Thomas Oswald

Rheometry of multi-port spaceborne antennas including mutual antenna capacitances and application to STEREO/WAVES

Electrolytic tank measurements were applied often to determine the antenna properties of spaceborne monopole antennas. This technique, called rheometry in the present context, yields the effective length vectors of antennas for the quasi-static frequency range. When receivers or preamplifiers are connected to the antennas, their input impedances and the capacitances of the cables change the effective length vectors. In the evaluation of former rheometry measurements this effect has been taken into account for each antenna individually, tacitly neglecting the mutual antenna capacitances. Our analysis shows that this neglect can cause errors in the measured effective axes (directions of effective length vectors) of up to about 10°. However, in order to apply direction finding techniques with a triaxial antenna system an accuracy of about 2° is required. We therefore extend rheometry to the measurement of the complete antenna capacitance matrix. An application of the presented technique to the WAVES antenna system onboard the STEREO spacecraft verifies that the mutual capacitances are crucial in this context. With the extended rheometry technique the antenna transfer and capacitance matrices can be completely determined, thereby yielding a full representation of the reception properties of the antenna system as part of an electronic circuit.

Calibration of Electric Field Sensors Onboard the Resonance Satellite

Strategies and results for calibrating electric field sensors (antennas), as used in radio astronomy, onboard the spacecraft “Resonance” are presented. Calibration is performed for four boom antennas and four cylindrical sensors at the boom tips. These antennas are devised for the measurement of electric fields and plasma parameters. It is shown that the electrical representations of the antennas, the effective length vectors, differ from their mechanical originals and are shortened and tilted by several degrees of angle. The knowledge of the acquired parameters is of great benefit to the Resonance mission. In particular, goniopolarimetry techniques like polarization analysis and direction finding depend crucially on the effective axes. For the first time, this kind of analysis is performed for a space-borne antenna system consisting of boom monopoles and cylindrical tip antennas.

In‐flight calibration of STEREO‐B/WAVES antenna system

The STEREO/WAVES (SWAVES) experiment on board the two STEREO spacecraft (Solar Terrestrial Relations Observatory) launched on 25 October 2006 is dedicated to the measurement of the radio spectrum at frequencies between a few kilohertz and 16 MHz. The SWAVES antenna system consists of 6 m long orthogonal monopoles designed to measure the electric component of the radio waves. With this configuration direction finding of radio sources and polarimetry (analysis of the polarization state) of incident radio waves is possible. For the evaluation of the SWAVES data the receiving properties of the antennas, distorted by the radiation coupling with the spacecraft body and other onboard devices, have to be known accurately. In the present context, these properties are described by the antenna effective length vectors. We present the results of an in-flight calibration of the SWAVES antennas using the observations of the nonthermal terrestrial auroral kilometric radiation (AKR) during STEREO roll maneuvers in an early stage of the mission. A least squares method combined with a genetic algorithm was applied to find the effective length vectors of the STEREO Behind (STEREO-B)/WAVES antennas in a quasi-static frequency range (Lantenna≪λwave) which fit best to the model and observed AKR intensity profiles. The obtained results confirm the former SWAVES antenna analysis by rheometry and numerical simulations. A final set of antenna parameters is recommended as a basis for evaluations of the SWAVES data.

Resonance spacecraft antenna calibration: Rheometry and numerical simulations

We report on the calibration effort for the monopole antennas onboard the Resonance spacecraft. The calibration is performed for four boom antennas and four cylindrical sensors at the boom tips. These antennas are devised for the measurement of electrical fields and plasma parameters. We apply two methods for the antenna analysis: first, electrolytic tank measurements (rheometry), which is a method to determine the effective length vectors of electrically short antennas; second, numerical computer simulations which enable us to study also the transition to higher frequencies. The accuracy of the applied methods is about 1 degree for directions of effective axes and some percent for effective lengths and antenna capacitances. It is shown that the electrical representations of the antennas, the effective length vector, differ from their mechanical originals, are shortened and tilted by several degrees of angle. The knowledge of the acquired parameters is of great benefit to the Resonance mission. In particular, goniopolarimetry techniques like polarization analysis and direction finding depend crucially on the effective axes. For the first time this kind of analysis is performed for a spaceborne antenna system consisting of boom monopoles and cylindrical tip antennas.

First results of the JUNO/Waves antenna investigations

Waves is the radio and plasma wave instrument onboard NASAs spacecraft JUNO. The instrument utilizes an electrically short dipole antenna for the measurement of electromagnetic field parameters. The instrument is devised for frequencies from 50 Hz up to 40 MHz. Waves antenna system properties are distorted because the highly conducting spacecraft body is in close vicinity of the antennas. In addition the antenna system is not tri-axial and goniopolarimetry techniques like polarization analysis and direction finding depend crucially on the true antenna properties. In the case of the Waves instrument, mentioned techniques have to rely on a rotating dipole method for detection of parameters like the wave incident direction. In this contribution we outline the first step to acquire the true antenna properties of the Waves antennas. We present the first results of our numerical investigations, the antenna monopole effective length vectors and antenna transfer matrices for the quasi static regime.