Short antennas on a large spacecraft
Résumé
Short dipole or monopole radio antennas are defined as being small in length relative to the wavelength of the frequency of operation. The reception properties of short linear antennas can be described by the so-called effective length vector which is pointing along the direction of minimum gain in the toroidal radiation pattern. We deal here with such antennas, and additionally the word 'short' also means a small antenna with respect to a large spacecraft body. Using numerical computer simulations we calculate the reception properties of an antenna system consisting of three short monopoles positioned on a large spacecraft body in the frequency range of several hundred kHz. It turns out that such a configuration has the major disadvantage that the angular separation between its three effective length vectors is quite small, which would lead to large errors in polarization and direction-finding measurements. We will show ways how to overcome this problem by changing the configuration to an antenna triad consisting of three short dipoles mounted on a boom. The calculations were employed to find a good configuration of the radio antennas for ESA's future JUICE (Jupiter Icy Moons Explorer) mission.