Антенны, СВЧ / OC / Broadband microstrip antennas

Figure 6.26 (a) C-shaped MSA gap-coupled to shorted RMSA and its measured (b) input impedance and (c) VSWR plots.

rise to broad BW. The transmission line model is used to analyze and design the antenna. The optimized dimensions in centimeters are shown in Figure 6.27(a). The substrate parameters are er = 4.8 and h = 0.052 cm. The reflection coefficient G plots for the loaded and unloaded C-patch are shown in Figure 6.27(b). The loaded configuration yielded a BW of 75 MHz at the center frequency of 1.345 GHz as compared to 15 MHz for unloaded C-patch. The radiation is in the broadside direction and the cross-polarization levels are 17 dB below the copolar levels.

Figure 6.27 (a) C-shaped MSA with matching network and (b) reflection coefficient G plots for ( —— ) loaded and ( - - - ) unloaded C-shaped MSA.

6.8.5 Circular Ring Containing a Shorted CMSA

The resonance frequency of a circular ring is much smaller than a CMSA as described in Chapter 2. However, the BW of a circular ring MSA is also small. To increase its BW without increasing its surface area, a shorted CMSA is placed inside the ring as shown in Figure 6.28(a). The CMSA is shorted so that its resonance frequency is close to that of the ring MSA. Only the shorted CMSA is fed using an SMA connector and the ring is gap-coupled. The optimized dimensions for er = 1.13, h = 0.5 cm, and

Figure 6.28 (a) Circular ring containing a shorted CMSA and (b) its input impedance plot: ( —— ) theoretical and ( - - - ) measured.

tan d = 0.001 are given in Figure 6.28(a). The radius of the shorting post is 0.033 cm. The measured and theoretical input impedance loci are shown in Figure 6.28(b). The theoretical results are obtained using the spectral domain electric field integral equation technique [28]. The measured BW is 6.8% at 1.9 GHz with a gain of 8.4 dB. This configuration is compact and gives wider BW as compared to the conventional CMSA.

6.8.6 Rectangular Ring Containing a Shorted RMSA

Similar to the gap-coupled circular ring MSA, a rectangular ring is gapcoupled to the shorted RMSA as shown in Figure 6.29(a) [29]. The optimized dimensions in centimeters for a glass-epoxy substrate are shown in Figure 6.29(a). The measured input impedance and VSWR plots are shown in Figure 6.29(b, c). The BW is 36 MHz centered at 842 MHz. This configuration gives a 30% reduction in the resonance frequency with twice the percentage BW as compared to the conventional RMSA.

6.8.7 Three Gap-Coupled Shorted C-Shaped MSA

Three compact shorted C-shaped patches are gap-coupled as shown in Figure 6.30(a) [30]. Only the central patch is fed; the other two patches are parasitically coupled. A large number of parameters are optimized to obtain broad BW. The most sensitive design parameters are the lengths of the fed and parasitic elements and the dimensions and the location of the apertures. The width of the partial short circuit and the location of the feed point have a strong effect on the input impedance of the antenna. The dimension of the ground plane has a significant effect on the performance of the antenna. Truncating the ground plane improves the isotropic characteristics of the radiation pattern, increases its sensitivity to horizontal and vertical polarized waves, and reduces the effect of human body on the antenna if mounted on a portable handset. The antenna is fabricated on a substrate with er = 2.2 and h = 0.23 cm and the optimized dimensions are shown in Figure 6.30. The ground plane size is only 0.1 cm larger from all the sides of the gap-coupled patches.

The measured input impedance and return loss | G| of the antenna are shown in Figure 6.30(b, c). The resonance frequency is around 900 MHz, and the BW is 40 MHz. Figure 6.31 shows the vertical and horizontal polarized radiation pattern at 900 MHz. The antenna is sensitive to both the polarizations, and the pattern is nearly isotropic. The variation of the pattern over the BW is very small. The total size of this antenna is much smaller than the conventional half-wavelength RMSA.

Figure 6.30 (a) Three gap-coupled shorted C-shaped MSA, and its (b) input impedance and (c) return loss | G | plots.

configuration does not increase the planar area, but the thickness of the antenna increases.

The side and top views of a stacked shorted RMSA configuration are shown in Figure 6.32(a, b). The two patches are shorted using a single shorting post. The bottom patch is fed by a coaxial feed, which is extended and soldered to both the patches. Both these patches are fabricated on a thin RT Duroid 5880 substrate of thickness 0.0125 cm and supported by foam with er = 1.07. The shorting post radius is taken as 0.0325 cm. The

Figure 6.31 Radiation pattern of three gap-coupled shorted C-shaped MSA at 900 MHz: ( —— ) vertical and ( - - - ) horizontal polarizations.

optimized dimensions of the configuration are given in Figure 6.32. The theoretical and measured input impedance loci are shown in Figure 6.32(c). The theoretical and measured BW is 32% and 34%, respectively. The measured radiation pattern in the E- and H-planes is shown in Figure 6.32(d), which is similar to that of the monopole antenna on a finite ground plane. The measured gain is more than 3.9 dB across the BW [31].

Similarly, other compact configurations using a single short or with a slot, such as rectangular ring and C-shaped patches, can be stacked to yield a broad BW [18].

6.10 Broadband MSAs with a U-Slot

It is observed in Section 6.6 that by cutting a slot inside or along the periphery of the RMSA, various compact configurations are realized with a reduced BW. If the resonance frequencies of the slot and the patch are close to each other, then broad BW could be obtained. However, care must be taken so that the polarization of the radiated field of the slot and the patch are similar, so that the pattern remains stable over the VSWR BW.

6.10.1 RMSAs with a U-Slot

A very promising configuration that yields broad BW is a RMSA with a U-shaped slot [32–36]. A resonant U-slot is cut symmetrically around the center of the patch. When a slot is cut inside the patch, the resonance frequency of the patch changes slightly in comparison with the resonance

Figure 6.32 Stacked shorted RMSA: (a) side and (b) top views, (c) input impedance plots [( —— ) theoretical and ( - - - ) measured], and (d) measured radiation pattern [( —— ) E-plane and ( - - - ) H-plane.]

frequency of the slot. Accordingly, the dimensions of the slot are chosen such that its resonance frequency is close to that of the rectangular patch with a slot. A thick foam substrate of thickness h = 2.7 cm, which corresponds to 0.08l at 0.9 GHz, is used to obtain a broad BW. All the dimensions, shown in Figure 6.33(a), are in centimeters. The measured input impedance and VSWR plots are shown in Figure 6.33(b, c). The measured BW is 47%. The wide BW is due to the appearance of two loops in the impedance plot representing the coupling between the rectangular patch and the U-slot. The resonance frequency of the RMSA with a U-slot is primarily determined by the length of the patch, and the position of the loop in the impedance plot primarily depends upon the total length of the U-slot. The loop in the lower

Figure 6.32 (Continued.)

frequency region is due to the resonance of the U-slot. The loop in the higher frequency region is due to the excitation of the second-order mode of the patch, when width W = 21.97 cm becomes equal to l.

It may be noted from Figure 6.33(b) that there is no appreciable inductive component associated with the input impedance. On the other hand, for the RMSA without a U slot, there would be large inductive reactance in the input impedance of the patch, because the substrate is electrically thick and even if the feed point is shifted to the edge of the patch, it is not possible to achieve impedance matching. So, a U-slot adds a capacitive component in the input impedance that compensates for the inductive component of the coaxial probe.

The radiation pattern in the E- and H-planes at 0.9 GHz is shown in Figure 6.34. The H-plane pattern remains in the broadside direction at all the frequencies within the BW. In the E-plane, on the other hand, the maxima shifts from the broadside as the frequency increases within the BW, and the pattern becomes conical at the second loop corresponding to 1.2 GHz. Therefore, the useful BW from the pattern point of view is slightly less than the impedance BW. The HPBW in the H-plane is 59° at 0.812 GHz and 57° at 1.1 GHz. In the E-plane, it is 65° at 0.812 GHz and 70° at 1.1 GHz.

Since the second loop in the impedance plot corresponds to the width W becoming equal to l , it can be shifted away by simply reducing W, which also reduces the size of antenna.

Figure 6.33 (a) RMSA with a U-slot and its measured (b) input impedance and (c) VSWR plots.

By using two U-slots in the RMSA as shown in Figure 6.35, and optimizing the slot dimensions, two loops appear in the Smith chart. These loops are due to the excitation of the inverted and noninverted slots. All the dimensions are given in Figure 6.35. The antenna is fabricated on a foam substrate with thickness h = 1.65 cm. The measured BW is 700 MHz (44%). The radiation pattern over the entire BW remains in the broadside direction [37]. Above configurations use coaxial feed. The coaxial feed can be replaced with an L-shaped probe, as described in Chapter 2, to further increase the BW [38].

Figure 6.34 Radiation pattern of an RMSA with a U-slot at 0.9 GHz: ( —— ) E-plane and ( - - - ) H-plane.

Figure 6.35 An RMSA with two U-slots.

6.10.2 CMSA with a U-Slot

Instead of cutting a U-shaped slot in the rectangular patch, it can be cut inside the circular patch as shown in Figure 6.36 [39]. The circular patch with a U-slot is fabricated on the low-cost printed circuit board (PCB) substrate in the inverted suspended configuration. As a result, the top layer acts as a protective superstrate layer. The inverted patch is supported by a foam substrate with h = 0.5 cm. An impedance BW for VSWR ≤ 2 is from