Active dipole antenna measurements on RPI/IMAGE satellite determine the plasma 
sheet characteristics

G.S. Sales, K. Bibl, P. Song, B.W. Reinisch, I. Galkin


The Radio Plasma Imager (RPI) aboard the IMAGE satellite was used to determine
the characteristics of the plasma sheath around the antenna when transmitting
whistler mode signals. Active antenna current measurements were made during
three sequential orbits on 29-30 Sept. 2004 while the RPI transmitter
systematically changed the transmission frequency from 8 to 23 kHz in steps of
100 Hz. This investigation was directed towards determining the input impedance
characteristics of the long thin transmitting dipole (short in terms of
free-space wavelengths) as the satellite, in a near polar orbit, moved through
the plasmasphere, over the polar cap region and out again through the nighttime
plasmasphere at altitudes ranging from 2 RE to 6 RE. The plasma frequency at the
satellite during this period ranged from 760 kHz to 40 kHz (when over the polar
cap) and most of the transmissions during these experiments were in the whistler
mode. Continuous antenna current measurements provided the tuning information.
The tuning inductance in the feed of the antenna was varied with each frequency
step and the location, in frequency, of the current peaks indicated an antenna
input capacitance of 550 ± 50 pF over a significant portion of the scanned
frequency range. The input capacitance of the RPI antenna (in free space) was
calculated to be 370 pF and measured, as the satellite left the plasmasphere
into the low-density magnetospheric cavity, to be 450 pF.

To simulate the growth and decay of the plasma sheaths surrounding the antenna,
a program was written that calculates the time variation of the charge and
current on a dipole antenna embedded in a plasma during each RF cycle. Using the
work of Shkarofsky (1972), and Wang and Bell (1969), this program finds the
antenna charge as function of applied voltage to calculate the time-dependent
radii of the surrounding ion and electron sheaths and the associated sheath
capacitance. An equivalent circuit four-capacitor and radiation resistance
(Balmain, 1964) model driven by a square-wave voltage was used to calculate the
antenna currents and radiated power. The results were compared with the actual
antenna current measurements. With this program it is possible to assume that
the antennaÕs wire elements were either insulated or bare. For this data set,
the model calculations were carried out at 18.8 kHz, where experimentally, a
maximum antenna current of about 300 mA was measured. The calculations indicate
a radiated power of 9.8 W. These results are consistent with the known RPI
performance.

The good agreement found between these experiments and the model calculations
make it possible to use this program to extend the simulations to the design of
new satellite transmitter and antenna systems that are planned for flights in
EarthÕs vicinity as well as in other planetary environments.


K.G. Balmain, ÒThe Impedance of a Short Dipole in a MagnetoplasmaÓ, IEEE Trans.
Antennas and Propagation, 12, pp. 605-617, Sept. 1964

I.P. Shkarowsky, ÒNonlinear admittance, currents, and charges associated with
high peak voltage drive on a VLF/ELF dipole antenna moving in the ionosphereÓ,
Radio Science, 7, pp. 503-523, April 1972.

T.N.C. Wang and T.F. Bell, ÒRadiation resistance of a short dipole immersed in a
cold magnetoionic mediumÓ, Radio Science, 4, pp. 167-177, Feb. 1969.

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Presented at the 28th URSI General Assembly, New Delhi, India, 
23 - 29 October, 2005