Radio communication between ground robots at low VHF frequencies is generally more reliable in complex environments compared to conventional systems operating at higher UHF frequencies. Unfortunately, high-gain directional antennas operating at low frequencies are impractically large for backpack sized robots. However, if a robot carrying a small, low VHF antenna having an omni-directional radiation pattern is positioned at a desired location close to a set of parasitic antenna elements (i.e., not connected to radios and their input is short-circuited), the overall multi-antenna system can form a parasitic array and create a directional radiation pattern with higher gain. In this paper, we consider a two element parasitic array formed by two ground robots and propose a technique by which this array can optimize its gain in a direction of interest online. It does this by finding the optimal spacing between the two antennas and passive antenna length, both of which depend on the electromagnetic characteristics of the ground. We simulate this array using full-wave FDTD and experimentally demonstrate in various outdoor environments and ground conditions that the robotic array can provide gain of over 2dB.