Антенны, СВЧ / OC / Broadband microstrip antennas

Figure 6.18 (a) H-shaped MSA and (b) bowtie MSA.

Table 6.9

Effect of the Slot Dimensions on the Performance of an H-Shaped MSA (L = 6 cm, W = 4 cm, er = 2.33, h = 0.159 cm, and tan d = 0.002)

feed point x is shifted toward the center for impedance matching. With an increase in w from 0.5 cm to 1.5 cm (keeping l = 1 cm), the resonance frequency decreases from 1.495 GHz to 1.061 GHz, but BW decreases drastically from 9 MHz to 2 MHz and h decreases from 73% to 32%. When the length of the slot l is increased from 1 cm to 4 cm (keeping w = 1.5 cm), the resonance frequency decreases slightly from 1.061 GHz to 0.981 GHz and then increases slightly to 1.001 GHz. The approximate resonance frequency can be calculated by equating (L e + 2w ) = l/2. However, for smaller slot dimensions, a better approximation is obtained by taking the average of this length and the length of the RMSA without a slot, which

gives (L e + w ) = l/2.

A bowtie MSA configuration is a variation of the H-shaped MSA [17]. In this case, two triangular shaped slots are cut along the nonradiating edges

of the RMSA as shown in Figure 6.18(b). The effects of varying the triangular slot dimensions are similar to that of the H-shaped MSA.

6.6.3 Rectangular Ring MSA

A rectangular ring MSA (RRMSA) is formed when a rectangular slot is cut in the center of the RMSA as shown in Figure 6.19(a) [16]. With an increase in the slot dimensions, the RMSA becomes RRMSA and then a printed loop antenna, and the resonance frequency decreases [18]. The results obtained using IE3D for various values of slot dimensions w and l are summarized in Table 6.10. For L = 6 cm and W = 4 cm, as (w × l ) are

Figure 6.19 (a) RRMSA and (b) RRMSA with short.

Table 6.10

Effect of the Slot Dimensions on the Performance of the RRMSA (L = 6 cm, W = 4 cm, er = 2.33, h = 0.159 cm, and tan d = 0.002)

228 Broadband Microstrip Antennas

increased from (0, 0) to (1 cm, 1 cm), the resonance frequency decreases from 1.606 GHz to 1.537 GHz, because the path length of the surface current increases due to an increase in the slot dimensions. The approximate resonance frequency of the RRMSA can be calculated by equating (L + w ) = l/2. When the slot dimensions are small, a better approximation is obtained by equating (L e + w /2) = l/2.

For the larger slot dimensions, the input impedance is very large and it is difficult to match with the 50-V coaxial line. Therefore, a shorting post is placed in the center of one of the sides, and then impedance matching is obtained by placing the feed near the shorting post as shown in Figure 6.19(b). This shorting does not alter the overall field distribution around the periphery, and hence the resonance frequency remains almost the same. The optimized feed point and the BW for different slot dimensions for the RRMSA with a short are also given in Table 6.10.

The radiation pattern of the RRMSA is similar to that of an RMSA. With an increase in the slot dimensions, the directivity of the antenna remains around 7.1 dB, but the efficiency decreases, resulting in a decrease in the gain of the antenna.

The comparison of RMSA and the other configurations, which are obtained by cutting a slot in the RMSA, is shown in Table 6.11. The outer dimensions are kept constant in all the cases and the total slot area is kept approximately the same. The C-shaped MSA is the most compact configuration but it has the worst efficiency, and the RRMSA gives the least reduction in frequency but has the higher efficiency and hence larger gain.

6.7 Slotand Short-Loaded MSAs

It has been observed in Sections 6.2–6.4 that by using shorting posts, the resonance frequency reduces. Also, by cutting a slot in the patch, the frequency

Table 6.11

Comparison of Various MSA Configurations with and Without Slot (L = 6 cm, W = 4 cm, er = 2.33, h = 0.159 cm, and tan d = 0.002)

shorting posts of diameter 0.1 cm. The antenna is fabricated on substrate with er = 2.2 and h = 0.0508 cm. The outer dimensions of the antenna are L = 0.955 cm and W = 1.1 cm. For slot length l = 0.455 cm and different values of slot depth w, the theoretical and measured reflection coefficients G are shown in Figure 6.21(b). As w increases from 0.1 cm to 1.1 cm, the resonance frequency decreases from approximately 5.0 GHz to 2.86 GHz (i.e., by more than 40%). For all the cases, the radiation pattern is in the broadside direction with a cross-polar level better than 17 dB.

6.8 Planar Broadband Compact MSAs

The compact MSAs described in earlier sections have a smaller BW and gain compared to the regularly shaped MSA due to their smaller size. The BW of these compact MSAs can be increased by using a thicker substrate or by using the planar multiresonator technique [22] or by stacking them in the multilayer configuration as described in Chapters 3 and 4, respectively. In the planar multiresonator technique, the compact coaxially fed element is gapor hybrid-coupled to the compact parasitic elements. These hybridcoupled configurations also give dual-frequency operation, which is discussed in Chapter 7.

6.8.1 Coupled Shorted RMSA

The BW of the RMSA has been increased by feeding one patch and gapcoupling other patches along the radiating or the nonradiating edges as described in Chapter 3. The problem is that the size of the antenna becomes too large. Therefore, if compact RMSAs are coupled, their overall size will still be small, but BW will increase.

6.8.1.1 Radiating Edge Gap-Coupled Shorted RMSA

Two shorted RMSAs that are gap-coupled along one of their radiating edges are shown in Figure 6.22(a). Both the patches are identical having L = 2.3 cm and W = 4.6 cm. A shorting post of diameter 0.1 cm is placed at the center of the extreme edges along the width of the two patches as shown in Figure 6.22(a). Only one patch is fed at x = 0.65 cm, and the other patch is parasitically coupled with a gap of s = 0.1 cm. The antenna is fabricated on a low-cost glass-epoxy substrate having er = 4.3, h = 0.159 cm, and tan d = 0.02 [23]. The measured input impedance and VSWR plots are shown in Figure 6.22(b, c). The loop is within the VSWR = 2 circle and

Figure 6.22 (a) Radiating edge gap-coupled shorted RMSA and its measured (b) input impedance and (c) VSWR plots.

the BW is 30 MHz (4%). This BW is 2.5 times of the corresponding shorted RMSA. The radiation pattern remains in the broadside direction with higher cross-polarization than the RMSA.

6.8.1.2 Nonradiating Edge Hybrid-Coupled Shorted RMSA

Two shorted RMSAs are coupled along the nonradiating edges as shown in Figure 6.23. The two RMSAs are identical having L = 4.6 cm and W = 2.3 cm. Only one patch is fed at y = 0.35 cm. When the patches are only gap-coupled with s = 0.1 cm, no loop is formed in the impedance plot, because of reduced coupling between the nonradiating edges. To enhance the coupling, either gap can be reduced or a shorting strip can be used.

Figure 6.23 Nonradiating edge hybrid-coupled shorted RMSA.

Since the field is not uniform along the nonradiating edges, the coupling depends upon the location of the shorting strip and its width. When the shorting strip is placed at P1, a broad BW of 36 MHz (4.9%) is obtained for the shorting strip width w 1 = 0.2 cm. A thicker shorting strip w 2 is required at P2 because the field is smaller at this location. For w 2 = 0.8 cm a BW of 30 MHz is obtained [23].

6.8.2 Broadband Gap-Coupled Shorted 908-Sectoral MSA

Two gap-coupled shorted 90°-sectoral MSAs of radius a = 3 cm with a gap s = 0.15 cm are shown in Figure 6.24(a). The antenna is fabricated on a glass-epoxy substrate. The shorting is done with closely spaced posts having a diameter of 0.04 cm. For y = 0.75 cm, the measured input impedance and VSWR plots are shown in Figure 6.24(b, c). The BW is 69 MHz at 1.358 GHz. The loop in the input impedance plot can be brought near the center of the Smith chart by reducing the parasitic patch dimensions slightly. For comparison, the response of a CMSA of radius a = 3 cm with a feed point at 1.1 cm from the center, is also given in Figure 6.24. The BW of the CMSA is 28 MHz at 1.375 GHz. Therefore, the BW of the gap-coupled configuration is almost 2.5 times of that of the CMSA, and its area is nearly half of that of the CMSA.

The measured radiation pattern in the E- and H-planes of the gapcoupled shorted 90°-sectoral MSA at the center frequency is compared with that of the CMSA in Figure 6.25. The copolar radiation pattern of the two antennas is similar. However, the cross-polar level of the gap-coupled antenna is higher than that of the CMSA [24].

Figure 6.24 (a) Broadband gap-coupled shorted 90°-sectoral MSA and its measured (b) input impedance and (c) VSWR plots: ( —— ) gap-coupled shorted 90°-sector and ( - - - ) CMSA.

Section 6.1 explained that the resonance frequency of the shorted 90°-sectoral antenna decreases with a decrease in the number of shorting posts and that it is at a minimum when a single shorting post is used in the middle of the uncoupled straight edge. When two sectoral MSAs with a single short are coupled with a gap of 0.15 cm, a BW of 41 MHz is obtained at 784 MHz, thereby realizing a compact configuration with a BW of 5.2% [24, 25].

6.8.3 C-Shaped MSA Coupled with a Shorted RMSA

Instead of using two identical compact patches, different compact patches could be coupled to increase the BW and retain the overall compactness of

Figure 6.25 Measured radiation pattern in the (a) E- and (b) H-planes: ( —— ) gap-coupled shorted 90°-sectoral MSA and ( - - - ) CMSA.

the antenna. A C-shaped MSA is gap-coupled to the shorted RMSA as shown in Figure 6.26(a). The antenna is fabricated on a low-cost glassepoxy substrate. Only the shorted RMSA is fed, and the C-shaped MSA is parasitically coupled. To excite the fundamental mode of the C-shaped MSA, a shorting strip is used. To obtain broad BW, various parameters to be optimized are patch dimensions, the number of shorting posts, the gap between the patches, the width and location of the shorting strip, and the feed-point location. For the optimized dimensions given in Figure 6.26(a), the measured input impedance and VSWR plots are shown in Figure 6.26(b, c). A BW of 34 MHz (5%) is obtained. The radiation pattern is in the broadside direction with a higher cross-polarization.

The area of this gap-coupled configuration is 27 cm2. On the other hand, with same area, an RMSA of L = 6 cm and W = 4.5 cm has a resonance frequency of 1.196 GHz and a BW of 22 MHz. Thus, the gap-coupled configuration gives a 43% reduction in the resonance frequency with more BW [26].

6.8.4 C-Shaped MSA with a Matching Network

Instead of placing a shorted RMSA within the slot of the C-shaped MSA, a LC resonant circuit also known as a matching network is connected as shown in Figure 6.27(a) [27]. The thin line represents the series inductance, whereas the thick line represents the shunt capacitance. The matching network is designed in such a way that it provides capacitive impedance below the resonance frequency of the unloaded patch, and inductive impedance above that frequency. As a result, the patch has two close resonance frequencies in the lower and upper sides of its unloaded resonance frequency giving