A reactively steered adaptive array (RESAA) has one element connected by a transmission line to a receiver and a number of closely spaced parasitic elements, each of which is terminated by an adjustable reactive load. The pattern is formed by control of the reactive loads. Experimental results and the theory are presented for a RESAA consisting of five microstrip rectangular patch elements resonant at 4.0 GHz. Using steepest descent control of the reactive loads in a power inversion mode (no reference), we find that a null with a depth of 30 dB (relative to the pattern maximum) and an angular width of about25degcan be steered towards an interferer. Typically, about 40 steps (iterations) are needed for forming the null. With the slow power meter and general purpose minicomputer that served as the controller, adaptation times of several seconds are required; extrapolation to a dedicated microprocessor controller predicts adaptation times of several milliseconds. Operation in a mode using a reference signal demonstrates that the pattern can be shaped to steer a null toward interference and a lobe towards a desired signal. The nulling bandwidth is approximately 40 MHz with this array. The advantages of a RESAA, as compared With a conventional adaptive array, include the elimination of the mixers and other hardware needed to perform the complex weighting of the output of each element at an intermediate frequency, and better pattern control for closely spaced elements. These advantages are obtained at the expense of a more complicated control algorithm.