Антенны, СВЧ / OC / Broadband microstrip antennas

Figure 6.36 CMSA with a U-slot.

3.77 GHz to 4.78 GHz (24%). The gain is about 7 dB at a frequency of 4 GHz.

6.10.3 TMSA with a U-Slot

Similar to the RMSA and CMSA with a U-slot, an ETMSA with a U-slot as shown in Figure 6.37(a) also yields a broad BW [40]. A foam substrate of thickness 1.43 cm (0.08l0) is used. The measured return loss G plot is shown in Figure 6.37(b), and the measured BW is 18%. The resonance frequency for the fundamental mode of ETMSA without a slot is approximately 1.8 GHz. Due to the presence of the slot, its frequency reduces and two adjacent modes are excited at 1.563 GHz and 1.746 GHz. The higher resonance frequency is more sensitive to the variation in the horizontal length of the slot, and the lower resonance frequency strongly depends upon the total length of the U-shaped slot. The radiation pattern is in the broadside direction over the entire BW. However, higher cross-polarization radiation is observed in the H-plane.

Similar results are obtained when a U-slot is cut inside an ITMSA with flare angle a = 53.9°. The length of the horizontal slot and width of

the parallel slot are reduced to adjust the occurrence of the two adjacent resonant modes for broadband operation. The measured BW is 304 MHz (17.8%) at the center frequency of 1,709 MHz. The radiation characteristics are similar to that of the ETMSA with a U-slot [40].

Figure 6.37 (a) TMSA with a U-slot and (b) its measured return loss plot.

6.11 Summary

Compact antennas are required for applications where limited antenna real estate is available. A compact MSA is realized by increasing the dielectric constant of the substrate, by using shorting posts, or by cutting a slot. Compact MSAs are realized by shorting along the zero potential line of the RMSA, CMSA, ETMSA, and 30°-60°-90° TMSA. The resonance frequency of the shorted RMSA decreases with a decrease in the shorting width and

is minimum for a single shorting post. Similarly, C- and H-shaped MSAs and RRMSAs are realized by cutting a slot in the RMSA. When both of these techniques (i.e., shorting and cutting slot) are combined, a very compact MSA is realized. Shorted C- and H-shaped MSAs fall in this category. However, the BW and gain of these antennas are significantly lower.

The BW of the compact MSAs is increased by using multiple resonators in planar configurations. Various shorted RMSAs, 90°-shorted sectors, and shorted C and ring antennas are gapor hybrid-coupled to achieve a broad BW. The multiresonator concept is also used in a stacked configuration to obtain a broad BW.

By cutting a U-slot in the rectangular, circular, and triangular MSA on a thick, low dielectric substrate, a broad BW is obtained. This antenna is most attractive as it yields a large BW without increasing the surface area and with stable radiation characteristics over the entire BW.

References

[1]Hirasawa, K., and M. Haneishi, Analysis, Design, and Measurement of Small and LowProfile Antennas, Norwood, MA: Artech House, 1992.

[2]James, J. R., and P. S. Hall, Handbook of Microstrip Antennas, Vol. 1, London: Peter Peregrinus, Ltd., 1989.

[3]IE3D 7.0, Zeland Software, Inc., Fremont, CA.

[4]Sanad, M., ‘‘Effect of the Shorting Posts on Short Circuit Microstrip Antennas,’’ IEEE AP-S Digest, 1994, pp. 794–797.

[5]Satpathy, S., K. P. Ray, and G. Kumar, ‘‘Compact Microstrip Antennas Using a Single Shorting Post,’’ Proc. NSAML, New Delhi, India, March 1998, pp. 69–72.

[6]Ray, K. P., et al., ‘‘Investigations on Shorted Rectangular Microstrip Antennas,’’ Proc. ELECTRO -2000, BHU, Varanasi, India, January 2001, pp. 153–156.

[7]Satpathy, S., K. P. Ray, and G. Kumar, ‘‘Compact Shorted Variations of Circular Microstrip Antennas,’’ Electronics Letters, Vol. 34, No. 2, 1998, pp. 137–138.

[8]Bahl, I. J., and P. Bhartia, Microstrip Antennas, Dedham, MA: Artech House, 1980.

[9]Waterhouse, R., ‘‘Small Microstrip Patch Antenna,’’ Electronics Letters, Vol. 31, No. 8, 1995, pp. 604–605.

[10]Satpathy, S., G. Kumar, and K. P. Ray, ‘‘Compact Shorted Variations of Triangular Microstrip Antennas,’’ Electronics Letters, Vol. 34, No. 8, 1998, pp. 709–711.

[11]Wong, K. L., and S. C. Pan, ‘‘Compact Triangular Microstrip Antenna,’’ Electronics Letters, Vol. 33, March 1997, pp. 433–434.

[12]Wong, K. L., and Y. F. Lin, ‘‘Small Broadband Rectangular Microstrip with Chip Resistor Loading,’’ Electronics Letters, Vol. 33, No. 19, 1997, pp. 1593–1594.

[13]Srinivasan, V., S. Malhotra, and G. Kumar, ‘‘Multiport Network Model for Chip- Resistor-Loaded Rectangular Microstrip Antennas,’’ Microwave Optical Technical Letters, Vol. 24, No. 1, 2000, pp. 11–13.

[14]Kosiavas, G., et al., ‘‘The C-Patch: A Small Microstrip Element,’’ Electronics Letters, Vol. 25, 1989, pp. 253–254.

[15]Sanad, M., ‘‘Double C-Patch Antennas Having Different Aperture Shapes,’’ IEEE AP-S Int. Symp. Digest, 1995, pp. 2116–2119.

[16]Palaniswamy, V., and R. Garg, ‘‘Rectangular Ring and H Shape Microstrip Antenna— Alternative Approach to Rectangular Microstrip Antenna,’’ Electronics Letters, Vol. 21, No. 19, 1985, pp. 874–876.

[17]George, J., et al., ‘‘New Compact Microstrip Antenna,’’ Electronics Letters, Vol. 32, No. 6, 1996, pp. 508–509.

[18]Bafrooei P. M., and L. Shafai, ‘‘Characteristics of Single and Double Layer Microstrip Square Ring Antennas,’’ IEEE Trans. Antennas and Propagation, Vol. AP-47, No.10, 1999, pp. 1633–1639.

[19]Srinivasan, V., ‘‘Multiport Network Model for Variations in Rectangular Microstrip Antennas,’’ Ph.D. thesis, Indian Institute of Technology Bombay, India, 2000.

[20]Satpathy, S., et al., ‘‘Compact Microstrip Antennas for Personal Mobile Communication,’’ Proc. IEEE TENCON, New Delhi, India, December 1998, pp. 245–247.

[21]Singh, D., P. Gardener, and P. S. Hall, ‘‘Miniaturized Microstrip Antenna for MMIC Application,’’ Electronics Letters, Vol. 33, No. 22, 1997, pp. 1830–1831.

[22]Kumar, G., and K. C. Gupta, ‘‘Broadband Microstrip Antennas Using Additional Resonators Gap-Coupled to the Radiating Edges,’’ IEEE Trans. Antennas Propagation, Vol. AP-32, 1984, pp. 1375–1379.

[23]Kapur, R., and G. Kumar, ‘‘Hybrid Coupled Shorted Rectangular Microstrip Antennas,’’ Electronics Letters, Vol. 35, No. 18, 1999, pp. 1501–1502.

[24]Ray, K. P., and G. Kumar, ‘‘Compact Gap-Coupled Shorted 90° Sectoral Microstrip Antennas for Broadband and Dual-Band Operations,’’ Microwave Optical Tech. Letters, Vol. 26, No. 3, 2000, pp. 143–145.

[25]Ray, K. P., ‘‘Broadband, Dual-Frequency and Compact Microstrip Antennas,’’ Ph.D. thesis, Indian Institute of Technology, Bombay, India, 1999.

[26]Deshmukh, A., and G. Kumar, ‘‘Hybrid-Coupled Compact Variation of Rectangular

Broadband Microstrip Antennas,’’ IEEE AP-S Int. Symp. Digest, July 2000,

pp.1422–1425.

[27]Fayyaz, N., and S. S. Naeini, ‘‘Bandwidth Enhancement of a Rectangular Patch Antenna by Integrated Reactive Loading,’’ IEEE AP-S Int. Symp. Digest, 1998,

pp.1100–1103.

[28]Kokotoff, D. M., R. B. Waterhouse, and J. T. Aberle, ‘‘An Annular Ring Coupled to Shorted Patch,’’ IEEE Trans. Antennas Propagation, Vol. AP-45, No. 5, 1997,

pp.913–914.

[29]Deshmukh, A., and G. Kumar, ‘‘Shorted Compact Broadband Microstrip Antennas,’’

Proc. NCC-2000 Symp., Indian Institute of Technology, Delhi, India, 2000, pp. 49–52.

[30]Sanad, M., ‘‘A Wideband Microstrip Antenna for Portable Cordless Telephones,’’

IEEE AP-S Int. Symp. Digest, 1995, pp. 1132–1135.

[31]Waterhouse R. B., ‘‘Broadband Stacked Shorted Patch,’’ Electronics Letters, Vol. 35, No. 2, 1999, pp. 98–99.

[32]Huynh, T., and K. F. Lee, ‘‘Single Layer Single Patch Wideband Microstrip Antenna,’’ Electronics Letters, Vol. 31, No. 16, August 1995, pp. 1310–1312.

[33]Lee, H. F., and W. Chen, Advances in Microstrip and Printed Antennas, New York: John Wiley & Sons, 1997.

[34]Lee, K. F., et al., ‘‘Experimental and Simulation Studies of the Coaxially Fed U-Slot Rectangular Patch Antenna,’’ IEE Proc. Microwaves, Antennas Propagation, Pt-H, Vol. 144, No. 5, 1997, pp. 354–358.

[35]Clenet, M., and L. Shafai, ‘‘Multiple Resonances and Polarization of U-Slot Patch Antenna,’’ Electronics Letters, Vol. 35, No. 2, 1999, pp. 101–103.

[36]Tong, K. F., et al., ‘‘A Broad-Band U-Slot Rectangular Patch Antenna on a Microwave Substrate,’’ IEEE Trans. Antennas Propagation, Vol. AP-48, No. 6, 2000, pp. 954–960.

[37]Guo, Y. X., et al., ‘‘Double U-Slot Rectangular Microstrip Antenna,’’ Electronics Letters, Vol. 34, No. 19, 1998, pp. 1805–1806.

[38]Luk, K. M., et al., ‘‘L-Probe Proximity Fed U-Slot Patch Antenna,’’ Electronics Letters, Vol. 34, No. 19, 1998, pp. 1806–1807.

[39]Luk, K. M., K. F. Lee, and W. L. Tam, ‘‘Circular U-Slot Patch with Dielectric Superstrate,’’ Electronics Letters, Vol. 33, No. 12, 1997, pp. 1001–1002.

[40]Wong, K. L., and W. H. Hsu, ‘‘Broadband Triangular Microstrip Antenna with U-Shaped Slot,’’ Electronics Letters, Vol. 33, No. 25, 1997, pp. 2085–2087.

When two or more resonance frequencies of a MSA are close to each other, one gets broadband characteristics. When these are significantly separated, dual-band operation is obtained. In general, all the methods described in Chapters 3 and 4 for increasing the BW of the MSAs can be utilized to obtain dual-band operations.

Various single and multilayer MSA configurations for dual-band operations are possible. In the single-layer MSA, dual-band operation can be achieved by utilizing the multiresonance characteristics of a single patch, by reactively loading the patch with quarter-wavelength stubs, by using shorting posts, by cutting slots, and by adding lumped elements, among other techniques. Multiresonators in both planar and stacked configurations yield dual-band operations. Both electromagnetic as well as aperture coupling mechanisms are used in multilayer configurations.

7.2 Tunable MSAs

The resonance frequency of the MSA can be tuned by changing its resonant dimension. For the fixed dimensions of a patch, the same effect is achieved by reactively loading the patch. This section discusses these tuning methods.

7.2.1 Stub-Loaded Tunable MSAs

One of the techniques for obtaining tunability is to add a stub to an MSA. For a RMSA, the stub may be placed either along one of its radiating or nonradiating edges. When the length of the stub is small, it yields tunability, whereas when it is comparable to l/4, it yields dual-frequency operation. The RMSA with a small stub for tuning the resonance frequency has been analyzed using MNM and IE3D [2–4]. Similarly, a CMSA with stubs also yields tunable and dual-band operation, which has been analyzed using an improved linear transmission line model [5–7].

7.2.1.1 Tunable RMSA with a Single Stub

An RMSA of length L = 3 cm and width W = 4 cm with a small stub of length l and width w is shown in Figure 7.1(a). The stub is placed at the center of one of the radiating edges of the RMSA. The substrate parameters are er = 2.55, h = 0.159 cm, and tan d = 0.001. To account for the fringing fields from the stub-loaded RMSA, the periphery is extended outward as shown in Figure 7.1(b, c). The effect of the stub is to increase the overall effective resonant length of the RMSA from L e to L e + Dl1. The value of

Figure 7.1 (a) RMSA with a single stub and its (b) edge extensions and (c) effective dimensions.

Dl1 is obtained by equating the effective area of the stub (= we ? le ) with that of the extension Dl1 along the effective width of the RMSA as given below.

The values of we , le , and We are obtained from expressions given in Appendix B. The resonance frequency of the stub-loaded RMSA is approximately obtained by modifying (2.9) for the RMSA as

where L e and ee are the effective length and effective dielectric constant corresponding to the rectangular patch.

The effect of the dimensions of the stub on the resonance frequency f 0 and BW for the feed point at x = 0.7 cm is given in Table 7.1. These

Figure 7.2 RMSA with two stubs.

the stub length l on the resonance frequency and BW of the antenna is given in Table 7.2. As in the case of single stub-loaded RMSA, with an increase in l, the resonance frequency decreases. In this case, the resonance frequency can be approximately calculated as:

It may be noted that Dl1 is added on both the sides of the effective RMSA. With an increase in l from 0.0 to 1.5 cm, the resonance frequency decreases from 2.975 GHz to 2.243 GHz and the BW decreases from 65 MHz to 22 MHz. In this case, the radiation pattern is similar to that of the RMSA without a stub. The cross-polar levels are much smaller than those of the single stub-loaded RMSA. Dual-band operation is obtained for l = 1.5 cm at 2.243 GHz and 3.604 GHz.

Table 7.2

Effect of Stub Length on the Resonance Frequency and BW of a Dual Stub-Loaded RMSA (L = 3 cm, W = 4 cm, w = 0.4 cm, x = 0.7 cm, er = 2.55, h = 0.159 cm, and tan d = 0.001)