k , is the wave number of microstrip line, where Z , = Y,,Au-'; (20) and Au is obtained from Eq. (14).
Results
The cavity-model theory, which was developed in the previous section, is generalized for an aperture-coupled microstrip antenna. We first verified it for the fundamental mode, by comparing it with the results of Himdi, Daniel, and Terret 151, and then by our measurement. In order to verify the theoretical results for the TM,, mode the designed antenna was excited at the TM,, mode, keeping the aperture at the center of the plane under the patch. The designed specifications were er = 2.55, h = 0.159 cm, t = 0.159 em, length of the antenna = 4 cm, width of the antenna = 3 cm, length of the aperture L , = 1.12 cm, width of the aperture W, = 0.155 cm, 5 0 4 feed line length = 4 cm, and L, = 2.0 cm. The theoretical resonance frequency at the TM,, mode was 3.39 GHz, and the measured frequency was 3.395 GHz. The theoretical and measured impedance loci at TM,, mode are plotted on a Smith chart in Figure 2. The theoretical 1:2 VSWR impedance bandwidth at the TM,, mode is 8%; that measured was 7.77%. The theoretical and measured conical radiation patterns at the TM,, mode are shown in Figure 3. The plots indicate good agreement between theory and experiment.