface, is a significant contribution either when the dipole and observation points are both located above the dielectric layer or when one is above and the other is within the layer. Index Terms--Asymptotic evaluation, near-earth wave propagation, two-layer half-space medium.

Previously insurmountable challenges posed by stringent requirements of simultaneous compact size, high bandwidth, high to moderate efficiency, and circular polarization operation at UHF have been surpassed by a unique design employing layered magnetodielectric materials. To achieve percentage bandwidth values in excess of 50% for an antenna with a maximum dimension of 0.15 [lambda] three approaches for bandwidth enhancement are combined in a proper fashion. A volumetric source, as opposed to printed planar or wire sources, inherently provides higher bandwidth and is used as the fundamental radiating element of the antenna. The radiating structure is made up of layered magnetodielectric material with proper design of permittivity and permeability values forming a magnetodielectric resonator antenna (MDRA). Noting that miniaturization and wave impedance in the MDRA are, respectively, proportional to the square-root of the product and ratio of the permeability and permittivity, moderate values of permittivity and permeability are used to enhance the bandwidth while achieving considerable miniaturization. The third method for bandwidth enhancement is based on the integration of a resonant feed and many parasitic elements into the MDRA structure. Square symmetry of the MDRA is used to obtain circular polarization operation. A prototype small UHF antenna operating over 240-420 MHz with a linear dimension smaller than 0.15 [lambda] at the lowest frequency is fabricated and tested; the results are summarized in this paper.

The Shuttle Radar Topography Mission (SRTM) provided data for detailed topographical maps of about 80% of the Earth's land surface. SRTM consisted of single-pass C- and X-band interferometric synthetic aperture radars (INSARs). In order to utilize SRTM data in remote sensing applications the data must be calibrated and validated. This paper presents The University of Michigan's SRTM calibration and validation campaign and our results using recently acquired C-band SRTM data of our calibration sites. An array of calibration targets was deployed with the intention of determining the accuracy of INSAR-derived digital elevation maps. The array spanned one of the X-band swaths and stretched from Toledo, OH to Lansing, MI. Passive and active targets were used. The passive targets included trihedrals and tophats. The locations in latitude, longitude, and elevation of the point targets were determined using differential GPS. We also acquired U.S. Geological Survey (USGS) digital elevation models (DEMs) to use in the calibration and validation work. The SRTM data used in this study are both Principal Investigator Processor (PI) data, which are not the refined final data product, and the ground data processing system (GDPS) data, which are a more refined data product. We report that both datasets for southeastern Michigan exceed the SRTM mission specifications for absolute and relative height errors for our point targets. A more extensive analysis of the SRTM GDPS data indicates that it meets the absolute and relative accuracy requirements even for bare surface areas. In addition, we validate the PI height error files, which are used to provide a statistical characterization of the difference between the SRTM GDPS and USGS DEM heights. The statistical characterization of the GDPS-USGS difference is of interest in forest parameter retrieval algorithms. Index Terms--Digital elevation model (DEM), Shuttle Radar Topography Mission, validation.

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 ([S.sub.11] < -8.5 dB) across an 8.5:1 frequency range is designed. The maximum dimension of this antenna is smaller than 0.37 [[lambda].sub.0] 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. Index Terms--Broadband antennas, loop antennas, ultrawideband (UWB) antennas.

In this paper, the behavior of wave propagation through coniferous forest stands at millimeter-wave frequencies is characterized both theoretically and experimentally. A coherent wave propagation model is used to simulate the propagation through foliage. The coherent model is composed of two components: a forest stand generator that makes use of a stochastic fractal model, and an electromagnetic model that makes use of Foldy's approximation and single scattering. An outdoor measurement system is designed and used for characterizing the channel behavior for a pine tree stand at Ka-band (35 GHz). In this experiment, 84 independent spatial samples of transmitted signal through the pine stand were collected to obtain the path-loss statistics. The comparison between measurement and simulation results showed that single scattering theory overestimates the wave attenuation through foliage. To improve the accuracy of the coherent model, partial multiple scattering occurred among the needles of highly dense leaf clusters must be included for the estimation of the coherent attenuation. Distorted Born approximation is used to macromodel the scattering pattern from needle clusters. This technique has comparable accuracy and requires much less computational resources than a full-wave solution, such as method of moment. By including multiple scattering effects of needle clusters in the simulation model, much better agreement is obtained for both mean and standard deviation of the path-loss. Index Terms--Distorted Born approximation (DBA), multiple scattering, path-loss, propagation model.

In this paper a novel wide-band propagation channel measurement system with high dynamic range and sensitivity is introduced. The system enables the user to characterize signal propagation through a medium over a very wide frequency band with fine spectral resolution (as low as 3 Hz) by measuring the attenuation and phase characteristics of the medium. This system also allows for the study of temporal, spectral and spatial decorrelation. The high fidelity data gathered with this system can also be utilized to develop empirical models or used as a validation tool for physics based propagation models which simulate the behavior of radio waves in different environments such as forests, urban areas or indoor environments. The mobility and flexibility of the system allows for site specific measurements in various propagation scenarios. Index Terms--Attenuation measurement, communication channels, electromagnetic measurements, microwave measurements, radio propagation.

In this paper, an approximate solution for electromagnetic scattering by a very thin planar homogeneous dielectric object is presented. This solution is obtained from a volumetric integral equation using Fourier transform and is shown to be uniformly valid from low to high frequencies at all incidence angles including edge-on incidence. Validity of the solution is demonstrated through a comparison with canonical objects such as an infinite dielectric slab, and a number of two-dimensional (2-D) and three-dimensional (3-D) dielectric scatterers. For 2-D, and 3-D scatterers, the approximate solution is compared with a method of moments solution. In all cases examined the approximate formulation provides very accurate results except for situations where the dielectric constant is very high. Index Terms--Scattering, thin dielectric disk.

In this paper, two accurate models for interdigital capacitors and shunt inductive stubs in coplanar-waveguide structures are presented and validated over the entire W-band frequency range. Using these models, a novel bandpass filter (BPF) and a miniaturized high-pass filter are designed and fabricated. By inserting interdigital capacitors in BPF resonators, an out-of-band transmission null is introduced, which improves rejection level up to 17 dB over standard designs of similar filters. A high-pass filter is also designed, using semilumped-element models in order to miniaturize the filter structure. It is shown that a fifth-order high-pass filter can be built with a maximum dimension of less than [[lambda].sub.g]/3. Great agreement between simulated and measured responses of these filters is demonstrated. Index Terms--Bandpass, coplanar waveguide (CPW), filter, high-pass, interdigital capacitor, millimeter wave, semilumped, W-band.

The goal of this study is to improve the bandwidth of a miniaturized antenna. The proposed technique combines a slot antenna and a dielectric resonator antenna (DRA) to effectively double the available bandwidth without compromising miniaturization or efficiency. With proper design it is observed that the resonance of the slot and that of the dielectric structure itself may be merged to achieve extremely wide bandwidth over which the antenna polarization and radiation pattern are preserved. In addition, using the DRA, a volumetric source, improves the radiation power factor of the radiating slot. A miniaturized antenna figure of merit (MAFM) is defined to simultaneously quantify aspects of miniaturized antenna performance including the degree of miniaturization, efficiency, and bandwidth. Figures for various common types of antennas are given and compared with that of the proposed structures. In order to determine the effects of varying design parameters on bandwidth and matching, sensitivity analysis is carried out using the finite-difference time-domain method. Numerous designs for miniaturized slot-fed dielectric resonator antennas are simulated and bandwidths exceeding 25% are achieved. Two 2.4 GHz antennas are built, characterized, and the results compared with theory. Index Terms--Dielectric resonator antenna (DRA), dual-resonance, slot antenna.

In this paper the design of a compact, efficient and electronically tunable antenna is presented. A single-fed resonant slot loaded with a series of PIN diode switches constitute the fundamental structure of the antenna. The antenna tuning is realized by changing its effective electrical length, which is controlled by the bias voltages of the solid state shunt switches along the slot antenna. Although the design is based on a resonant configuration, an effective bandwidth of 1.7:1 is obtained through this tuning without requiring a reconfigurable matching network. Four resonant frequencies from 540-890 MHz are selected in this bandwidth and very good matching is achieved for all resonant frequencies. Theoretical and experimental behavior of the antenna parameters is presented and it is demonstrated that the radiation pattern, efficiency and polarization state of the antenna remain essentially unaffected by the frequency tuning. Index Tells--Electronic tuning, miniaturization, PIN diode switch, reconfigurable antenna, resonant antenna.

A wide-band slot antenna element is proposed as a building block for designing single- or multi-element wide-band or dual-band slot antennas. It is shown that a properly designed, off-centered, microstrip-fed, moderately wide slot antenna shows dual resonant behavior with similar radiation characteristics at both resonant frequencies and therefore can be used as a wide-band or dual-band element. This element shows bandwidth values up to 37%, if used in the wide-band mode. When used in the dual-band mode, frequency ratios up to 1.6 with bandwidths larger than 10% at both frequency bands can be achieved without putting any constraints on the impedance matching, cross polarization levels, or radiation patterns of the antenna. The proposed wide-band slot element can also be incorporated in a multi-element antenna topology resulting in a very wide-band antenna with a minimum number of elements. It is also shown that, by using only two of these elements in a parallel feed topology, an antenna with good radiation parameters over a 2.5:1 bandwidth ratio can be obtained. Index Terms--Broad-band antennas, multiple band antennas, slot antennas.

The concept of a novel reactive impedance surface (RIS) as a substrate for planar antennas, that can miniaturize the size and significantly enhance both the bandwidth and the radiation characteristics of an antenna is introduced. Using the exact image formulation for the fields of elementary sources above impedance surfaces, it is shown that a purely reactive impedance plane with a specific surface reactance can minimize the interaction between the elementary source and its image in the RIS substrate. An RIS can be tuned anywhere between perfectly electric and magnetic conductor (PEC and PMC) surfaces offering a property to achieve the optimal bandwidth and miniaturization factor. It is demonstrated that RIS can provide performance superior to PMC when used as substrate for antennas. The RIS substrate is designed utilizing two-dimensional periodic printed metallic patches on a metal-backed high dielectric material. A simplified circuit model describing the physical phenomenon of the periodic surface is developed for simple analysis and design of the RIS substrate. Also a finite-difference time-domain (FDTD) full-wave analysis in conjunction with periodic boundary conditions and perfectly matched layer walls is applied to provide comprehensive study and analysis of complex antennas on such substrates. Examples of different planar antennas including dipole and patch antennas on RIS are considered, and their characteristics are compared with those obtained from the same antennas over PEC and PMC. The simulations compare very well with measured results obtained from a prototype [lambda]/10 miniaturized patch antenna fabricated on an RIS substrate. This antenna shows measured relative bandwidth, gain, and radiation efficiency of BW = 6.7%, G = 4.5 dBi, and [e.sub.r] = 90%, respectively, which constitutes the highest bandwidth, gain, and efficiency for such a small size thin planar antenna. Index Terms--Antenna miniaturization, finite-difference time-domain (FDTD), impedance surfaces, meta-substrates, perfectly magnetic conductor (PMC), periodic structures, planar antennas.

A method is introduced for designing bandpass frequency-selective surfaces (FSSs) using arrays of antenna-filter-antenna (AFA) modules. An AFA module is a filter with radiation ports, which is obtained by integrating two antennas and a nonradiating resonant structure in between. AFA modules are designed based on circuit models and microwave filter design techniques. Three types of these AFA modules are designed using microstrip antennas and coplanar-waveguide resonators, and are used to form FSSs with three- and four-pole shaped bandpass response at 35 GHz. FSS structures are formed by arraying these modules in a periodic grid with an optimal cell size. The proposed concept and the design method are validated using numerical simulation (finite-element method), as well as experimental results. Index Terms--Antenna-filter-antenna (AFA), bandpass filter, frequency-selective surface (FSS), high-order FSS.

In this paper, new methods for further reducing the size and/or increasing the bandwidth (BW) of a class of miniaturized slot antennas are presented. This paper examines techniques such as parasitic coupling and inductive loading to achieve higher BW and further size reduction for this class of miniaturized slot antennas. The overall BW of a proposed double resonant antenna is shown to be increased by more than 94% compared with a single resonant antenna occupying the same area. The behavior of miniaturized slot antennas, loaded with series inductive elements along the radiating section is also examined. The inductive loads are constructed by two balanced short circuited slot lines placed on opposite sides of the radiating slot. These inductive loads can considerably reduce the antenna size at its resonance. Prototypes of a double resonant antenna at 850 MHz and inductively loaded miniaturized antennas at around 1 GHz are designed and tested. Finally the application of both methods in a dual band miniaturized antenna is presented. In all cases measured and simulated results show excellent agreement. Index Terms--Slot antennas, electrically small antennas, parasitic antennas, multifrequency antennas.

In this paper, the unique features of periodic magneto-dielectric meta-materials in electromagnetics are addressed. These materials, which are arranged in periodic configurations, are applied for the design of novel EM structures with applications in the VHF-UHF bands. The utility of theses materials are demonstrated by considering two challenging problems, namely, design of miniaturized electromagnetic band-gap (EBG) structures and antennas in the VHF-UHF bands. A woodpile EBG made up of magneto-dielectric material is proposed. It is shown that the magneto-dielectric woodpile not only exhibits band-gap rejection values much higher than the ordinary dielectric woodpile, but also for the same physical dimensions it shows a rejection band at a much lower frequency. The higher rejection is a result of higher effective impedance contrasts between consecutive layers of the magneto-dielectric woodpile structure. Composite magneto-dielectrics are also shown to provide certain advantages when used as substrates for planar antennas. These substrates are used to miniaturize antennas while maintaining a relatively high bandwidth and efficiency. An artificial anisotropic meta-substrate having [[micro].sub.r] [[epsilon].sub.r], made up of layered magneto-dielectric and dielectric materials is designed to maximize the bandwidth of a miniaturized patch antenna. Analytical and numerical approaches, based on the anisotropic effective medium theory (AEMT) and the finite-difference time-domain (FDTD) technique, are applied to carry out the analyzes and fully characterize the performance of finite and infinite periodic magneto-dielectric meta-materials integrated into the EBG and antenna designs. Index Terms--Anisotropic effective medium theory (AEMT), antenna miniaturization, electromagnetic band-gap (EBG) materials, finite-difference time-domain (FDTD) technique, magneto-dielectrics, meta-materials, periodic structures.

In this paper, a scaled measurement system designed for wireless channel characterization is proposed and discussed, and experimental results are reported. The system consists of a vector network analyzer, miniaturized W-band transmitter and receiver probes, scaled buildings, and a precise computer-controlled positioner. The scaled propagation measurement system is designed to operate coherently at the 90.8-92.8-GHz band, and it provides a dynamic range of 85 dB. As the measurement wavelength in this system ([congruent to]3 mm) is approximately two orders of magnitude smaller than the actual wavelength in most commercial and military wireless systems, a typical dimension of the propagation environment can be smaller than the actual environment proportionally. Hence, propagation measurements can be done very efficiently under laboratory conditions using a scaled model of a city block. This system is intended for verifying the results of wave-propagation prediction software. It can also be used to investigate scattering models for different types of buildings and complex objects, data collection for wireless communication, and many other studies such as diversity methods (frequency and polarization). Index Terms--Coplanar waveguide (CPW), millimeter wave, scaled model, subharmonic mixer, wave propagation, W-band.

A new class of low insertion-loss miniaturized tilters using slot-line resonators is proposed. Miniaturization is achieved by terminating the slot line with a double-spiral inductive termination at both ends. Using this miniaturized resonator, both positive and negative couplings may be realized, and therefore, both standard coupled-line and cross-coupled quasi-elliptic filters are realizable. The unloaded Q of these slot-line filters is considerably higher than that of miniaturized microstrip filters of comparable dimensions due to the inherent higher Q of the slot line. To demonstrate the validity of the design procedures and the performance characteristics, two different types of tilters were fabricated and tested. One is a four-pole Chebyshev filter and the other is a quasi-elliptic tilter where, in each case, the full-wave simulations show very good agreement with measurements. Index Terms--Microstrip filters, microwave filters, miniaturized filters, quasi-elliptic filters, slot-line filters.

In this paper, exact and approximate multipole representations for fields of a horizontal or a vertical infinitesimal dipole above impedance surfaces are derived from the exact image formulation. These multipole expansions are appropriate for applications in the method of moments through a formulation known as the fast multipole method. Unlike the existing approximate formulations, these new multipole representations of the Green's function for the half-space impedance surface problem do not impose any restrictions on the location of source or observation points and are computationally very efficient. Index Terms--Green's function, method of moments (MoM), multipole half-space.