Антенны, СВЧ / OC / Broadband microstrip antennas

Figure 7.20 (a) Input impedance and (b) VSWR plots of double stub-loaded RMSA for dual-band operation.

A small loop may be noted at 2.813 GHz, but the radiation pattern at this frequency is conical. By using unequal stub lengths, the size of the smaller loop can be increased to yield triple-frequency response [29].

7.3.2.3 Single Stub-Loaded CMSA

A CMSA with a single stub is shown in Figure 7.21(a). When the stub length is small, its resonance frequency is tunable as described in Section 7.2.1.4. However, when the stub length is comparable to l/4, dual-band operation is achieved [5]. For a = 3 cm, er = 2.33, h = 0.159 cm, and tan d = 0.001, the theoretical and the measured input impedance and VSWR plots for stub length l = 1.85 cm and width w = 0.4 cm are shown in Figure 7.21(b, c). The theoretical (obtained using the linear transmission line model) and the experimental dual-band resonance frequencies for different stub lengths are given in Table 7.8. The agreement between the theoretical and the experimental resonance frequencies are within 1% at the lower frequency f 1 and within 4% at the higher frequency f 2. The measured f 2 /f 1 ratio increases from 1.350 to 1.527 as l decreases from 3.40 cm to 1.60 cm.

The measured radiation pattern of the antenna in the E- and H-planes remains in the broadside direction at f 1. However, at f 2 for some cases, a dip in the broadside direction in the E-plane is noted as given in Table 7.8. It is observed that as f 2 /f 1 approaches near the ratio of 1.6588 (the ratio of the first two roots of the derivative of the Bessel function or 3.054/ 1.84118), the dip in the broadside in the E-plane becomes more pronounced.

Figure 7.22 (a) Double stub–loaded CMSA and its (b) input impedance and (c) VSWR plots: ( - - - ) theoretical and ( —— ) experimental.

length. The theoretical and experimental input impedance and VSWR plots for stub lengths l1 = 1.75 cm and l2 = 2.0 cm are shown in Figure 7.22(b, c). The comparison between the theoretical and experimental dual-band resonance frequencies for various combinations of stub lengths l1 and l2 is given in Table 7.9. When the stub lengths are varied, the change in f 1 is less as compared to that in the f 2. As the stub length is increased, the BW at both the resonance frequencies decreases because of the decrease in the resonance frequency, which decreases h /l 0.

For a double stub-loaded CMSA, the radiation fields are in the broadside direction at f 1, whereas there is a dip in the broadside direction in the

E-plane at f 2. The variation of dip in the broadside in the E-plane at f 2 is also given in Table 7.9. As the ratio f 2 /f 1 approaches 1.6588, the dip in the E-plane is more pronounced, indicating that higher resonance corresponds to the higher order TM21 mode of the CMSA. When the length of the two stubs are nearly equal, the ratio f 2 /f 1 is close to 1.6588, and hence there is a larger dip in the broadside at f 2. As f 2 /f 1 deviates from 1.6588, the amount of the dip reduces and finally disappears.

7.3.3 Notch-Loaded MSAs

Reactive loading to the patch could also be provided by cutting a notch or spur line in one of the edges of the MSA to obtain dual-band operation. It creates the same effect as the stub loading, with the advantage of reduced size [30–34].

A square MSA with a rectangular notch of depth w along its nonradiating edge is shown in Figure 7.23(a). The antenna is similar to that of a C-shaped MSA, but it is fed along the diagonal, which excites orthogonal polarization at the first and second resonance frequencies. The antenna parameters are as follows: side length of the square patch L = 2.5 cm, notch length l = 1.0 cm, er = 1.79, h = 0.08 cm, and tan d = 3.2 × 10−4. The antenna is fed along the diagonal by a 50-V coaxial line at x = −y = 0.523 cm. For the square patch without the notch (w = 0), the resonance frequency is 4.3 GHz. The variation of the resonance frequencies f 1 and f 2 as a function

Figure 7.23 (a) Square MSA with a notch and (b) variation of resonance frequencies with w : ( —— ) f 2 theoretical and ( ? ? ? ) f 2 experimental, and ( - - - ) f 1 theoretical and ( x x x ) f 1 experimental.

at higher frequencies to improve the matching. The effects of successively adding more posts (each approximately 0.05 cm in diameter) at the positions indicated in Figure 7.24 are shown in Table 7.10. Since all these posts are located at the nulls of the TM30 mode, f 2 remains constant at around 1,865 MHz, while f 1 varies from 613 MHz to 891 MHz. The ratio f 2 /f 1 varies from 3.0 to 2.1, which could be lowered by using more shorting posts.

The radiation pattern remains in the broadside direction at both the frequencies with the same polarization. However, a sidelobe at f 2 appears in the E-plane pattern, which is due to the large spacing (normalized with respect to the free-space wavelength) between the two radiating edges of the patch. It can be easily reduced or even removed by using substrate with higher er .

7.3.4.2 Dual-Band RMSA Using a Single Short

A compact dual-frequency RMSA using a single shorting post is shown in Figure 7.25(a). The patch dimensions are L = 3.73 cm and W = 2.487 cm, and the feed point is kept within a few millimeters from the shorting post. The substrate parameters are er = 4.4 and h = 0.0762 cm. The diameter of the shorting post is 0.064 cm, and the feed diameter is 0.126 cm. When the location d s of the shorting post is changed, the two resonance frequencies vary as shown in Figure 7.25(b). The lower resonance frequency f 1 varies in the range of 722–950 MHz and the higher resonance frequency f 2 varies in the range of 1,900–2,310 MHz. This yields a frequency-tuning range of 2.0–3.2. When the shorting post is near the edge of the patch, it gives a maximum separation of frequencies. The antenna has a broadside radiation pattern at both the frequencies [36, 37].

Table 7.10

Variation of Resonance Frequencies with a Number of Shorting Posts of Multiple Shorted RMSA