Rheometry experiment for a wire antenna aboard spacecraft at low frequencies

Abstract

For satellite observation of the electric fields of waves, a dipole antenna using a wire (wire antenna) is often used. An antenna with probes on both ends (probe antenna) is used for observation of the static electric field. For calibration of these antennas, the effective length of the antenna is a problem. In general, in AC wave observation, the effective length of the wire antenna is half the total length L and hence L/2. It has been verified that this is correct at frequencies of several hundred hertz or higher. However, in the GEOTAIL antenna, the same antenna is co-used for wave observation and the static field observation. Calibration is performed with an effective length of L/2 for wave observation with an effective length of L for static electric field measurement. Reliable data are obtained in both cases. This paper reports a rheometry experiment in which the output voltage of a small antenna is measured in water for the wire antenna, obtaining the frequency dependence of the effective length in the low-frequency region. It is found that the frequency dependence of the effective length depends on the antenna configuration. When a wire antenna with only its side surfaces insulation coated is placed in a conductive medium such as water or plasma, it is found that the effective length approaches L at low frequencies and L/2 at higher frequencies. This result is theoretically analyzed by means of an equivalent circuit and it is found that the phenomenon depends on the balance among surrounding medium, the insulation coating of the wire, and the input impedance of the receiver. © 2007 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 90(9): 45–53, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.20353