Many wireless ad hoc networks exploit MAC protocols with conventional omni-directional antennas. Smart antennas, however, improve performance by avoidance of radio interference and effective use of limited resources of mobile nodes. This paper proposes a MAC protocol using a directional beam form for ad hoc networks called SWAMP. SWAMP consists of two types of access modes, i.e., OC-mode and EC-mode, which are selected according to the relative position between a sender and a receiver. OC-mode forms a beam in the restricted area and uses omni-NAV shorter than an ordinary NAV to improve spatial utilization. In EC-mode, a higher gain and narrower beam is used to extend a transmission range. Moreover, SWAMP includes a method of obtaining information of neighbors indispensable for beam forming. Simulation results show the throughput advantages over omni-directional antennas.