Microstrip antennas, also called patch antennas, are very popular in the microwave frequency range because of their simplicity and compatibility with circuit board technology. They are usually operated near half-wave resonance to obtain a nominally real-valued input impedance. The fringing fields act to extend the effective length of the patch, thus, the length of the patch is usually less than a half wavelength in the dielectric medium. The pattern of a square patch antenna is broad with maximum direction normal to the plane of the antenna.
The center frequency of a patch antenna can be lowered by removing a square area of metal from its center. The resonant frequency and impedance bandwidth decreases while the input impedance increases by increasing the cut-out area. The reduction in resonant frequency for the same cut-out area is greater for thinner substrates