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This paper reports the design, construction and electromagnetic performance of a new freestanding frequency selective surface (FSS) structure which generates coincident spectral responses for dual polarization excitation at oblique angles of incidence. The FSS is required to allow transmission of 316.5-325.5 GHz radiation with a loss [less than or equal to] 0.6 dB and to achieve [greater than or equal to] 30 dB rejection from 349.5-358.5 GHz. It should also exhibit crosspolarization levels below -25 dB, all criteria being satisfied simultaneously for TE and TM polarizations at 45[degrees] incidence. The filter consists of two identical, 30 mm diameter, 12.5 [micro]m thick, optically flat, perforated metal screens separated by 450 [micro]m. Each of the [approximately equal to] 5000 unit cells contains two nested, short circuited, rectangular loop slots and a rectangular dipole slot. The nested elements provide a passband spectral response centered at 320 GHz in the TE and TM planes; the dipole slot increases the filter roll-off above resonance. The FSS was fabricated from silicon-on-insulator wafers using precision micromachining and plating processes including the use of deep reactive ion etching (DRIE) to pattern the individual slots and remove the substrate under the periodic arrays. Quasi-optical transmission measurements in the 250-360 GHz range yielded virtually identical copolarized spectral responses, with the performance meeting or exceeding the above specifications. Experimental resuits are in excellent agreement with numerical predictions. Index Terms--Dichroic filters, freestanding mesh, frequency selective surfaces (FSSs), millimeter and submillimeter waves, quasioptical filters.

The mutual coupling between dipole antenna array elements using a planar meta-surface as superstrate is experimentally investigated. The meta-surface is based on grids of short metal strips and continuous wires. A comparison between the mutual coupling when the dipoles are radiating in free space and in presence of the superstrate is presented. On average, between 3 to 14 dB reduction of the mutual coupling is achieved when the superstrate is used. The effect of the mutual coupling on the radiation performance of the array is studied by spherical near-field measurements of the radiation pattern when one driven dipole is fed and the others are matched with 50 [OMEGA] loads. The back-projected field on the aperture and on the E-plane is shown. Index Terms--Dipole antenna array, meta-surface, mutual coupling.

Two different electromagnetic bandgap (EBG) superstrate array antenna configurations, intended for the Wide Area Augmentation Service space segment, are presented in this paper. The described antenna configurations take advantage of the directivity enhancement produced by a semireflective sheet placed parallel to a metallic ground plane. The first design presented is realized using a 2 x 2 circularly polarized (CP) patch array illuminating an EBG superstrate composed of a square pattern of circular holes etched in a thin metallic sheet. The second design consists of a 2 x 2 CP helix array feeding a hexagonal pattern of holes etched into a metallic EBG superstrate. Both configurations have been designed, breadboarded, and measured, and excellent agreement between simulations and measurements has been recorded. The accurate control of the antenna pattern phase center variation with both the frequency and the antenna field of view, necessary for the intended navigation antenna application, has been the principal challenge of this work. The EBG technology designs presented here are simpler than conventional navigation antennas and can lead to cost reduction, beamforming network simplification, and height reduction while offering similar radio-frequency performances to equivalent products realized in conventional technology. Index Terms--Antennas, electromagnetic bandgap (EBG) structures, phase center control, Wide Area Augmentation System (WAAS).

A novel type of short monopole antenna is proposed. Its structure consists of a periodically slotted coaxial line radiating in linear polarization (LP). The structure can be easily modified by the addition of a thin layer of dipoles in order to generate circular polarization (CP). The number of slots along the structure, their periodicity and the tuning of the terminating load allow control of the antenna main beam' angle and width. One advantage of the proposed design over conventional monopoles is that the antenna does not require a ground plane. Both left-hand and right-hand CP are possible with the same design, simply by inverting the symmetry of the printed dipoles. Design details for the annular slotted coaxial line and for the LP and CP mast antennas are given. Prototypes in both LP and CP were fabricated and their characteristics were measured. Simulations and measurement are in good agreement. The proposed CP mast antenna is compliant with the requirements of the XM digital satellite radio system, with a frequency bandwidth of more than 1.7% in which VSWR < 2, and an AR < 4 dB in the elevation range of [+ or -]30[degrees] < [theta] < [+ or -]70[degrees]. Index Terms--Coaxial transmission line discontinuities, mobile antenna, monopole antenna, printed dipoles, slot arrays.

The design, fabrication, and characterization of a 500-GHz electromagnetic bandgap (EBG) based heterodyne receiver array is presented. The array contained seven planar dipole antennas that were photolithographically defined on a common 20-[micro]m-thick quartz substrate. Each antenna incorporated a Schottky diode and was connected to coplanar transmission lines that conveyed the down-converted 500-GHz signals to detectors. The quartz substrate was backed by a silicon EBG woodpile structure, which reduced antenna crosstalk and increased directivity. An off-axis parabolic mirror completed the beam-forming network. Index Terms--Electromagnetic bandgap (EBG) technology, imaging array, millimeter-wave technology.

Satellite positioning systems, like GPS and GALILEO, require ground reference antennas with high quality radiation performance. The use of a radiator integrated with an artificial magnetic conductor (AMC) appears attractive in order to obtain an antenna with low back radiation and good axial ratio performance near the horizon, which is essential for the considered applications. The presented crossed-dipole antenna integrated on a wideband AMC has a simple feed network consisting mainly of a self balanced feed. It also has a low mechanical profile and is simple to manufacture as a multilayer printed circuit board. Detailed numerical analyses and measurements are presented which clearly demonstrate its potential as a ground reference antenna. Index Terms--Artificial magnetic conductor (AMC), GALILEO, GPS, low profile, wideband.

The performance of multibeam focal plane arrays feeding a single aperture is usually reduced due to conflicting requirements on the feed elements. Dense packing is usually required to minimize the beam separation, while typically large feed apertures are needed to provide the high feed directivity to reduce spillover losses from the reflector. In this paper the use of dielectric super-layers to shape the radiation pattern of each feed is demonstrated. The shaping is obtained by exciting, according to design, a pair of TE/TM leaky waves. The spillover from the reflector is reduced without physically increasing the dimensions of each single element aperture. A prototype of a feed array composed of 19 waveguides arranged in a hexagonal lattice was designed, manufactured and tested. The measured embedded patterns provided an increase of the edge of coverage gain, with respect to the free space case, of at least 0.6 dB in an operating bandwidth (BW) of [approximately equal to] 12%. Moreover when reactive loading of adjacent feeds is adopted the increase in the edge of coverage with respect to the free space case was demonstrated to be larger than 1.6 dB over a 3% BW. Index Terms--Leaky wave antennas, leaky waves, reflector antenna feeds, reflector antennas.

The impact of mutual coupling between neighboring radiators in an imaging array configuration in the presence of a dielectric super-layer is investigated. The super-layer generally aims at increasing the directivity of each element of the array. However, here it is shown that the directivity of the embedded element patterns are reduced by a high level of mutual coupling. Thus a trade off between directivity enhancement and close packing of the array elements must be found depending on the bandwidth and the pattern requirements. Index Terms--Electromagnetic bandgap (EMB) materials, leaky waves (LWs), leaky wave (LW) antennas, reflector antenna feeds.

The design of a multifrequency dipole antenna array based on a resonant meta-surface superstrate is proposed. The behavior of a single element that is closely placed to a meta-surface is experimentally investigated. The proposed meta-surface is based on resonating unit cells formed by capacitively loaded strips and split ring resonators. By tuning a dipole antenna to the pass band of the meta-surface, the physical area is effectively illuminated enhancing the radiation performance. The gain, radiation efficiency and effective area values of the whole configuration are compared to the ones obtained with a single dipole without superstrate. Radiation efficiency values for the proposed configuration of more than 80% and gain values of more than 4.5 [+ or -] 1 dB are obtained. Based on this configuration, simulated results of a multifrequency antenna array are presented. Distinctive features of this configuration are high isolation between elements (20 dB for a distance of [[lamda].sub.0]/4), and low back radiation. Index Terms--Effective area, meta-surfaces and multifrequency arrays, metamaterials.

A dedicated low profile antenna for geodesic applications is presented. This type of antenna requires mitigation of multipath signals to achieve sub-centimeter level of precision. Typically, bulky and heavy choke ring structures have been used to reduce the effects of multipath interference. The antenna presented here consists of a low temperature co-fired ceramic (LTCC) patch antenna immersed in an electromagnetic bandgap (EBG) substrate. The EBG substrate reduces the effects of multipath by blocking the propagation of surface waves. The result is an antenna showing good multipath rejection and a stable phase center. The advantage of printed EBG substrates is that they can be realized in a low-weight, low-price, and low-profile fashion. First prototypes have been manufactured and tested, and its performance has been compared to that of a more classical choke ring antenna. Index Terms--Antenna, electromagnetic bandgap (EBG) materials, GALILEO, GPS.

A thin artificial magnetic conductor (AMC) structure is designed and breadboarded for radar cross-section (RCS) Reduction applications. The design presented in this paper shows the advantage of geometrical simplicity while simultaneously reducing the overall thickness (for the current design [lambda]/16). The design is very pragmatic and is based on a combination of AMC and perfect electric conductor (PEC) cells in a chessboard like configuration. An array of Sievenpiper's mushrooms constitutes the AMC part, while the PEC part is formed by full metallic patches. Around the operational frequency of the AMC-elements, the reflection of the AMC and PEC have opposite phase, so for any normal incident plane wave the reflections cancel out, thus reducing the RCS. The same applies to specular reflections for off-normal incidence angles. A simple basic model has been implemented in order to verify the behavior of this structure, while Ansoft-HFSS software has been used to provide a more thorough analysis. Both bistatic and monostatic measurements have been performed to validate the approach. Index Terms--Artificial magnetic conductor (AMC), electromagnetic band gap structures, radar cross-section.

The design, manufacture and characterization of a sub-harmonic mixer operating in the millimeter wavelength range is described. The mixer combines for the first time electromagnetic band gap (EBG) technology (which improves the radiation features of the dipole antenna used to couple radiation to the mixer), with conventional local oscillator (LO) waveguide circuitry. The mixer is designed to operate in an RF frequency band around 250 GHz when supplied with a LO frequency between 115 and 135 GHz. The fixed IF frequency spans 2.5-3.5 GHz. Performance predictions were made using a combination of the finite elements (FE) method to compute the embedding impedance of the diodes and the harmonic balance analysis (HBA) to predict the noise temperature. A prototype mixer has been fabricated and tested. Best mixer performance (double side band (DSB) mixer noise temperature and conversion loss of 3000 K and 11.5 dB, respectively) was measured with a LO power level of about 5 mW. Good agreement is observed with the predicted performance. Index Terms--Electromagnetic band gap (EBG) technology, millimeter wave mixers, millimeter wave technology, subharmonic mixer.

We describe the use of electromagnetic bandgap (EBG) super-layers to improve the shape of reflectors illumination function. Following an investigation of the leaky wave pole singularities of the EBG Green's function, the shape of the radiation patterns of small apertures in ground planes are optimized. The maximization of the reflector aperture efficiency can be obtained by properly tuning the super layers geometrical parameters. A prototype of such feed has been designed, manufactured and tested. The results indicate that the inclusion of the EBG, increases the aperture efficiency of the feed+reflector system to values higher than 80%, over a 10% bandwidth. These low profile feeds are compatible with printed circuit board and/or integrated technology. Index Terms--Aperture efficiency, electromagnetic bandgap (EBG) materials, leaky waves, reflector antenna feeds.

The sensitivity of the characteristic band edge frequencies of three different 500-GHz electromagnetic-bandgap crystals to systematic variations in unit cell dimensions has been analyzed. The structures studied were square bar woodpiles made with dielectric having [[epsilon.sub.r]] [approxi,ately equal to] 12 and [[epsilon.sub.r]] = 37.5 and two wide bandgap [[epsilon.sub.r]] = 37.5 crystals designs proposed by Fan et al. and Johnson and Joannopoulos. These [[epsilon.sub.r]] values correspond to high-resistivity silicon and a zirconium-tin-titanate ceramic, respectively. For the woodpiles, the fractional frequency bandgap varied very little for dimensional deviations of up to [+ or -] 5% from the optimum. The bandgaps of the Fan and Johnson and Joannopoulos structures were affected to a greater extent by dimensional variations, particular sensitivity being exhibited to the air-hole radius. For all crystals, the effect of increasing the amount of dielectric in the unit cell was to shift the bandgap edges to lower frequencies. Both silicon and ceramic woodpiles, along with a ceramic Fan structure, were fabricated and dimensionally characterized. Mechanical processing with a semiconductor dicing saw was used to form the woodpiles, while the Fan structure required both dicing and UV laser drilling of circular thru-holes. Good agreement with predicted normal incidence transmissions were found on the low-frequency side of the bandgap in all cases, but transmission values above the upper band edge were lower than expected in the ceramic structures. Index Terms--Electromagnetic.bandgap (EBG) materials, fabrication technologies, sub-millimeter-wave devices.

We show how the design of integrated arrays can significantly benefit from planar circularly symmetric (PCS) electromagnetic band gap (EBG) structures. Using this technology, a phased array that scans up to 40[degrees] in one dimension and that is characterized by relatively large bandwidth (BW [approximately equal to] 15%) is designed, manufactured and tested. The specific advantages coming from the use of PCS-EBGs are two fold. On one hand the losses associated to surface waves are significantly reduced. On the other hand each element of the array has a larger effective area that leads to a higher gain for the complete array when compared with a standard technology. Additional benefits are the low cross-polarization levels, the good front to back ratio considering that the antenna does not include a backing reflector, and the low profile. Index Terms--Electromagnetic band gap (EBG), phased arrays, printed technology, surface waves.

In this paper, the mutual coupling between dipole antennas placed on top of a woodpile structure is experimentally investigated. The coupling between dipoles backed by a woodpile substrate is compared with that obtained when the dipole antennas are placed on a dielectric substrate and when they radiate into free space. On average, between 2 and 7 dB reduction of the mutual coupling is achieved with respect to the dielectric substrate case when the woodpile substrate is used. The free space coupling is also reduced in the H-plane configuration case. However, the E-plane coupling slightly increases due to modification of the radiation pattern. Imaging array applications could potentially benefit from the use of EBG substrates due to the improved isolation between pixels. Index Terms--Electromagnetic bandgap (EBG) technology, imaging array, mutual coupling, planar antennas.

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, [[lambda].sub.sw]/2. Index Terms--Electromagnetic bandgap (EBG), planar circularly symmetric electromagnetic bandgap (EBG), planar printed antennas, slot antennas, surface waves, wideband antennas.

This paper discusses the design and analysis of planar circularly symmetric (PCS) electromagnetic band gap (EBG) structures for reducing the surface waves excited by printed antennas on dense dielectric substrates. The key advantage of the circularly symmetric geometries is that a surface wave generated by a source located at its center experiences the same band gap effect in all radial directions. To obtain simple design rules of the PCS-EBGs for the optimization of the bandwidth, an equivalence is established between 2-D-EBGs and PCS-EBGs. Integrated planar printed antennas with bandwidths up to 20% are designed, manufactured and tested. Index Terms--Circular structures, dispersion diagrams, electromagnetic band gap (EBG), printed antennas, surface waves.

An analysis including the temperature dependence of the permittivity and loss tangent of three low-cost and high-permittivity materials (zirconium--tin--titanate, alumina, and titanium--dioxide) in the terahertz frequency range is presented. Such dielectric materials find varied applications in microwave and terahertz systems and components. Their effective use under varying environmental conditions or in space applications requires a detailed knowledge about temperature dependencies. Here, measurements using broad-band terahertz time-domain spectroscopy are presented in the temperature range from 10 to 323 K. It is shown that zirconium-tin-titanate and alumina provide a good thermal stability of the permittivity, whereas the permittivity of titanium--dioxide exhibits a strong dependence on the temperature. Index Terms--Ceramics, dielectric materials, dielectric thermal factors, measurement, submillimeter waves, titanium compounds.

The work presented in this paper concerns a theoretical study on frequency selective surfaces (FSS) with application to artificial magnetic conductors or high-impedance surfaces (HIS). Current realizations of HIS are based on a planar FSS at the interface of a metal-backed dielectric slab either including vertical vias or not. A stable resonance was found for the case of series-resonance grids without vias in the slab. The resonance turns out to be unique in theory for all angles of incidence and both polarizations of plane waves illuminating the HIS. It was shown that vias destroy the stabilization effect and introduce a frequency shift. The analytical model was validated by HFSS simulations. Index Terms--Artificial magnetic conductor, frequency-selective surface (FSS), high-impedance surface, self-resonant grid, transmission line model.

A promising way to reduce the problems created by surface waves in printed phased array antennas is to use an electromagnetic bandgap (EBG) substrate instead of a standard dielectric. Two metallo-dielectric EBG substrates well suited for phased array applications have been designed and tested. One of them has a compact unit cell and exhibits a broad bandwidth performance. The other EBG structure has a narrower bandwidth but is simple to manufacture and has a low mechanical profile. Single radiating elements and a small linear array have been manufactured on a standard dielectric substrate and on the EBG substrates. The measurements show a significant improvement in radiation efficiency as well as of the front to back ratio for the antennas manufactured on the EBG substrates compared to the ones obtained for the antennas manufactured on the standard dielectric substrate. These results clearly demonstrate the effectiveness of the EBG substrates in reducing the surface and guided waves. Index Terms--Electromagnetic bandgap (EBG) materials, metamaterials, periodic structures, planar antennas.

The characterization of planar electromagnetic band-gap structures requires the calculation of the dispersion diagram of the modes supported by the periodic structure and the phase of the reflection coefficient under plane-wave illumination. In this paper, we present a novel method for the calculation of the dispersion diagram. The electromagnetic analysis is based on the method of moments/boundary integral-resonant mode expansion (MoM/BI-RME) method and leads to the formulation of a homogeneous matrix problem. The solution of this problem is performed by an iterative procedure: for a given value of the propagation phase constant, the frequency range is scanned to find the frequencies where the field equation has a nontrivial solution. The search of these frequencies is based on the tracking of the eigenvalues in the complex plane, and proved more efficient than other classical methods (direct search of the determinant zeros, singular value decomposition). The reflection coefficient can be readily determined by using the MoM/BI-RME method, already developed for the analysis of the scattering from frequency selective surfaces. The method is applied to the characterization of the classical uniplanar compact photonic-bandgap structure, and analysis results show the accuracy of the method, its efficiency, and its convergence properties. Index Terms--Eigenvalues and eigenfunctions, electromagnetic band-gap structures, integral equations, periodic structures.

A novel hybrid technology where amorphous silicon (a-Si:H) solar cells are either integrated or physically combined with printed slot antennas is presented. This basic idea is demonstrated with the help of two innovative designs where the solar cells are directly grown on a stainless steel ground-plane or glued onto a standard copper layer printed on a dielectric substrate. To show the compatibility of solar cells and antennas, the concept is used to create a linearly polarized slot antenna, a circularly polarized slot antenna and a slot array. The concept offers advantages in terms of surface coverage, volume, weight and electric performance when compared with a simple juxtaposition of antennas and solar cells. Index Terms--Circularly polarized array, slot antennas, SOLANT, solar antenna, solar cells.

A method is proposed for the analysis of slot antennas excited by coplanar waveguides. First, a standard integral equation for the continuity of the magnetic field is formulated. Then the appropriate equivalent magnetic currents of the method of moments are represented in terms of entire-domain basis functions which synthesize the resonant behavior of the slot and the field in proximity of the feeding source and of the bends. In order to define these basis functions, canonical geometries are identified, whose Green's functions have been found analytically. The accuracy and the effectiveness of the method in terms of convergence rate and number of unknowns is demonstrated by comparison with a standard fine meshing full-wave analysis. Index Terms--Coplanar waveguides, entire domain basis functions, method of moments, slot antennas, submillimeter wave antennas.

Low-loss high dielectric-constant materials are analyzed in the terahertz frequency range using time-domain spectroscopy. The dielectric constant and loss tangent for steatite, alumina, titania loaded polystyrene, and zirconium--tin--titanate are presented and compared to measurements on high-resistivity silicon. For these materials, the real part of the dielectric constant ranges from 6 to 90. All of the samples were found to have reasonable low-loss tangents. Applications as photonic crystal substrates for terahertz frequency antenna are envisaged. Index Terms--Antennas, dielectric materials, measurements, submillimeter waves.

This paper presents a rapid, versatile, and practical technique for the manufacture of layer-by-layer photonic crystals in the millimeter- and submillimeter-wave regions. Mechanical machining is used to derive a rugged layer-by-layer structure from high-resistivity silicon wafers. Unlike traditional anisotropic etching techniques, this method does not rely on any particular crystal orientation of the substrate and allows greater flexibility in the photonic crystal design. Automatic alignment of alternating layers is achieved via careful placement of the separation cuts. Using this ability, two configurations of photonic crystals are realized and their RF characteristics are measured and presented. Firstly, a symmetrical photonic crystal is studied as an initial demonstration of the technique. This is followed by an asymmetrical example, where a different frequency response is observed for the two orthogonal polarization of the incident radiation. Two measurement techniques are used to characterize the photonic crystals and the merits of each are discussed. Theoretical predictions are seen to agree well with the measured behavior. Index Terms--Photonic crystal, planar antennas, submillimeter-wave antennas, surface-wave modes.

This paper is devoted to the phenomenon of resonant transmission in three-dimensional electromagnetic band gap structures (EBGs), achieved by introducing a defect into an EBG structure with the corresponding resonance frequency of the defect lying within the initial EBG region. It is shown that the resonance frequency of an EBG air cavity (acceptor defect) can either increase or decrease with increasing cavity dimensions. In particular, this paper demonstrates both experimentally and theoretically that the resonance frequency of a woodpile air cavity, formed by the partial removal of a woodpile bar, increases with increasing cavity size; it increases continuously or saturates depending on the field polarization with respect to the cavity orientation. The results obtained in this paper can be used for designing EBG devices based on resonant transmission. [ABSTRACT FROM AUTHOR]