Oriented, single phase thin films (∼5000 A thick) of Ba0.5Sr0.5TiO3 (BST) have been deposited onto (100) MgO and (100) LaAlO3 (LAO) substrates using pulsed laser deposition. The capacitance and dielectric Q (1/tan δ) of as-deposited and annealed films have been measured from 1 to 20 GHz as a function of electric field (0–80 kV/cm) at room temperature using interdigitated Ag electrodes deposited on top of the film. For films deposited onto MgO, it is observed that, after a postdeposition anneal (1000–1200 °C), the dielectric constant decreases and the dielectric Q increases. For films deposited onto LAO, a postdeposition anneal (⩽ 1000 °C) resulted in a significant increase in the dielectric constant and a decrease in Q. The observed dielectric properties of the BST films are attributed to the changes in film stress, which affects the extent of ionic polarization.

The effect of annealing on the microwave properties of Ba0.5Sr0.5TiO3 thin films

Simulations and measurements are presented for an all-metal flared-notch array element in both single and dual-polarization configurations. The ultrawideband radiator exhibits an operational bandwidth of 12:1 for broadside scan and 8:1 bandwidth at a 45-degree scan in all planes, maintaining active VSWR<;2. The feed consists of a direct coax-to-slot-line transition that mounts directly into the base of the radiator. The all-metal flared-notches are machined from common metal stock and fed via SMA coaxial connectors. No soldering is required for any part of the design-including the feed-and assembly is simple and modular. The array parts are machined using a high-precision wire-EDM cutting technology, ensuring that measurements (in the 700 MHz-9 GHz range) are repeatable and give close agreement with theory, even through multiple assembly cycles of the modular construction system. This paper presents results for a 32-element linear array of horizontal elements and also an 8 × 8 planar array of dual-polarized elements, comparing measurements with full-wave simulations of the complete finite array structures.

Ultrawideband All-Metal Flared-Notch Array Radiator

The effect of in-plane strain on the nonlinear dielectric properties of $\mathrm{SrTi}{\mathrm{O}}_{3}$ epitaxial thin films is calculated using density-functional theory within the local-density approximation. Motivated by recent experiments, the structure, zone-center phonons, and dielectric properties with and without an external electric field are evaluated for several misfit strains within $\ifmmode\pm\else\textpm\fi{}3%$ of the calculated cubic lattice parameter. In these calculations, the in-plane lattice parameters are fixed, and all remaining structural parameters are permitted to relax. The presence of an external bias is treated approximately by applying a force to each ion proportional to the electric field. After obtaining zero-field ground state structures for various strains, the zone-center phonon frequencies and Born effective charges are computed, yielding the zero-field dielectric response. The dielectric response at finite electric field bias is obtained by computing the field dependence of the structure and polarization using an approximate technique. The results are compared with recent experiments and a previous phenomenological theory. The tunability is found to be strongly dependent on the in-plane lattice parameter, showing markedly different behavior for tensile and compressive strains. Our results are expected to be of use for isolating the role of strain in the tunability of real ultrathin epitaxial films.

Tunability of the dielectric response of epitaxially strained SrTi O 3 from first principles

An efficient recursive eigenfunction solution for the problem of scattering by a multilayer chiral circular cylinder, with or without a surface impedance center cylinder, is presented. For an M layer cylinder, the solution requires the multiplication of M4*4 matrices, versus the solution of a 4M*4M matrix equation in a standard approach. The problem of a transverse electric (TE) incident plane wave differs from that of a transverse magnetic (TM) wave only by a plus/minus sign at one step in the solution. The addition of the surface impedance center cylinder requires only one additional matrix multiplication. Numerical results, including echo width and internal fields, are presented for several sample cylinders. >

Scattering by a multilayer chiral cylinder

A fundamentally different approach to broadband array design has been developed and validated with measured hardware. Traditional array design is based on designing an isolated element with the desired bandwidth and radiation characteristics and does not account for the mutual coupling that occurs when this element is placed in an array environment. It is well known that undesired mutual coupling in an array can cause dramatic changes in antenna element impedance and radiation patterns. Harris' new approach treats the array aperture as a periodic surface so that the performance of elements within the array environment is assured. The approach is similar to that used to design a frequency selective surface, another periodic, highly-coupled structure. The resultant array apertures are inherently low-profile in cross-section, amenable to conformal implementations, and are able to maintain efficient characteristics over large scan volumes with bandwidths approaching a decade. This paper will present an introduction to this design methodology supported by predicted and measured results from several array designs.

A New Approach to Broadband Array Design using Tightly Coupled Elements

An integral equation and method-of-moments (MM) solution to the problem of scattering by an inhomogeneous chiral cylinder of arbitrary cross section is presented. The volume equivalence theorem for chiral media is developed and used to formulate a set of coupled integral equations for the electric and magnetic volume polarization currents representing the chiral cylinder. These coupled integral equations are solved using a standard pulse basis and point-matching MM solution. Numerical results, including echo width and internal fields, are presented for the scattering by chiral slabs and circular cylinders. These results are compared to exact solutions when available. >

Scattering by a chiral cylinder of arbitrary cross section

A 5:1 bandwidth stripline notch array antenna is designed from parametric investigations of flare and feed dimensions. The finite-difference time-domain (FDTD) method is employed to perform the parametric studies. Both linear and planar single-polarization arrays are considered with half-wavelength element spacing at the highest frequency. The linear array elements depend upon E-plane element mutual coupling to achieve wideband behavior. Edge elements, which cannot benefit from full E-plane coupling, are shown to maintain good transmit performance with the application of amplitude tapering. The planar array is shown to have a scanability (active VSWR <2) averaging 51/spl deg/ off broadside in the E-plane and exceeding 60/spl deg/ in the H-plane. As an infinite planar array, the antenna is predicted to have a bandwidth exceeding 7:1 on broadside. Measurements are in good agreement with the computations.

Design of a 5:1 bandwidth stripline notch array from FDTD analysis

Right and left circular vector potentials are developed and used in a spectral-domain solution for a microstrip transmission line on a chiral substrate. These vector potentials have properties similar to those of the usual magnetic and electric vector potentials, except that they result in circular rather than linearly polarized fields, thereby simplifying field expansions in chiral media. The chiral microstrip line does not have bifurcated modes like other chiral guided wave structures; however, the chiral substrate causes a significant asymmetry in both the fields and currents. >

A microstrip line on a chiral substrate

Thin (0.3–50 μm) films of SrxBa(1−x)TiO3 (SBT) with x=0.35 − 0.8 have been deposited by pulsed laser deposition onto single crystals of MgO, LaAlO3 and thin films of YBa2Cu3O7−δ. The SBT films were characterized morphologically, structurally and electrically at frequencies ≤ 13 GHz. On MgO, smooth, oriented single phase films were obtained at substrate temperatures ≥ 825°C. Films as thick as 10 μm were exclusively (100) oriented. At thicknesses approaching 50 μm SBT films were phase pure but polycrystalline. The dielectric constant of SBT films, as determined from patterned structures using microstrip geometries, was ∼20% of that observed in bulk SBT (emax(film)=1100 for x=0.5 at 200 K). The temperature dependence of the dielectric constant was broad in comparison to the sharply peaked behavior of the bulk material. The ferroelectric thin film properties described, while significantly different from bulk material, are very encouraging for use in active devices at microwave frequencies.

M. S. Kluskens

Papers

Scattering by a multilayer chiral cylinder

Scattering by a chiral cylinder of arbitrary cross section

Design of a 5:1 bandwidth stripline notch array from FDTD analysis

A microstrip line on a chiral substrate

SrxBa(1−x)TiO3 thin films for active microwave device applications