I. Introduction
Advanced communication, radar, passive imaging and remote sensing instruments are extending frequency and bandwidth requirements to 200 GHz and beyond. The high frequencies of mm-waves as well as their propagation characteristics make them an excellent choice to satisfy actual requirements imposed by modern wireless communication systems, such as small profile, high data rates, low cost, and short radio links. As is well known, the signal wavelength becomes shorter as the frequency increases, so smaller antennas can be integrated with other circuits. Printed antennas are commonly adopted for these systems since they are light in weight, the low profile, the low in cost, and are easy to fabricate and integrate [1], [2].