Investigation into Polarization of Unloaded and Loaded Microstrip Square-Ring Antennas

The polarization characteristic of unloaded and loaded square-ring microstrip antennas is investigated. Several different loading types like single-stub, dual-stub, notch, gap and shorting-pin are considered and their effects are studied. Loading enables feeding using a 50-Omega probe. The simulation and measurement results show that the loading techniques excite a loaded TMy ₁₁ mode that is orthogonal to the unloaded TMx ₁₁ mode. This indicates that by loading the ring antenna its polarization can be switched adaptively. However, the purity of the loaded and unloaded modes depends on the loading type. For some loading types like gap and shorting-pin, the excitation of the unloaded mode seems negligible, in comparison to the loadings by stub and notch. For the stub and notch loaded antennas the unloaded mode is also present, and its excitation efficiency is frequency dependent. Thus, their polarization plane, which is due to both loaded TMy ₁₁ and unloaded TMx ₁₁ modes, also becomes frequency dependant. The results of this investigation can be useful in selecting the loading methods for high-impedance microstrip ring antennas, and control of their impedance and polarization. The knowledge of the antenna polarization is essential in communications, and its dependence on the loading type can be used as an important parameter in design of adaptive antennas and sensors.     

Characteristics of aperture coupled microstrip antennas withvarious radiating patches and coupling apertures

A full wave moment method is applied to the analysis of aperture coupled microstrip antennas, in which all components of the electric and equivalent magnetic surface currents are considered. The electric current distributions on the rectangular patch for different coupling aperture positions are presented with their radiation patterns. The effects of the coupling aperture shape and size on the input impedance and radiation performance are also discussed. As an example of new radiators, slotted patches are studied, and it is shown that they can be used to achieve dual-frequency operation     

Phase shift bandwidth and scan range in microstrip arrays by the element frequency tuning

In this paper, the far-field phase shift properties of microstrip patch antennas are investigated. It is shown that, similar to reflectarrays, the resonant nature of microstrip patches can be used to change the phase of the radiated field. This phase change can be caused by the dimensional change of the microstrip patch, or by a reactive loading of its cavity such as an aperture on its ground plane. However, the available phase shift is limited by the antenna impedance bandwidth. The problem is initially investigated for conventional patch antennas, determining the available phase shift range. It is then studied for a wideband E-slot microstrip antenna, showing a considerably larger phase shift range. Then, a micro-electro-mechanical (MEM) based ground plane membrane, activated by an electrode from below, is proposed to adaptively generate and control the required phase shifts. It provides a low loss, continuously variable phase shifter that can be used at high frequencies for beam scanning in small arrays.     

Integral equation formulation of radiation from rectangular waveguides and horns

The radiation field patterns of rectangular waveguides and horns of finite dimensions are obtained numerically. The electric field integral equation is formulated to relate their radiation patterns to the surface current distribution. These currents are determined numerically by reducing the integral equation to a matrix equation, using the moment method. The computed currents are then used to calculate the radiation patterns and cross-polar fields of the finite wavegiudes and horns. The method is numerically efficient and can be applied to the computation and optimization of the antenna feed configurations.     

Radiation Characteristics of Bent-Wire Antennas

A mathematical model, which predicts the far-field power pattern of a wire antenna bent in one plane, is developed, and the classical approximation of a sinusoidal current distribution along the arc length is employed, neglecting all end and mutual coupling effects. General expressions for the complex Poynting vector, electric- and magnetic-field intensities are also derived in terms of the retarded vector potential. Although the theory for parabolically-bent antennas compares favorably with experiment, it is shown that the agreement is more favorable when the current distribution is computed by the method of moments. In contrast to the figure-eight ?-plane pattern of the conventional straight dipole, the circularly and parabolically bent antennas operating at the first resonance are shown to have radiation patterns approaching omnidirection as the bending curvature is optimized, thus lending themselves to use as replacements for many common dipole applications for gain standards, field probing, reflector feeds, as well as mobile and broadcast antennas.     

Effect of bank slope on the flow patterns in river intakes

In this work, we studied the dimensions of stream tube in the vertical as well as inclined bank conditions. Data were collected from both a physical model and a 3-D numerical model（SSIIM 2）. Equations for predicting stream tube dimensions were presented and compared with existing formulae. In comparison with vertical bank, it is found that inclining bank causes the bottom stream tube width to be greater than at the surface. The strength of secondary current formed at the entrance of branch channel is reduced. These changes in flow pattern can reduce the amount of sediment delivery into the intake.     

Realization theory for two-time-scale distributions through approximation of Markov parameters

In this article, the problem of approximating the Markov parameters of a two-time-scale (TTS) distribution is studied. It is shown that the Markov parameters of a TTS distribution can be approximated in terms of the Markov parameters of its fast distribution only. This is an O(ε^1?i ) approximation, which deteriorates by a factor of 1/ε at each step as higher order Markov parameters are computed. In order to use the Markov parameters of the slow distribution in the approximation scheme, an inversion map is introduced by which a TTS distribution and its slow distribution are mapped into two new distributions. It is then shown that every Markov parameter of the inverted distribution approximates that of the inverted slow distribution with an O(ε) accuracy. An approximate expression for the Hankel matrix of the Markov parameters of a TTS distribution is also obtained. This expression is in the form of a telescopic series and involves the Markov parameters of the fast distribution only.     

Qualitative robust fuzzy control with applications to 1992 ACCbenchmark

Robust control has long been the purview of quantitative linear control techniques, while qualitative symbolic control has been deemed more suitable to obtaining complex control objectives that require only low-output precision. The intelligent techniques of fuzzy control have, however, shown promise in obtaining results comparable to those obtained from H_∞ and H₂ robust control techniques. Often though, these fuzzy control techniques ignore the original intent of fuzzy logic: implementation of symbolic linguistic control laws based on qualitative models of the plant and control behaviors. We show that robust control objectives, even for simple plants, can be achieved by first developing qualitative behaviors that stabilize the plant and then superimposing tracking behaviors that achieve control objectives. Specifically, by superimposing qualitative stability and tracking behaviors, we can achieve robustness and tracking stability comparable to the best published linear compensators for the 1992 ACC robust control benchmark     

Analysis and Design of a Micromachined Electric-Field Sensor

In this paper, the design and experimental results for a novel micromachined electric-field sensor are presented. The operation of the sensor is based on chopping an incident field with a shutter and measuring the induced charge on two sets of electrodes situated below the shutter. Employing of thermal actuators to move the shutter allows for substantial reduction in drive-signal amplitude as compared to electrostatic actuators which has consequently resulted in less interference from the drive signal. Moreover, the drive and sense signals are separated in the frequency domain owing to the inherent nonlinearity of thermal actuators, further improving the accuracy and resolution of the measurements. The relatively small displacements produced by thermal actuators are mechanically amplified using a novel lever mechanism. The sensor is capable of measuring fields as small as 42 V/m using actuation signals on the order of 60 mV.     

Performance of offset prime focus reflectors as a function of feed orientation

Radiation characteristics of the offset paraboloidal reflectors illunminated by a tilted prime-focus feed are studied employing the physical optics current method. A family of curves is generated which indicates the optimum orientation of the axis Of the feed for maximum antenna efficiency as a function of the offset anglebetaand the half-anglealphasubtended by the rim of the reflector at its focus. A comparative study on efficiency and sidelobe levels is presented for different excitation angles of the feed axis.