Антенны, СВЧ / OC / Broadband microstrip antennas

8.4.5 Short or Chip Resistor-Loaded Square MSA

Instead of adding stubs or cutting slots or notches, in the square patch shorting posts or chip resistors could be used to obtain CP [15–17]. The resonance frequency of a square MSA changes when two shorting posts are placed symmetrically on either side of the center along the feed-axis as described in Section 7.2.3. When two shorting posts are placed closer to the center along the x -axis, the resonance frequency for the TM10 mode (oriented along the x -axis) changes slightly but it remains unaffected for the TM01 mode. When this configuration is fed diagonally as shown in Figure 8.18(a), it yields RHCP. When the feed is along the opposite diagonal, LHCP is obtained.

The measured resonance frequency of a square MSA of length L = 3 cm with er = 4.4 and h = 0.16 cm is 2.355 GHz. Two shorts are placed at a distance s = 0.48 cm and the patch is fed along the diagonal at (x , y ) = (−0.5, −0.5) cm to excite two orthogonal modes. The AR variation with frequency is shown in Figure 8.18(b). The BW for AR ≤ 3 dB is 22 MHz (0.9%) and the BW for VSWR ≤ 2 is 116 MHz (4.9%).

Both AR and VSWR BWs of the antenna increase when the shorting posts are replaced by chip resistors of value 4.7V, which is due to increase in the ohmic losses. The feed-point is shifted toward the corner to (−0.7, −0.7) cm and the separation between the resistors is increased to 0.78 cm. The BWs for AR ≤ 3 dB and VSWR ≤ 2 increase to 44 MHz and 164 MHz, respectively. However, the gain of the antenna decreases by 0.7 dB, when shorting posts are replaced by 4.7-V resistors. Therefore, the BW

Figure 8.18 (a) Square MSA with two shorts (or resistors) and (b) variation of AR with frequency: ( - - - ) shorts and ( —— ) resistors.

Figure 8.20 Single-feed modified CMSA: (a) elliptical MSA and CMSA with (b) two notches,

(c) two stubs, and (d) a rectangular slot in the center.

when the ellipticity ratio (major axis to minor axis lengths) varies from 1.01 to 1.10 depending upon the substrate parameters. The feed is placed at an angle of 45° from the x -axis, which excites the two orthogonal modes with an equal amplitude and a 90° phase difference. In Figure 8.20(b), two notches are cut along the periphery of the circular patch at the diametrically opposite points. The feed is placed at an angle of 45° from the notch axis to excite the orthogonal modes in phase quadrature. Instead of cutting notches, two small stubs could be added as shown in Figure 8.20(c). Also, instead of cutting two notches or adding two stubs, only one notch or a stub could also be used. In Figure 8.20(d), a rectangular slot is cut at the center of the circular patch to yield CP. For all these antennas, the performance is similar to that of the modified square MSAs.

A modified ETMSA also generates CP [23–27]. Some of the modified ETMSA configurations are the nearly ETMSA (isosceles triangle with S1 /S2 = 1.01 to 1.10), tip-truncated ETMSA, ETMSA with a rectangular slot, and ETMSA with a notch (also called slit) along the periphery, as shown in Figure 8.21. Depending upon the feed-point location and dimensions, LHCP or RHCP is obtained.

other frequencies by using frequency-scaling techniques described in Section 2.2.7. Alternatively, a single-feed modified MSA can be designed using the following procedures.

8.4.8 Design Procedure for Single-Feed Circularly Polarized MSAs

A square MSA with two corners chopped and a nearly square MSA are shown in Figure 8.23(a, b). To obtain CP with minimum AR, the dimensions of the perturbations are very critical as described earlier.

The approximate value of the total perturbation area Ds = l 2 for a two-corner-chopped square MSA can be obtained from the following relation [1, 29, 30]

Figure 8.23 (a) Square MSA with two corners chopped, (b) nearly square MSA, and CMSA with (c) notches and (d) stubs.

332 Broadband Microstrip Antennas

Q 0 = unloaded quality factor of the patch.

For the nearly square MSA, the perturbation area Ds = l ? L is given

by

The value of Q 0 depends on the dimension of the patch and substrate parameters such as er , h, and tan d. The plot of Q 0 and efficiency h as a function of h /l0 for a square patch of L = 0.914 cm and er = 2.55 is given in Figure 8.24(a). The percentage variations of Ds /S versus Q 0 for both the configurations are shown in Figure 8.24(b). These curves are for only one value of er . For other values of er , Q 0 (er ) can be approximately calculated

For the given substrate parameters, the value of Q 0 is obtained first. The value of the corresponding efficiency should be higher than 90%. Next,

Figure 8.24 (a) Variation of Q 0 ( —— ) and h ( - - - ) with h /l 0 and (b) variation of Ds /S with Q 0 : ( —— ) corner-chopped square, ( - - - ) modified circular, and ( – ? – ) nearly square.

the percentage of perturbation Ds /S required for CP is calculated from (8.10) or (8.11), or read from the plots in Figure 8.24. Finally, the feed point is selected for the desired sense of CP polarization and input impedance matching.

Similarly, for the CMSA with two types of perturbation shown in Figure 8.23(c, d), the perturbation area Ds = 2l 2 can be obtained from [1, 29, 30]

where K 11 = 1.84118 = first derivative of the Bessel function of order 1. The variation of Ds /S with Q 0 of CMSA is plotted in Figure 8.24. Once Ds /S is obtained, the procedure to design a single-feed CMSA for CP radiation is the same as that of the square MSA with perturbation. A similar concept can be extended to other shapes of the single-feed CP MSA.

8.5 Compact Circularly Polarized MSAs

The size of the above CP MSA configurations is large in the UHF frequency range. By combining the techniques to realize compact MSAs covered in Chapter 6 and CP MSAs discussed in previous sections, several compact CP MSA configurations are possible. Some of these are described in the following sections.

8.5.1 Modified Square Ring MSA

A nearly square ring MSA is shown in Figure 8.25(a). The outer dimensions of the ring correspond to that of a nearly square MSA, and a square slot is cut in the center of the patch, which increases the path length of the surface current thereby reducing the resonance frequency. The patch is fed along the diagonal, and the feed point is very close to the slot [29, 31]. The MNM method has been used to analyze the antenna. A nearly square MSA of lengths L 1 = 8.35 cm and L 2 = 8.2 cm, having a square slot of length l = 2.0 cm cut in the center with er = 2.5 and h = 0.159 cm, yields LHCP. The BW for AR ≤ 6 dB is 1.5% at f 0 = 1.043 GHz. The antenna can be made more compact by increasing the slot dimensions, but then impedance matching is not possible for a 50-V coaxial feed point at any location on

Figure 8.27 Square ring MSA with an inside offset polarizer.

is considered. The substrate parameters are er = 4.3, h = 0.16 cm, and tan d = 0.02. When a large slot is cut inside the square patch, the input impedance is large at the two orthogonal points inside the ring, so a quarter-wave transformer is used to transform this impedance to 100V. A 50-V coaxial feed point is located on the 100-V line, such that it provides equal amplitudes with a 90° phase difference to the two orthogonal points at the center of the two adjacent inner edges of the square ring patch. The measured and theoretical (obtained using IE3D) resonance frequencies are 830 MHz and 828 MHz, respectively. The BW for AR ≤ 3 dB is 8 MHz. This antenna has size reduction of 40% as compared to the square MSA for the same frequency of operation.

8.5.2 Modified Square MSA with Slits at the Edges

A compact CP MSA is also realized by cutting two pairs of slits of unequal lengths lx and ly of narrow width w (w << lx , ly ), at the four edges of the square patch as shown in Figure 8.28(a). For the feed-point location shown in Figure 8.28, RHCP is obtained when lx > ly , and LHCP is obtained when lx < ly . With an increase in lx and ly , the resonance frequency of the antenna decreases with a corresponding decrease in the BW. For L = 2.62 cm, lx = 0.97 cm, ly = 0.90 cm, w = 0.1 cm, er = 4.4 and h = 0.16 cm, RHCP is obtained at 2.272 GHz. The BW for AR ≤ 3 dB is 32 MHz (1.4%).

The gain of this compact CP MSA can be increased by placing a l/4 thick substrate of a high dielectric constant on top of the patch, which is