Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmsm23a0481e&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #SM23A-0481
Physics
7819 Experimental And Mathematical Techniques, 7855 Spacecraft Sheaths, Wakes, Charging, 7894 Instruments And Techniques, 6909 Electromagnetic Metrology, 6984 Waves In Plasma
Scientific paper
The "effective length" is an important parameter of a wire antenna. The output voltage V for an electric antenna subject to an imposed electric field E (measured in V/m) is written as V=heff × E where heff is the effective length. It is very difficult to determine the exact value of an effective length of a wire antenna onboard satellite by ground testing, because a wire antenna which observes low frequency plasma waves down to several 100 Hz becomes very long. For example, "Akebono" spacecraft has 60 m tip-to-tip antennas and "Geotail" has 100m tip-to-tip antennas. Additionally, the antenna characteristics in geospace are different from those in free space. In previous studies, the effective lengths have been usually assumed to be L for DC (static) electric fields and L/2 for AC (wave) electric fields, where L is the tip-to-tip length of a wire dipole antenna. This means that the effective length depends on the frequency of wave. In this study, we determine the characteristics of the effective length of the wire antenna at low frequencies by an experimental measurement called "Rheometry," and then by a theoretical analysis using the equivalent circuit. The result of the measurement and theoretical analysis confirms that the effective length depends on the structure of the antenna at low frequencies. According to the experiment, we can sea the same characteristic as those described above, that the value of effective length is L at low frequencies and L/2 at high frequencies. Theoretically, when the wire antennas is arranged to be parallel to the direction of the electric field, the electrostatic potential increases linearly along the wire. This is simulated as a voltage supplier which is distributed along the wire. When we calculate the voltage output of the antenna using this equivalent circuit, the result of calculation is consistent with the experimental measurement. In particular, the transition frequency of the effective length is well explained. Also we calculate the effective length of the wire antenna aboard GEOTAIL spacecraft by adding the effect of plasma sheath. We will show and discuss the calculation result.
Esaki S.
Higashi Ryoichi
Imachi T.
Matsumoto Haru
Nagano Isamu
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