Use of Admittance-Wall Models in Analysis of Radiating Line Antennas
Tammen, David John
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https://hdl.handle.net/2142/72025
Description
Title
Use of Admittance-Wall Models in Analysis of Radiating Line Antennas
Author(s)
Tammen, David John
Issue Date
1993
Doctoral Committee Chair(s)
Mayes, Paul E.
Department of Study
Electrical Engineering
Discipline
Electrical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Electronics and Electrical
Physics, Radiation
Abstract
This thesis describes the development of analytical models for two radiating line antenna structures: the traveling-wave microstrip antenna and the Annular Sector Radiating Line (ANSERLIN) antenna. The models provide data and insights into the operation of the antennas without requiring large amounts of computation. It was shown that the general characteristics of the radiation patterns of ANSERLIN elements can be obtained using only a small number of circularly polarized spherical modes in the field expansion. A first-order model is obtained by solving for the fields of a closed waveguide with perfect electric conducting (PEC) top and bottom walls and perfect magnetic conducting (PMC) side walls. The propagation constant obtained from these fields of the closed waveguide is then used in the equivalent magnetic current filaments to obtain the radiated fields. Use of the propagation constant of the closed waveguide fields does not include the fringing and radiation behavior. A second-order model was developed in which the radiation loss and fringing fields are included by putting a small complex admittance on the side walls rather than having PMC side walls. Two formulations were developed to include the radiation loss in the representation: a coupled-mode formulation and an eigenvalue formulation. The coupled-mode formulation, which is similar to that described by Schelkunoff for a waveguide with lossy conductors, is used to obtain the complex-valued propagation constant. In the eigenvalue formulation, the eigenvalues obtained from the solution of the characteristic equation of the waveguide with admittance side walls are used to obtain the complex-valued propagation constant. This complex-valued propagation constant, which includes the fringing and radiation behavior, is then used in the equivalent magnetic current filaments to obtain the radiated fields. Good agreement was found between measured results and results computed using the eigenvalue formulation for thin antenna structures. The agreement was not as good for thicker structures or for results obtained using the coupled-mode formulation. A study of the modal distribution as a function of frequency was done for the ANSERLIN antenna. For a given geometry, it was found that as the frequency was increased more coupling into the higher-order circularly polarized free space modes occurred. This shifting of the mode mixtures with changing frequency explains the beam tilt observed for the ANSERLIN antenna.
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