Abstract—A scheme for a multi-antenna jammer, jamming a SISO primary link working in TDD mode is presented. The jammer relies on reciprocity to estimate the channel to a terminal in the primary link blindly. The jammer can then beamform noise towards this user to degrade the performance of the primary link. Simulations show that, for certain scenarios, the proposed jammer can do the same damage to the primary link as an omnidirectional jammer, but with close to two orders of magnitude lower output power.