Microstrip antennas and antenna arrays are analyzed using the “add-on” technique. The goal of this work is to provide a way of handling more than 1000 unknowns while maintaining agreement with the moment method. The problem of a 1008-unknown finite array, with 11 elements and connecting and feeding lines, is an example of the capability of the method. In a CPU time of one hour and a half for each frequency point, accurate impedance results are obtained from the resultant standing-wave pattern along the feeding microstripline. The accuracy of the method is established by comparison with published results when possible, and by the computation of the root mean square (rms) error in the boundary condition on the conductor.