Alireza Motevasselian

A Circularly Polarized Cylindrical Dielectric Resonator Antenna Using a Helical Exciter

A novel circularly polarized cylindrical dielectric resonator antenna excited by an external tape helix is presented. The helix is fed by a coaxial line through a small hole on a finite size ground plane. The configuration offers a compact and easy to fabricate feeding network providing a 3 dB axial-ratio bandwidth of 6.4%. A prototype of the proposed configuration is fabricated and measured. Measured and simulated return loss, axial-ratio, radiation pattern, and realized gain are presented and discussed together with design guidelines.

A Helix Excited Circularly Polarized Hollow Cylindrical Dielectric Resonator Antenna

In this letter, we use a helix to excite two orthogonal HE11δ modes in phase quadrature in a hollow cylindrical dielectric resonator antenna to generate circular polarization. As compared to our previous work, it is shown that replacing the cylindrical dielectric resonator antenna with a hollow cylindrical-shaped one leads to a significant enhancement in the 3-dB axial-ratio bandwidth. A design guideline is provided, and three design cases are presented. A prototype for one of the designs is fabricated and measured. Measured and simulated data are presented and discussed. It is shown that the 3-dB axial-ratio bandwidth increases as the wall thickness of the hollow cylindrical dielectric resonator decreases. Furthermore, a reduction in the wall thickness requires a longer helix exciter for similar performance .

SPECTERAL DOMAIN ANALYSIS OF RESONANT CHARACTERISTICS AND RADIATION PATTERNS OF A CIRCULAR DISK AND ANNULAR RING MICROSTRIP ANTENNA ON UNIAXIAL SUBSTRATE

A full-wave analysis for determining the resonantfrequency, quality factor and far-zone radiation patterns of a circulardisk and annular ring microstrip patches, printed on a uniaxialanisotropic su ...

Partially Transparent Jaumann-Like Absorber Applied to a Curved Structure

A Jaumann absorber with its metal backing replaced with a combined low-pass and polarizer FSS is investigated with respect to its absorption and its polarization-dependent low-frequency transparenc ...

Using multiple modes to reconstruct conductor locations in a cylindrical model of a power transformer winding

In the present thesis, we propose and investigate a new approach to diagnose the effects of the various degradation mechanisms, including thermal degradation at hot spots, winding deformations due to the mechanical forces from short circuit currents, partial discharges due to local electric field surges, and increased moisture levels in the cellulose insulation due to decomposition, that affect electric power transformers during their normal operation in an electric power grid.Although the proposed diagnostics method can in principle be used to detect various degradation mechanisms mentioned above, we focus in the present thesis on mechanical deformations of transformer winding structures. Such mechanical deformations are most often caused by mechanical forces from short circuit currents, but they may also be caused by initial manufacturing errors and inconsistencies not detected by the power transformers’ suppliers quality assurance processes.We model a transformer winding surrounded by the transformer-tank wall and the magnetic core as a two-dimensional parallel plate waveguide or as a three-dimensional coaxial waveguide, where one metallic boundary (plate or cylinder) represents the wall of the transformer tank and the other metallic boundary (plate or cylinder) represents the iron core that conducts the magnetic flux. In between there is a set of parallel or coaxial conductors representing the winding segments.The new principle proposed in the present thesis is to insert a number of antennas into a transformer tank to radiate and measure microwave fields interacting with metallic structures and insulation. The responses from the emitted microwave radiation are expected to be sensitive to material properties that reflect the changes caused by any harmful deterioration processes mentioned above. Specifically, we investigate the mechanical deformations of transformer winding structures by determining the locations of the individual winding segments or turns, using measurements of the scattered fields at both ends of the winding structure. We solve the propagation problem using conventional waveguide theory, including mode-matching and cascading techniques.The inverse problem is solved using modified steepest-descent optimization methods. The optimization model is tested by comparing our calculated scattering data with synthetic measurement data generated by the commercial program HFSS.A good agreement is obtained between the calculated and measured positions of winding segments for a number of studied cases, which indicates that the diagnostics method proposed in the present thesis couldbe potentially useful as a basis for the design of a future commercial on-line winding monitoring device. However, further development of the theoretical analysis of a number of typical winding deformations, improvements of the optimization algorithms and a practical study with measurements on an actual power transformer structure are all needed to make an attempt to design a commercial winding monitoring device feasible.

Online power transformer diagnostics using multiple modes of microwave radiation to reconstruct winding conductor locations

Abstract A novel approach to diagnostics of transformer winding deformations, caused by mechanical forces from short-circuit currents, is presented. We employ a simple model of a transformer as a two-dimensional parallel plate waveguide. The upper plate represents the transformer tank wall and the lower plate represents the iron core which carries the magnetic flux. Between the two plates, we model the transformer winding by a set of parallel conductors. We utilize commercial simulation software to simulate the generation and measurement of microwave radiation at both ends of the winding structure. The radiation interacts with the metallic structures in the model waveguide. The measured responses from the model waveguide are expected to be sensitive to mechanical deformations of the transformer winding. We use conventional waveguide theory to solve the direct propagation problem, and an optimization method to solve the inverse problem. In particular, we determine the locations of winding segments, and obtain a good agreement between reconstructed and true conductor positions.

Radar cross section reduction of aircraft wing front end

In this paper we present a comparison of two methods for radar cross section reduction (RCSR) of a wing-front profile. The aim is to reduce RCS over the frequency range 1–16 GHz in a forward sector of the traveling direction. We achieved a 5 dB reduction of the two dimensional mono-static radar cross section (RCS) for 68% (or more) of the frequency band for the polarization with the electric field parallel to wing surface (TMz). It is shown that the Jaumann screen has a better RCSR performance than a shape optimized screen in the present case. The total thickness of the Jaumann screen is only 4.5 mm. The 5 dB 2D-RCSR frequency range for the orthogonal polarization in which the electric field is orthogonal to axis of periodicity (TEz) is over 43% of frequency band.

A partially transparent Jaumann absorber applied to an aircraft wing profile

A planar Jaumann absorber [1] is constructed by placing one or more resistive sheets spaced a quarter wavelength (λ/4) apart in front of a metal plate in a stratified medium. Non-planar Jaumann absorbers has been examined in [2] and it has been demonstrated that they effectively reduce the monostatic radar cross-section over a wide frequency band on an aircraft wing. In the present paper we examine the possibility to make the absorber partially transparent as to allow placing an antenna inside the wing structure under a Jaumann like absorber designed for the 2–16 GHz frequency band. As a first step toward transparency we replace the metal plate of the Jaumann absorber by a frequency selective structure (FSS). The FSS is designed to be as transparent as possible at antenna operation frequency while keeping its metallic behavior over radar cross-section reduction (RCSR) frequency range.

Miniaturization of a Circular Patch Microstrip Antenna Using an Arc Projection

A semicircle arc projection is introduced to miniaturize the patch in circular patch microstrip antennas. The idea has been inspired from the effectiveness of a cuboid ridge in size reduction of a rectangular patch microstrip antenna. A prototype of the proposed antenna together with a prototype of a conventional circular patch antenna has been fabricated and measured. The two antennas are compared, and the results are presented and discussed. A parametric study is also carried out on the characteristics of the arc projection.

Bandwidth enhancement of helix excited cylindrical dielectric resonator antennas by means of stacking

Stacking method is used to enhance the 3 dB axial ratio and impedance bandwidth of the circularly polarized cylindrical dielectric resonator antennas excited by an external tape helix. In this method, a dielectric cylinder with lower permittivity than the basement cylindrical dielectric resonator is placed concentrically on top of the basement cylinder. The new configuration offers an axial ratio bandwidth up to 11% and impedance bandwidth of 31%.