Антенны, СВЧ / OC / Broadband microstrip antennas

radiation pattern characteristics as described above. The input impedance and VSWR plots for the three-radiating-edge gap-coupled RMSA is shown in Figure 3.7(b, c). The loop size is large compared to the two gap-coupled RMSA, because two coupled patches are resonant at the same frequency and hence the coupling is greater. The loop size is reduced by increasing the gap between the fed and the parasitic patches. The input impedance and VSWR plots for two different values of s (0.15 cm and 0.2 cm) are also shown in Figure 3.7. The loop in the impedance plot for s = 0.15 and 0.2 cm is completely inside the VSWR = 2 circle, yielding BWs of 209 MHz and 171 MHz, respectively. The BW for s = 0.15 cm is larger than that for s = 0.20 cm because of the larger loop size.

The radiation patterns in the E- and H-planes near the two-band edge frequencies (2.89 GHz and 3.09 GHz) for s = 0.15 cm are shown in Figure 3.8. The variation of the pattern in the H-plane over the entire BW is very small. The HPBW in the H-plane remains nearly the same as that of the single RMSA, because there are no parasitic patches in this plane. In the E- plane, the beam maxima is in the broadside direction. However, as the frequency increases, the sidelobes begin to appear due to the large overall length of the antenna in this plane. Also, at the higher frequency, the parasitic patches are resonant, which means that they experience phase delay with respect to the central patch. Hence, the beam will try to shift in the +u and −u directions. The net result is that sidelobes are formed in the E-plane pattern along these directions. The HPBW in the E-plane is smaller than that of the two gap-coupled configurations due to increase in the aperture area. The gain of the three gap-coupled RMSA is 9.4 dB at 3 GHz, which is 2.7 dB more than the single RMSA. Thus, by adding two parasitic patches,

Figure 3.8 Radiation pattern of three gap-coupled RMSA at frequencies (a) 2.89 and

(b) 3.09 GHz: ( —— ) E-plane and ( - - - ) H-plane.

the gain of the antenna increases from 6.7 dB to 9.4 dB, and the BW increases from 65 MHz to 209 MHz at the expense of increase in the size of the antenna.

The BW of the three gap-coupled RMSA can be increased by increasing h, as in the case of single RMSA. However, as shown in Figure 3.1, if the BW of an individual patch is large, then the difference between the two patch dimensions should also be large to obtain broader BW. So, when h is increased from 0.159 cm to 0.318 cm, and if the parasitic patch of length L 1 = 2.9 cm is placed along both the radiating edges of the RMSA, then the BW will not be optimum. Also, the coupling between the fed and parasitic patches depends upon the s /h ratio and not on the s value alone, so the gap has been increased from 0.15 cm to 0.3 cm for three gap-coupled RMSA. For the feed at x = 1.4 cm, the input impedance plot is shown in Figure 3.9. As expected, in this case, the BW is not optimum and the loop is not inside the VSWR = 2 circle, instead it is formed in the lower frequency region. To bring the loop within the VSWR = 2 circle, the length L 1 of the parasitic patch is decreased, so the loop will be formed in the higher frequency region and therefore moves in the clockwise direction. For L 1 = 2.75 cm, the input impedance and VSWR plots are also shown in Figure 3.9. The loop is completely inside the VSWR = 2 circle, yielding a BW of 335 MHz (11.3%). The gain of the antenna at 3 GHz is 9.2 dB. The gain is slightly decreased as compared to that of the antenna with h = 0.159 cm due to an increase in the surface waves.

Figure 3.9 (a) Input impedance and (b) VSWR plots of three gap-coupled RMSA with h = 0.318 cm for two values of L 1: ( - - - ) 2.9 cm and ( —— ) 2.75 cm.

Figure 3.11 Radiation pattern of a three gap-coupled RMSA with er = 1 and h = 0.5 cm at two frequencies. (a) 2.775 GHz and (b) 3.225 GHz: ( —— ) E-plane and ( - - - ) H-plane.

Figure 3.12(a). The length L 1 is taken slightly greater than the length L 2, so that the two parasitic patches will be resonant at different frequencies. If these frequencies are close to each other, broad BW is obtained. The input impedance and VSWR plots for L = 3.0 cm, W = 4.0 cm, L 1 = 3.0 cm, s = 0.1 cm, x = 1.4 cm with er = 2.55, h = 0.159 cm, tan d = 0.001, and two values of L 2 (2.8 cm and 2.85 cm) are shown in Figure 3.12(b, c). Two loops are formed in the impedance plots, which are due to the different resonance frequencies of the two parasitic patches. Both the loops are inside the VSWR = 2 circle. A larger BW of 328 MHz is obtained for L 2 = 2.8 cm as compared to the BW of 289 MHz for L 2 = 2.85 cm. For L 2 = 2.85 cm, the separation between the two loops is less as compared to that for L 2 = 2.8 cm because of the smaller difference in length with respect to L 1 = 3 cm. Similar separation in the higher frequency range may also be noted from VSWR plot. If the difference between the lengths of the two parasitic patches is further reduced, then the second loop is formed within the first loop. If both parasitic lengths are reduced from L 1 = 3 cm and L 2 = 2.85 cm to L 1 = 2.95 cm and L 2 = 2.8 cm (i.e., the difference between the two parasitic patches is same), then the shape of the loops in the impedance plot remains the same, except that their position is shifted clockwise due to the decrease in both the parasitic lengths.

For the three gap-coupled RMSA with L 1 = 3 cm and L 2 = 2.8 cm, the radiation pattern at three frequencies (near the center and the two bandedge frequencies) is shown in Figure 3.13. In the H-plane, the radiation pattern remains nearly the same over the entire BW. However, in the E-plane, the radiation pattern varies with frequency. At 2.85 GHz, the central patch

is resonant, so the maximum radiation is in the broadside direction. At 3.01 GHz, the radiation from the parasitic patch of length L 1 is dominant, so the beam shifts to +30° from the broadside. At 3.17 GHz, the parasitic patch of length L 2 is resonant, so the beam shifts along the −55° direction. The shift in the pattern is more in the −u direction because it experiences larger phase delay from the feed point as it is resonant at a higher frequency. However, along the broadside direction, the beam remains within 5 dB of its maximum value at all the frequencies within the BW. So, the broad impedance BW is useful only if this amount of pattern variation is acceptable.

Similarly, broad impedance BW from 2.597 GHz to 3.194 GHz is obtained for er = 1 with h = 0.5 cm, L = 5 cm, L 1 = 4.5 cm, L 2 = 4 cm, W = 3 cm, s = 0.3 cm, and x = 2.2 cm. This impedance BW of 597 MHz is larger than the impedance BW of 502 MHz for two parasitic patches of

Figure 3.13 Radiation pattern of a three gap-coupled RMSA with L 1 = 3.0 cm and L 2 = 2.8 cm at three frequencies (a) 2.85 GHz, (b) 3.01 GHz, and (c) 3.17 GHz: ( —— ) Eu in the f = 0° and 90° planes and ( - - - ) Ef in the f = 90° plane.

equal length (L 1 = L 2 = 4 cm). However, the beam maxima varies from the broadside to the +u and −u directions with an increase in the frequency within the BW.

The theoretical and experimental input impedance and VSWR plots for a three gap-coupled RMSA with unequal parasitic patches are shown in Figure 3.14 [7]. The dimensions of the antenna are L = 2.7 cm, L 1 = 2.615 cm, L 2 = 2.51 cm, W = 3.9 cm, s = 0.165 cm, and x = 1.23 cm, and the substrate parameters are er = 2.55, h = 0.159 cm, and tan d = 0.001. The two loops in the impedance plot are inside the VSWR = 2 circle. The theoretical and measured BWs are 294 MHz and 331 MHz, respectively. The theoretical and measured radiation patterns at three frequencies in the E-plane are given in Figure 3.15(a, b). As discussed earlier, the E-plane pattern varies over the BW. However, along the broadside direction, it remains within approximately 2 dB in the entire frequency range. In the

Figure 3.14 (a) Input impedance and (b) VSWR plots of a three-radiating-edge gap-coupled RMSA with unequal parasitic patches: ( —— ) theoretical and ( - - - ) experimental.

H-plane, there is not much variation in the pattern within the BW. Hence, the theoretical measured pattern is given only at 3.2 GHz in Figure 3.15(c).

In the above cases, if the difference in patch lengths is large, and the gap between the fed and parasitic patches is small, then dualand tripleband operations can be obtained as described in Chapter 7.

3.3.2 Nonradiating-Edge Gap-Coupled RMSAs

Instead of placing two parasitic rectangular patches along the radiating edges of the centrally fed RMSA, these could be placed along its nonradiating edges as shown in Figure 3.16(a) [9]. The gaps between the fed and the parasitic patches should be smaller as compared to those in the radiating edge-coupled RMSA. Because the field varies sinusoidally along the nonradiating edge, the coupling will be smaller as compared to the coupling along the radiating edge, where the field is uniform.

Initially, the parasitic patches are identical. For L = 3 cm, W = 4 cm, s = 0.05 cm, length of the parasitic patches L 1 = L 2 = 2.9 cm, and feed at x = 1.1 cm, with substrate parameters er = 2.55, h = 0.159 cm, and tan d = 0.001, the input impedance and VSWR plots are shown in Figure 3.16(b, c). These dimensions are similar to that of the three-radiating-edge gap-coupled RMSA except for the smaller value of air gaps. There is a single loop in the impedance plot, and the BW is 159 MHz (5.3%). The effect of various parameters on the performance of the antenna is similar to that of the

Figure 3.15 Radiation pattern of a three gap-coupled RMSA with unequal parasitic patches: (a) theoretical and (b) experimental pattern in E-plane at three frequencies—( - - - ) 3.19 GHz, ( —— ) 3.29 GHz, and ( – - – ) 3.39 GHz; (c) H-plane pattern at 3.2 GHz: ( —— ) theoretical and ( - - - ) experimental.

radiating-edges gap-coupled RMSA. When the substrate thickness h is increased from 0.159 cm to 0.318 cm, the various parameters, such as, L 1, L 2, s, and x have to be optimized to obtain broad BW. For L 1 = L 2 = 2.7 cm, s = 0.05 cm, and x = 1.4 cm, the input impedance and VSWR plots are also shown in Figure 3.16(b, c). The BW is 319 MHz (10.5%).

The radiation patterns near the two band-edge frequencies (2.88 GHz and 3.19 GHz) for the thicker substrate are shown in Figure 3.17. In the E-plane, the radiation pattern is similar to that of a single RMSA. As the frequency increases within the BW, the HPBW decreases and the sidelobe level increases. In the H-plane, at the lower frequency, the pattern is maximum in the broadside direction. As the frequency increases, three lobes are formed in the pattern due to the phase delay experienced by the resonant parasitic patches. The gain of the antenna is 8.1 dB at 3 GHz.

Figure 3.16 (a) Nonradiating-edges gap-coupled RMSA with two parasitic patches and its (b) input impedance and (c) VSWR plots for two values of h : ( —— ) 0.159 cm and ( - - - ) 0.318 cm.

Since the width of each patch is larger than the length, the overall width of the gap-coupled antenna becomes too large. The width of the each patch is therefore reduced from 4 to 3 cm for a thicker substrate. For L = 3 cm, W = 3 cm, L 1 = L 2 = 2.7 cm, s = 0.05 cm, and x = 1.4 cm, the input impedance and VSWR plots are shown in Figure 3.18. The BW is 390 MHz (12.7%), which is comparable to that of the antenna with W = 4 cm. However, the gain of the antenna is reduced from 8.1 dB to 7.5 dB because of the decrease in the aperture area.

The BW of the antenna increases when the two parasitic patches are of unequal lengths. For L = 3 cm, W = 3 cm, L 1 = 2.8 cm, L 2 = 2.6 cm,

Figure 3.17 Radiation pattern of nonradiating-edge gap-coupled RMSA for h = 0.318 cm at frequencies (a) 2.88 GHz and (b) 3.19 GHz: ( —— ) Eu in the f = 0° and 90° planes and ( - - - ) Ef in the f = 90° plane.

Figure 3.18 (a) Input impedance and (b) VSWR plots of nonradiating-edge gap-coupled RMSA with W = 3 cm for equal and unequal parasitic lengths: ( - - - ) L 1 = L 2 = 2.7 cm and ( —— ) L 1 = 2.8 cm and L 2 = 2.6 cm.

s = 0.05 cm, and x = 1.0 cm, the input impedance and VSWR plots are shown in Figure 3.18. The two loops present in the impedance plot are due to the different resonance frequencies of the two parasitic patches. The BW of the antenna is 447 MHz (14.5%), which is more than that of the antenna with equal parasitic patches.

For unequal parasitic elements, the radiation patterns at three frequencies (2.86, 3.08, and 3.29 GHz) are shown in Figure 3.19. In the E-plane, there is a small variation in the pattern over the BW, but in the H-plane,