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Raveendranath U. A single patch antenna provides a maximum directive gain of around dBi. It is relatively easy to print an array of patches on a single large substrate using lithographic techniques. Patch arrays can provide much higher gains than a single patch at little additional cost; matching and phase adjustment can be performed with printed microstrip feed structures, again in the same operations that form the radiating patches. The ability to create high gain arrays in a low-profile antenna is one reason that patch arrays are common on airplanes and in other military applications.
Such an array of patch antennas is an easy way to make a phased array of antennas with dynamic beamforming ability. An advantage inherent to patch antennas is the ability to have polarization diversity.
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Patch antennas can easily be designed to have vertical, horizontal, right hand circular RHCP or left hand circular LHCP polarizations, using multiple feed points, or a single feedpoint with asymmetric patch structures. The most commonly employed microstrip antenna is a rectangular patch which looks like a truncated microstrip transmission line. It is approximately of one-half wavelength long.
When air is used as the dielectric substrate, the length of the rectangular microstrip antenna is approximately one-half of a free-space wavelength. As the antenna is loaded with a dielectric as its substrate, the length of the antenna decreases as the relative dielectric constant of the substrate increases. The resonant length of the antenna is slightly shorter because of the extended electric "fringing fields" which increase the electrical length of the antenna slightly.
An early model of the microstrip antenna is a section of microstrip transmission line with equivalent loads on either end to represent the radiation loss. The dielectric loading of a microstrip antenna affects both its radiation pattern and impedance bandwidth. As the dielectric constant of the substrate increases, the antenna bandwidth decreases which increases the Q factor of the antenna and therefore decreases the impedance bandwidth.
This relationship did not immediately follow when using the transmission line model of the antenna, but is apparent when using the cavity model which was introduced in the late s by Lo et al. These slots act as an array and have the highest directivity when the antenna has an air dielectric and decreases as the antenna is loaded by material with increasing relative dielectric constant. This can be explained thusly: For the same value of L F , the position of the feeding point is closer to the bottom edge of parasitic element, compared with that of the main patch.
Microstrip Antenna Maintains Low Profile
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. It can also be seen from Fig. It can be attributed to the decrease of the resonant region at lower frequency. As L F increases from Hence, the bandwidth increases with optimal feed point position. As mentioned above, the parameters L P , L U , and G H are important and sensitive in tuning the two resonant frequencies. The widths G V and G F of the gaps, together with the feed point position L F , 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 and MHz with return loss values of The proposed antenna achieves a dB return-loss bandwidth of 67 MHz to MHz , whereas the return loss for the corresponding rectangular patch antenna is beyond the acceptable range of operation.
The simulated results in Fig. Figure 9 shows a prototype of the fabricated antenna. The measured impedance bandwidth is 66 MHz to MHz and two separate resonant frequencies at and MHz with return loss values of The measured antenna performance shows excellent agreement with the simulated return-loss curve.
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A few shifts in the bandwidth and positions of resonant frequencies are due to the fabrication tolerances. The measured and simulated normalized radiation patterns at the first and second resonant frequencies are plotted in Fig. The proposed low-profile antenna exhibits good broadside radiation patterns in the E-plane x-z plane and H-plane y-z plane at each resonant frequency. In the E-plane, the measured 3-dB beamwidth is 83 deg. It can be seen that the beam peaks of the E-plane are slightly shifted from the z-direction due to the feeding position on the main patch.
In the H-plane, the measured 3-dB beamwidth is 93 deg.
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Symmetrical radiation patterns are obtained in the H-plane due to the bilateral symmetry of the antenna configuration. In addition, it is obvious that low cross-polarization levels are obtained in the two orthogonal planes. The antenna gain versus frequency was also measured, with measured peak gain of 5. As is well-known, the impedance bandwidth of a conventional microstrip antenna with a thin substrate less than 0. To enhance its impedance bandwidth, a U-shaped parasitic patch was employed, and summaries of conventional and modified with patch microstrip antennas can be compared in Tables 1 and 2.
Table 1 shows that the modified antenna provides a wider operating bandwidth than a conventional microstrip antenna—as much as 2. But Table 2 shows that the modified antenna exhibits similar radiation patterns to those of the conventional antenna, except with a slight decrease in gain. The modified U-slot antenna obtains maximum gain due to a larger ground plane.
The proposed low-profile antenna is well suited for conformal communications terminals. Mintong Li, Associate Professor. Schaubert, D.
Pozar, and A. AP, June , pp. Pues and A. AP, No.
Crop and D. Wi, Y.
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