K-band aperture-coupled stacked microstrip antennas are studied and numerical results based on the solution of integral equations solved in the spectral domain are presented. The effects of varying physical parameters of the structure are investigated with the goal of designing millimeter-wave wideband microstrip antennas. Antennas with different characteristics are then analyzed and compared with experimental data. Bandwidths in excess of 20% are obtained and applications to phased array antennas are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>