Mobile ad hoc networks (MANETs) consisting of nodes with single omnidirectional antennas and limited transmit power can support wireless connectivity over long distances through coherent operation. Although designed for short-range networked communication, the individual radio nodes in a MANET can be coordinated to form a sparse aperiodic array with a high power directional gain. By targeting a distant receiver with the array beam synthesized by a synchronized transmission, the MANET can ensure greater signal quality at the receiver. The sparse nature of the array coupled with the mobility-driven randomization of node positions makes the array gain somewhat uncontrollable outside of the main beam. Our work investigates methods of shaping the pattern of a coherent distributed transmit array using open-loop beamforming. A null-forming algorithm based on a partitioning of the network nodes into two sub-arrays is presented. Simulation results demonstrate the attenuation of the array factor in the desired null direction. A stochastic optimization step is also introduced.