An array of three microstrip patch antennas, each connected by a matching network to an impatt diode, has been investigated. Coherent radiation from the array was obtained at 10.2GHz by feeding only the centre element with an injection-locking signal, which then appeared at the input to the other two elements by free-space mutual coupling. The three impatts injection-locked successfully, producing a broadside beam with a width of 38°. An RF efficiency of 86% was measured.