Development of an Antenna Emulator Device for the JuventasRadar Experiment on the HERA Mission

The Juventas Radar (JuRa), a monostatic radar system designed to probe Didymos binary
system internal structure using a BPSK coded signal. A key component in ensuring the effective
operation of the JuRa system is the accurate matching of the dipole antenna impedance to the
power amplifier and receiver. In the transmitting path, the power amplifier performance is sensitive
to the loading impedance. For high efficient power amplifier this is a non-linear effect which is
challenging to model and simulate, especially for modulated signals. Calibration data obtained in a
standard 50-ohm environment would be therfore inaccurate.
Traditional deployment of antennas in an anechoic chamber for testing is not feasible due to the low
center frequency of 60 MHz as coupling with close objects and multiple reflections change
its frequency response. To address this challenge, we developed an antenna emulator device.
The antenna emulator device is engineered to present an equivalent antenna-like impedance
towards the transmitter, mimicking the behavior of the actual antennas used in the JuRa system
when deployed in space. This device is crucial for testing and calibration because deploying actual
antennas is impractical. Thus, the emulator provides a realistic impedance environment, ensuring
that the power amplifier operates under conditions that closely resemble those it will encounter in
space.
Additionally, the emulator is equipped with input/output interfaces for monitoring TX signals and
signal injection for RX testing. However, these interfaces exhibit a inadvertently non-flat frequency
response, necessitating the development of a mathematical model to correct for these discrepancies
through post- and pre-processing techniques. The mathematical model developed for the emulator's
input/output interfaces involves linear pre-and post-processing techniques, based on combination of
multiple measurement of the antenna emulator device and instrument.
The publication will detail the design process of the antenna emulator, including the theoretical
considerations and practical challenges (e.g. strong coupling with the solar panels) addressed during
its development. To facilitate testing during the assembly and integration (AIT) phases, we
developed a modified version of the emulator that can be used with non-deployed antennas. This
version allows for testing during the AIT without risking damaging the power amplifier by large
voltage standing wave ratio.
Finally the principles and techniques developed for the Juventas Radar antenna emulator can be
applied to other missions with low-frequency antennas facing similar testing challenges. By
providing a practical solution to the problem of testing low-frequency antennas without
environmental interference, our approach offers a valuable tool for future planetary and
interplanetary space missions.