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Browsing by Author "Alsawaha, Hamad Waled"

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    New Designs for Wideband Hemispherical Helical Antennas
    Alsawaha, Hamad Waled (Virginia Tech, 2008-07-29)
    A unique property of spherical and hemispherical helical antennas is that they provide very broad half-power beamwdiths and circular polarization over a narrow bandwidth. In this thesis, new designs for hemispherical helical antennas are introduced that provide significant improvement in bandwidth, while maintaining the directivity and half-power beamwidth of the basic design. In the basic design, a simple wire of circular cross section is wound on the surface of a hemisphere, whereas in the proposed new designs a metallic strip forms the radiating element. Furthermore, the metallic strip may be tapered and tilted relative to the hemispherical surface, allowing wider bandwidth to be achieved. The antenna is fed by a coaxial cable with the inner conductor connected, through a matching section, to the radiating strip and its outer conductor connected to a ground plane. Radiation properties of the proposed hemispherical helical antennas are studied both theoretically and experimentally. A commercial software, based on the method of moments, is used to perform the numerical analysis of these helices. Three-dimensional far-field patterns, axial ratio, directivity, and voltage standing-wave ratio (VSWR) are calculated for several designs. The impacts of tapering as well as tilting of the metallic strip on radiation characteristics are examined. Also, matching of the proposed hemispherical antennas to 50â ¦ transmission lines is addressed. A 4.5-turn hemispherical helix with tapered radiating element and zero degree tilt angle, (metallic strip is perpendicular to the hemisphere axis of symmetry) provides the largest overall bandwidth. A nonlinearly tapered matching section is incorporated into the design in order to reduce the VSWR. For this design, an overall measured bandwidth of about 24% at a center frequency of 3.35 GHz is achieved. Over this bandwidth, the axial ratio remains below 3 dB, the VSWR is less than 2, and the directivity is about 9 ±1 dB. A half- power beamwidth of 70° is also obtained. A prototype of the best design was fabricated and tested using the VT indoor antenna range. Radiation patterns, the scattering parameter S₁₁, and the axial ratio were measured. The measured and simulated results agree reasonably well. In particular, agreements between measured and calculated far-field patterns and VSWR are quite remarkable. This compact, low profile antenna might find useful applications in avionics, global positioning systems (GPS), and high data rate wireless communication systems.
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    Synthesis of Ultra-Wideband Array Antennas
    Alsawaha, Hamad Waled (Virginia Tech, 2014-01-20)
    Acquisition of ultra-wideband signals by means of array antennas requires essentially frequency-independent radiation characteristics over the entire bandwidth of the signal in order to avoid distortions. Factors contributing to bandwidth limitation of arrays include array factor, radiation characteristics of the array element, and inter-element mutual coupling. Strictly speaking, distortion-free transmission or reception of ultra-wideband signals can be maintained if the magnitude of the radiated field of the array remains constant while its phase varies linearly with frequency over the bandwidth of interest. The existing wideband-array synthesis methods do not account for all factors affecting the array bandwidth and are often limited to considering the array factor and not the total field of the array in the synthesis process. The goal of this study is to present an ultra-wideband array synthesis technique taking into account all frequency-dependent properties, including array total pattern, phase of the total radiated field, element field, element input impedance, and inter-element mutual coupling. The proposed array synthesis technique is based on the utilization of frequency-adaptive element excitations in conjunction with expressing the total radiated field of the array as a complex Fourier series. Using the proposed method, element excitation currents required for achieving a desired radiation pattern, while compensating for frequency variations of the element radiation characteristics and the inter-element mutual coupling, are calculated. An important consideration in the proposed ultra-wideband array design is that the "phase bandwidth", defined as the frequency range over which the phase of the total radiated field of the array varies linearly with frequency, is taken into account as a design requirement in the synthesis process. Design examples of linear arrays with desired radiation patterns that are expected to remain unchanged over the bandwidth of interest are presented and simulated. Two example arrays, one with a wire dipole as its element and another using an elliptically-shaped disc dipole as the element are studied. Simulation results for far-field patterns, magnitude and phase characteristics, and other performance criteria such as side-lobe level and scanning range are presented. Synthesis of two-dimensional planar arrays is carried out by employing the formulations developed for linear arrays but generalized to accommodate the geometry of planar rectangular arrays. As example designs, planar arrays with wire dipoles and elliptical-shaped disc dipoles are studied. The simulation results indicate that synthesis of ultra-wideband arrays can be accomplished successfully using the technique presented in this work. The proposed technique is robust and comprehensive, nonetheless it is understood that the achieved performance of a synthesized array and how closely the desired performance is met also depends on some of the choices the array designer makes and other constraints, such as number of elements, type of element, size, and ultimately cost.