In the frame of the European Space Agency's 2016 ExoMars mission, the Electromagnetic Investigation of the SubSurface (EISS) ground-penetrating radar has been designed and developed to perform deep soundings of the Martian subsurface from the surface. The EISS is designed to take advantage of the potential for bistatic radar investigations of the Martian subsurface between the fixed station (Lander) and the mobile platform (rover) and to characterize the 3-D structure and stratigraphy of the subsurface at depths ranging from 100 m to a few kilometers out to a 1-km radius around the lander. The EISS makes use of an electric dipole antenna made of two identical 35-m resistively loaded monopoles to transmit (and also receive in a monostatic mode) the high-frequency signal. However, the EISS's most innovative capability is its potential for bistatic operation, made possible by the accommodation of a small magnetic sensor on the rover (as initially planned for the ExoMars mission) which can measure the magnetic field (all three components) of the received waves whatever the direction and orientation of the rover. The aim of this paper is to show that the two monopoles of the antenna must be deployed on the surface in nearly opposite directions but not aligned to ensure good volume coverage around the transmitter. This paper is based on Finite Difference in Time Domain (FDTD) electromagnetic simulations. The simulated data have been used to study the impact of the angle between these two monopoles on the instrument performance.