Антенны, СВЧ / OC / Broadband microstrip antennas

Figure 5.6 (a) Single-stacked RMSA and its (b) input impedance and (c) VSWR plots.

Figure 5.6(b, c). A loop is formed in the impedance plot, which is within the VSWR = 2 circle. The BW of the antenna is 405 MHz (14.2%). The radiation is in the broadside direction, and the variation in the pattern is very small over the entire BW. At 2.85 GHz, the gain is 9.1 dB. The BW of the stacked RMSA is nearly 3.1 times of that of the single RMSA with comparable gain, at the expense of an increase in the antenna thickness from 0.5 cm to 0.9 cm.

In the planar configuration, the total length of the antenna increases but the height of the antenna remains the same as that of the single RMSA. In the stacked configuration, on the other hand, the total length of the antenna remains the same as that of the single patch, while its thickness increases. By combining these two techniques (i.e., by stacking multiple resonators on each other), the overall BW or gain of the antenna increases due to the increase in both the length and thickness of the antenna. Several of these configurations are described in the following sections. For comparing

178 Broadband Microstrip Antennas

and h1 = 0.4 cm, there are two loops in the impedance plot, which are due to the parasitic patches in the planar and stacked configurations. However, the loops are not inside the VSWR = 2 circle because of the loading effect. Nevertheless, it provides a good starting point. The dimensions of the antenna are optimized to bring both the loops inside the VSWR = 2 circle. For L 1 = 3.9 cm, L 2 = 4.1 cm, h1 = 0.4 cm, and s = 0.3 cm, the input impedance and VSWR plots are shown in Figure 5.7(b, c). The two loops are inside the VSWR = 2 circle. The loop in the lower frequency region is due to L 2 and the second loop is due to L 1. The BW of the antenna is 704 MHz (23.5%). This BW is much higher than the BW of the singlestacked patch or three gap-coupled planar RMSA.

The variation of gain with frequency is shown in Figure 5.8. The gain is more than 10 dB in the lower frequency range, but it decreases sharply at the higher frequency due to the splitting of the main beam into three lobes.

The radiation characteristics of the antenna are improved when the resonance frequency of the top and the bottom parasitic patches is nearly the same. This will only give a single loop instead of two loops in the impedance plot, which will decrease the overall BW of the antenna. However, the radiation from all the parasitic patches will be around the same frequency, which will enhance the gain within the BW. For L 1 = L 2 = 3.95 cm, s = 0.3 cm and h1 = 0.4 cm, the input impedance and VSWR plots are

Figure 5.8 Variation of gain with frequency for 3B1T RMSA for two cases: ( —— ) L 1 = 3.9 cm, L 2 = 4.1 cm and ( - - - ) L 1 = L 2 = 3.95 cm.

shown in Figure 5.9. There is only one loop in the impedance plot, and the BW is decreased to 585 MHz. The variation of gain with frequency for this case is also shown in Figure 5.8. The gain of the antenna does not fall off at the higher frequency within the impedance BW as in the previous case when L 1 ≠ L 2.

5.2.2One Rectangular Patch at the Bottom and Three Patches on the Top

Instead of placing three patches on the bottom layer and one on the top layer, the configuration with one rectangular patch on the bottom layer with three gap-coupled patches on the top layer (1B3T RMSA) is shown in Figure 5.10(a) [4]. Only the bottom patch is fed, and the three parasitic gap-coupled patches on the top layer are aligned symmetrically with respect to the radiating edges of the bottom patch for symmetrical radiation pattern. The dimensions of the three top parasitic patches are chosen to be equal, so that their resonance frequencies are the same to yield a relatively flatter gain curve in the upper frequency range within the VSWR BW, as observed in the previous case. For L 1 = 3.6 cm, s = 0.25 cm, and h1 = 0.4 cm, the input impedance and VSWR plots are shown in Figure 5.10(b, c). Only one loop appears in the impedance plot as the parasitic patches are identical. The BW is 782 MHz (26.1%). In general, the BW of the 1B3T RMSA is greater than that of the 3B1T RMSA, because in this case, all the three parasitic patches are at the higher height from the ground plane. The variation of gain with

Figure 5.9 (a) Input impedance and (b) VSWR plots of 3B1T RMSA with L 1 = L 2 = 3.95 cm.

Figure 5.10 (a) One patch on bottom layer and three gap-coupled rectangular patches on the top layer, its (b) input impedance and (c) VSWR plots, and (d) variation of gain with frequency for ( —— ) 1B3T and ( - - - ) 3B3T RMSAs.

frequency is shown in Figure 5.10(d). The gain remains more than 10 dB over the entire BW. Thus, 1B3T configuration has better performance than that of the 3B1T RMSA.

5.2.3Three Rectangular Patches at the Bottom and Three Patches on the Top

After studying the BW and gain of 3B1T and 1B3T RMSA configurations, the next natural choice would be to look at the configuration where three patches are placed at the bottom layer and three patches are symmetrically stacked on the top layer as shown in Figure 5.11 [4]. Only the central bottom rectangular patch is fed; all the other rectangular patches are parasitically coupled. All the top patches are centered over the bottom rectangular

to be equal to 2 cm. For s = 0.1 cm, h1 = 0.4 cm, and x = 2.4 cm, the input impedance and VSWR plots are shown in Figure 5.12(b, c). There is only one loop in the impedance plot, because the length of all the parasitic patches is same. The BW of the antenna is 920 MHz (30.7%). The 1B5T RMSA has a much wider BW with a flatter gain response as compared to that of the five gap-coupled RMSA at the expense of an increase in the overall height of the antenna.

The BW and gain comparison of all the above configurations are given in Table 5.1. The gain is given at a single frequency, which is close to the center frequency. The stacked multiresonator RMSA configurations have a better performance than that of the planar multiresonator or single-stacked RMSA. For example, the BWs of three gap-coupled and 1B1T RMSA configurations are 463 MHz and 405 MHz, respectively, whereas a much larger BW of 782 MHz is obtained for 1B3T RMSA, which is a combination of these two configurations. Also, the gain of 1B3T RMSA is much flatter over the entire VSWR BW as compared to the other two configurations. Out of all these configurations, 1B5T RMSA yields a maximum BW of 920 MHz with a flatter gain response.

5.2.5One Rectangular Patch at the Bottom and Two Patches on the Top

In all the three and five gap-coupled stacked configurations (1B3T, 3B1T, 3B3T, and 1B5T), the overall length of the antenna is very large, and in some cases, the radiation pattern varies over the BW, which makes them unsuitable for many applications. These can be taken care of by using two identical patches on the top layer with one fed patch on the bottom layer.

Table 5.1

Comparison of Various Planar and Stacked RMSA Configurations

L = 5.0 cm, W = 3.0 cm, h = 0.5, cm, x = 2.4 cm, and various values of h1, the optimized dimensions of L 1 and ox, along with BW and a gain at 3.0 GHz are given in Table 5.2. With an increase in h1, the effective length of the top patch increases due to an increase in the fringing fields; hence the length L 1 of the top patch is reduced so that the loop in the impedance plot is within VSWR = 2 circle. Also, with an increase in h1, the coupling between the bottom and the top patch decreases. The coupling is increased by changing the value of ox (center-to-center horizontal spacing between the lower and upper patches in the x -direction), and maximum coupling is obtained when ox is approximately equal to L /2. This is because the edges of the fed patches act as magnetic currents, giving maximum coupling to the top patches when its edges coincide with the maximum magnetic field region (i.e., the center) of the top patches.

As h1 increases from 0.4 cm to 1.1 cm, the length L 1 is to be decreased from 3.8 cm to 3.2 cm, so that the loop is within the VSWR = 2 circle and the value of ox is decreased from 4.7 cm to 2.5 cm to increase the coupling. The center frequency of the antenna remains at around 3.0 GHz. For two values of h1 (0.4 cm and 0.9 cm), the input impedance and VSWR plots are shown in Figure 5.13(b, c). As h1 is increased from 0.4 cm to 0.9 cm, the BW of the antenna increases from 650 MHz to 1,043 MHz, because the total height of the antenna increases. When h1 is increased further, the BW decreases slightly because the loop size decreases due to the decrease in the coupling, which cannot be increased, as the offset ox has been optimized for maximum coupling.

The radiation pattern in the E- and H-planes of the 1B2T RMSA for h1 = 0.9 cm at 2.5 GHz and 3.5 GHz (near the two band-edge frequencies)

Table 5.2

Variation of BW and Gain of 1B2T RMSA with h1 (L = 5 cm, W = 3 cm, x = 2.4 cm, and h = 0.5 cm)