We examine the conical spiral antenna (CSA) for use in ground-penetrating radars (GPR). When this antenna is isolated in free space, its performance, e.g., input impedance, is nearly frequency independent over a broad range of frequencies, and it has a nearly unidirectional far-field pattern with circular polarization. An antenna with these characteristics would be useful for application in GPRs. However, in a GPR the antenna is placed close to the ground, and the near field of the antenna in the air and ground is of primary importance, not the far field. It is not clear that the desirable characteristics of the CSA in free space will be preserved in this new situation. This is the motivation for the research described. The FDTD is used to analyze the CSA over the ground. The model contains all of the details of the antenna, dissipative soil, and buried object. The time-domain results from the FDTD analysis are Fourier transformed and used to determine frequency-domain characteristics, such as, the input impedance, and the amplitude and state of polarization for the near field. First, to verify the analysis, computed impedances and far-field patterns of a CSA in free space are compared with previously measured results. Then, the model is used to investigate the near field of the CSA in free space. Characteristics of the far field, such as frequency independence and circular polarization, are shown to also apply to the near field. Next, the properties of the CSA are examined when it is placed over a typical dissipative ground. The input impedance is found still to be fairly frequency independent, and the near field within the ground possesses some of the same characteristics as it did in free space. Finally, the model is used for a preliminary examination of the CSA in a monostatic GPR.
Study of the conical spiral antenna for use in ground-penetrating radars: initial results
2000
11 Seiten, 8 Quellen
Conference paper
English
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