I. Ederra

Thin AMC Structure for Radar Cross-Section Reduction

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 ). 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 Sievenpipers 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.

Measurement of the dielectric constant and loss tangent of high dielectric-constant materials at terahertz frequencies

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.

Broadband Radar Cross-Section Reduction Using AMC Technology

This paper presents the design, fabrication, and characterization of a planar broadband chessboard structure to reduce the radar cross-section (RCS) of an object. The chessboard -like configuration is formed by combining two artificial magnetic conductor (AMC) cells. The bandwidth limitations intrinsic to AMC structures are overcome in this work by properly selecting the phase slope versus frequency of both AMC structures. A 180 ° phase difference has been obtained over more than 40% frequency bandwidth with a RCS reduction larger than 10 dB. The influence of the incidence angle in the working bandwidth has been performed. A good agreement between simulations and measurements is achieved.

Resonant Meta-Surface Superstrate for Single and Multifrequency Dipole Antenna Arrays

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 plusmn 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 lambda0/4), and low back radiation.

Electromagnetic response and homogenization of grids of ferromagnetic microwires

This contribution presents an analytical formulation for the electromagnetic response of grids of ferromagnetic microwires, where the electromagnetic fields produced by the structure are found by means of the local field method. In addition, a circuit analogy is introduced for a better understanding of the grid response, where a single ferromagnetic microwire is modeled as an impedance-loaded wire, and the transmission-line approach is used for the whole grid. Moreover, the homogenization of the structure is considered to provide more physical insight into internal polarizations of the grid. Contrary to the previous experiments of left-handed transmission in grids of ferromagnetic microwires, it is found that such structures can be modeled as artificial dielectric slabs with a frequency dispersive permittivity.

EBG Superstrate Array Configuration for the WAAS Space Segment

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 2times2 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 2times2 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.

Coupling Reduction Between Dipole Antenna Elements by Using a Planar Meta-Surface

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.

Manufacturing Tolerance Analysis, Fabrication, and Characterization of 3-D Submillimeter-Wave Electromagnetic-Bandgap Crystals

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 epsiv rap12 and epsivr=37.5 and two wide bandgap epsivr=37.5 crystals designs proposed by Fan and Johnson and Joannopoulos. These epsivr 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 plusmn5% 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

Magnetotunable left-handed FeSiB ferromagnetic microwires.

The magnetotunable left-handed characteristics of Fe(77.5)Si(12.5)B(10) glass-coated ferromagnetic microwires are analyzed in array and single microwire configuration, employing a rectangular waveguide working in X band. While the negative permeability is ascribed to the natural ferromagnetic resonance (NFMR) of the highly and positive magnetostrictive microwire, the negative permittivity features of the medium are attributed to the interaction of the microwires with the metallic rectangular waveguide. The dependence of the NFMR frequency on the applied external magnetic field enables the design of magnetotunable left-handed systems with wide-frequency band.

A Metamaterial T-Junction Power Divider

A metamaterial based compact microstrip T-junction power divider working at 10GHz is proposed. The metamaterial unit cell consists of microstrip gaps and via holes whose behavior is equivalent to the combination of series capacitors and shunt inductors respectively, that is, a dual transmission line (high-pass) configuration. By adjusting the parameters of these structures, the characteristics of the Metamaterial-medium can be set to achieve a desired phase shift. To validate the design, a T-junction power divider is fabricated and measured. A 70% reduction of the length of the impedance transformer, without significant performance degradation, has been achieved