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In the parametric studies, six critical parameters with strong effect on the performances are given and discussed. The other parameters, such as the patch width (WP and WU), the ground size (L and W), substrate permittivity, and the substrate thickness (H) are kept the same as mentioned above. The return loss curves of the proposed antenna are shown in Figs. 2-7 as a function of frequency for different design parameters.

Microstrip Antenna Maintains Low Profile, Fig. 6

Figures 2 and 3 illustrate the influences of the patch lengths (LP and LU) for the resonant frequencies. It can be seen from Fig. 2 that the position of lower resonant frequency is obviously shifted downward as LP increases, but that of higher resonance frequency is slightly shifted. The variation in the case of increasing LU is contrary to that of increasing LP, as shown in Fig. 3. Comparing the two results shows that the position of lower resonant frequency is primarily controlled by the length (LP) of main patch; on the other hand, the length (LU) greatly affects the position of higher resonance frequency.

Microstrip Antenna Maintains Low Profile, Fig. 7

Figure 4 shows the effect of the vertical gap (GLV) on antenna performance. As can be seen, a resonant mode at the higher frequency gradually emerges with increasing vertical gap, indicating that the energy in the main patch can couple to the parasitic patch from the radiating edge via a suitable gap. In addition, the two resonant frequencies are away from each other as GV increases. The curves show that the vertical gap plays an important role in introducing a resonance at the higher frequency.

Figure 5 shows the effect of horizontal gap (GH) on antenna performance. It depicts that an increase in the horizontal gap causes considerable shift downwards in the higher resonant frequency and a moderate shift in the lower resonant frequency. It should be noted that the two resonant frequencies are close to each other with increasing GH, contrary to the effect of GV. As a result, the bandwidth decreases while the horizontal gap increases from 1.8 to 2.6 mm. This also shows a matching effect on the performance of the antenna.

Figure 6 shows variations in return loss with frequency at different feed-gap (GF) spacings. The variation in the positions of the two resonance frequencies is moderate with decreasing feed gap, while the return losses between them decrease significantly. The influence of GF indicates that when two resonant frequencies are excited simultaneously in the designated frequency region, the optimal performance of antenna can be obtained by tuning GF.

As shown in Fig. 7, the position of the feed point (LF) has a crucial effect on antenna performance. Obviously, it affects the amplitude of return loss at higher resonance frequency. This can be explained thusly: For the same value of LF, the position of the feeding point is closer to the bottom edge of parasitic element, compared with that of the main patch. Therefore, with a slight variation in feeding position, the impedance variation of higher resonant frequency is acute, whereas that of lower resonant frequency is relatively stable.

Microstrip Antenna Maintains Low Profile, Fig. 8

It can also be seen from Fig. 7 that when LF increases from 14.5 to 15.5 mm, the position of the lower resonant frequency shifts upward. It can be attributed to the decrease of the resonant region at lower frequency. As LF increases from 14.5 to 15.5 mm, the resonant region of the lower frequency changes from both the main and parasitic patches to only the main patch, with the parasitic patch finally working at the higher frequency. Hence, the bandwidth increases with optimal feed point position.

As mentioned above, the parameters LP, LU, and GH are important and sensitive in tuning the two resonant frequencies. The widths (GV and GF) of the gaps, together with the feed point position (LF), have great influence on the achievable bandwidth. Figure 8 shows return-loss curves for the proposed antenna and for a conventional microstrip antenna. The lower and higher resonances appear at 2435 and 2379 MHz with return loss values of 23.01 dB and 28.34 dB, respectively.

The proposed antenna achieves a 10-dB return-loss bandwidth of 67 MHz (2419 to 2486 MHz), whereas the return loss for the corresponding rectangular patch antenna is beyond the acceptable range of operation. The simulated results in Fig. 8 indicate that the EM coupling between the main patch and parasitic element strongly affect the obtainable impedance bandwidth.

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