Антенны, СВЧ / OC / Broadband microstrip antennas

Figure 7.36 (a) Dual-band gap-coupled RMSA and its (b) input impedance and (c) VSWR plots for three values of gap s : ( —— ) 0.1 cm, ( – ? – ) 0.05 cm, and ( - - - ) 0.01 cm.

coupling, a copper strip is placed across the gap as shown in Figure 7.37(a). Because of the hybrid (gap and direct) coupling, the loop size increases, and it does not completely remain inside the VSWR = 2 circle, thereby yielding a dual-band response [51]. The measured results for different shorting strip widths w are summarized in Table 7.14. With an increase in w from 0.1 cm to 1.0 cm, the lower frequency f 1 decreases from 716 MHz to 621 MHz, while the upper frequency f 2 remains between 752 MHz and

Figure 7.37 Hybrid-coupled shorted RMSA coupled along one of its (a) radiating edges and (b) nonradiating edges.

Table 7.14

Measured Results of Radiating Edge Hybrid-Coupled Shorted RMSA for Different w

753 MHz, resulting in an increase in the f 2 /f 1 ratio from 1.05 to 1.21. For all these cases, the VSWR remains less than 2 at both the frequencies.

A nonradiating edge hybrid-coupled shorted RMSA is shown in Figure 7.37(b). Along the nonradiating edge, the field variation is sinusoidal. Therefore, the width as well as the position of the copper strip determines the coupling between the fed and the parasitic patches [51]. The coupling is maximum when the strip is placed at the farthest point from the short, as shown in Figure 7.37, where the field is maximum. For L = 2.3 cm, W = 4.6 cm, s = 0.1 cm, y = 0.5 cm, er = 4.3, h = 0.16 cm, and tan d = 0.02, the results for various coupling strip widths w are given in Table 7.15. When w is reduced from 1.0 cm to 0.2 cm, f 2 stays around 637 MHz, while f 1 increases from 560 MHz to 617 MHz, resulting in a variation of the f 2 /f 1 ratio from 1.14 to 1.03.

300 Broadband Microstrip Antennas

feed is directly connected to the top patch and the bottom patch is electromagnetically coupled. Two circular discs with an equal diameter of 7.0 cm are fabricated on a substrate with er = 2.32 and h = 0.159 cm. For D1 = 0, when D2 is varied from 0 to 0.1 cm, the higher resonance frequency f 2 increases from 825 MHz to 910 MHz, whereas the lower resonance frequency f 1 remains fixed around 775 MHz. This yields f 2 /f 1 variation from 1.06 to 1.17. When D2 = 0 and D1 is varied, then both the resonance frequencies change. Hence, the convenient way to tune f 2 /f 1 is to fix the values of D1 and patch dimensions for given f 1 and then vary D2 to realize f 2.

Similar results of dual-band operation can be obtained by stacking rectangular, annular ring, and triangular patches [56–61]. Instead of using regularly shaped patches, various compact antennas described in Chapter 6 may also be stacked to realize compact dual-band antennas [62, 63].

7.3.8.2 Dual-Band ACMSAs

Instead of using a coaxial feed for the stacked patches, these can also be excited using an aperture-coupling technique to yield a dual-band response. Similarly, compact configurations described in Chapter 6 and other coaxialfed dual-band configurations covered in this chapter can be aperturecoupled [64, 65]. To give an example, an RMSA with two slots described in Section 7.3.6 could be fed by a microstrip line through an aperture in the ground plane as shown in Figure 7.40. A rectangular aperture is cut in the ground plane and a microstrip line feed is used on the other side of the ground plane. The dimensions of the antenna are shown in Figure 7.40. The upper substrate where the square patch is located has er 1 = 2.2 and h1 = 0.16 cm. The feed transmission line on the lower substrate with er = 2.55 and h = 0.08 cm has a width of 0.22 cm and extends by length ls = 2.06 cm beyond the slot as shown in Figure 7.40(b). The two equal slots with lengths l are equidistant from the center and are at distance d s from the respective edges. This configuration is analyzed using the cavity model and the input impedance is calculated using the theorem of energy conversion [65].

The position and length of the slots primarily change the third-order resonance frequency f 2 and to a lesser extent the fundamental TM10 resonance frequency f 1. For l = 4.6 cm and d s = 0.15 cm, the theoretical resonance frequencies are 1.810 GHz and 2.610 GHz, against the respective measured frequencies of 1.835 GHz and 2.590 GHz. The variation of the f 2 /f 1 ratio with the slot length l and distance d s are shown in Figure 7.40(c, d). As l increases from 2.5 to 4.5 cm, the f 2 /f 1 ratio decreases from 2.1 to 1.4. As the separation between slots increases (i.e., d s decreases from 0.4 cm to

Figure 7.40 An aperture-coupled RMSA with two slots: (a) top and (b) side views. Variation of f 2 /f 1 with (c) slot length l and (d) slot position d s .

0.1 cm), the f 2 /f 1 ratio decreases from approximately 2.0 to 1.4. Consequently, a low value of the f 2 /f 1 ratio requires longer slots located closer to the two edges.

7.3.8.3Aperture-Coupled Coplanar Parallel Dipoles for Multifrequency Operation

Coplanar parallel dipoles fed by aperture coupling could be used to obtain multifrequency operation. The dipoles of different lengths are fed by a microstrip line through a rectangular slot cut in the ground plane. In general, this antenna consists of 2N dipoles of N different lengths, which are symmetrically excited through the aperture at N frequencies [66]. Either the longest identical pair of dipoles could be placed in the center of the slot and smallest

Table 7.16

Theoretical and Measured Resonance Frequencies of Aperture-Coupled Coplanar

Parallel Dipoles

This configuration yielded low BWs at the second and third frequencies, because proper matching is achieved only at the first frequency and the matching is not very good at higher frequencies. The theoretical and measured ratios of extreme frequencies are 1.426:1 and 1.385:1, respectively. Similarly, the configuration with three pairs of dipoles arranged with longest pair near the center and the smallest pair at the edge also yields triple-frequency operation. The radiation pattern is in the broadside direction at all the three frequencies, and the antenna is attractive for its simplicity.

7.4 Summary

In multichannel applications, the antenna is required to operate over a broad BW to cover all the channels. However, at any given time, it requires a small BW to cover a single channel. In this case, a tunable MSA is required. If the antenna is designed to operate at two far-away frequencies, then a dual-band MSA could be used.

Various techniques have been described to tune the resonance frequency of the MSA. The tuning of the resonance frequency around the 20% range is achieved by loading the MSA with external reactive components such as stubs, shorting posts, and varactor and PIN diodes. The frequency is also tuned by varying the thickness of the air gap between the patch substrate and the ground plane. This tunable configuration is useful if the increased thickness of the antenna is acceptable.

Dual-frequency operation with the same or orthogonal polarizations at two frequencies is described. Different methods such as higher order modes, stub loading with lengths comparable to l/4, loading the patch with lumped components, shorting posts, and slots are discussed. By using shoring posts, two modes of the rectangular patch TM10 and TM30 are excited at

which the radiation pattern is in the broadside direction. The separation between the two resonance frequencies is controlled by changing the position of the single shorting post. When a lower dual-frequency ratio is required, then RMSA or CMSA with a single short can be used, whereas a single short TMSA yields a higher dual-frequency ratio. The separation between the two frequencies can be reduced significantly by using slots inside the patch.

Dual-frequency operation is also achieved by coplanar or stacked multiresonator configurations, when the separation of the two frequencies is in the range of 10–20%. A stacked geometry offers the advantages of operating in the same mode and the flexibility of tuning the separation by means of an air gap. Aperture-coupled coplanar parallel dipoles yield multifrequency operation. This configuration has the advantage that individual resonance frequencies can be easily tuned by changing the length of the dipoles.

References

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

[2]Plessis, M. Du., and J. H. Cloete, ‘‘Tuning Stub for Microstrip Patch Antenna,’’

IEEE AP-S Int. Symp. Digest, June 1993, pp. 964–967.

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

[4]IE3D 7.0, Zeland Software, Inc., Fremont, CA, 2000.

[5]Ray, K. P., and G. Kumar, ‘‘Tunable and Dual Band Circular Microstrip Antenna with Stubs,’’ IEEE Trans. Antennas Propagation, Vol. AP-48, July 2000, pp. 1036–1039.

[6]Babu, S., and G. Kumar, ‘‘Parametric Study and Temperature Sensitivity of Microstrip Antennas Using an Improved Linear Transmission Line Model,’’ IEEE Trans. Antennas Propagation, Vol. AP-47, 1999, pp. 221–226.

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

[8]Ray, K. P., and G. Kumar, ‘‘Stub Loaded Circular Microstrip Antennas,’’ Proc. APSYM98, Kochi, India, December 1998, pp. 84–87.

[9]Ray, K. P., and G. Kumar, ‘‘Circular Microstrip Antenna with Double Stubs,’’ Proc. ISRAMT-99, Malaga, Spain, December 1999, pp. 381–384.

[10]Daniel, A. E., and G. Kumar, ‘‘Multiport Network Model for Tunable Rectangular

Microstrip Antennas,’’ Proc. NSMAL, New Delhi, India, December 1995,

pp. 572–577.

[11]Schaubert, D. H., et al., ‘‘Microstrip Antennas with Frequency Agility and Polarization Diversity,’’ IEEE Trans. Antennas Propagation, Vol. AP-29, 1981, pp. 118–123.

[12]Bhartia, P., and I. J. Bahl, ‘‘A Frequency Agile Microstrip Antenna,’’ IEEE AP-S Int. Symp. Digest, 1982, pp. 304–307.

[13]Daryoush, A. S., K. Bontzos, and P. R. Hercsfeld, ‘‘Optically Tuned Patch Antenna for Phased Array Applications,’’ IEEE AP-S Int. Symp. Digest, 1986, pp. 361–364.

[14]Lee, K. F., K. Y. Ho, and J. S. Dahele, ‘‘Circular-Disc Microstrip Antenna with an Air Gap,’’ IEEE Trans. Antennas Propagation, Vol. AP-32, 1984, pp. 880–884.

[15]Dahele, J. S., and K. F. Lee, ‘‘Theory and Experiment on Microstrip Antennas with Air Gap,’’ IEE Proc. Microwaves, Antennas Propagation, Pt. H, Vol. 132, 1985,

pp.455–460.

[16]Lee, K. F., and J. S. Dahele, ‘‘The Two Layered Annular Ring Microstrip Antenna,’’ Int. J. Electronics, Vol. 61, 1986, pp. 207–217.

[17]Das, N., and S. K. Chowdhury, ‘‘Rectangular Microstrip Antenna on a Ferrite Substrate,’’ IEEE Trans. Antennas Propagation, Vol. AP-30, No. 3, 1982, pp. 499–502.

[18]Das, N., S. K. Chowdhury, and J. S. Chaterjee, ‘‘Circular Microstrip Antenna on a Ferrite Substrate,’’ IEEE Trans. Antennas Propagation, Vol. AP-31, No. 1, 1983,

pp.188–190.

[19]Pozar, D. M., and V. Sanchez, ‘‘Magnetic Tuning of a Microstrip Antenna on a Ferrimagnetic Substrate,’’ Electronics Letters, Vol. 24, June 1988, pp. 729–731.

[20]Mishra, R. K., S. S. Patnaik, and N. Das, ‘‘Tuning of Microstrip Antenna on Ferrite Substrate,’’ IEEE Trans. Antennas Propagation, Vol. AP-41, No. 2, 1993, pp. 230–233.

[21]Maci, S., and G. B. Gentilli, ‘‘Dual Frequency Patch Antenna,’’ IEEE AP Magazine, Vol. 39, December 1997, pp. 13–17.

[22]Chen, J. S., and K. L. Wong, ‘‘A Single-Layer Dual-Frequency Rectangular Microstrip Patch Using a Single Probe Feed,’’ Microwave Optical Tech. Letters, Vol. 11, No. 2, 1996, pp. 83–84.

[23]Antar, Y. M. M., A. I. Ittipiliboon, and A. Bhattacharya, ‘‘A Dual-Frequency Antenna Using a Single Patch and an Inclined Slot,’’ Microwave Optical Tech. Letters, Vol. 8, No. 6, 1995, pp. 309–310.

[24]Srinivasan, V., K. P. Ray, and G. Kumar, ‘‘Orthogonal Polarized Microstrip Antennas,’’ Proc. NSAML-2000, Delhi, India, March 2000, pp. 43–46.

[25]Deepukumar, M., et al., ‘‘Broadband Dual Frequency Microstrip Antennas,’’ Electronics Letters, Vol. 32, August 1996, pp. 1531–1532.

[26]Murakami, Y., et al., ‘‘Dual Slot Coupled Microstrip Antenna for Dual Frequency Operation,’’ Electronics Letters, Vol. 29, October 1993, pp. 1906–1907.

[27]Richards, W. F., S. E. Davidson, and S. A. Long, ‘‘Dual Band Reactively Loaded Microstrip Antenna,’’ IEEE Trans. Antennas Propagation, Vol. AP-33, May 1985,

pp.556–561.

[28]Davidson, S. E., S. A. Long, and W. F. Richards, ‘‘Dual Band Microstrip Antennas with Monolithic Reactive Loading,’’ Electronics Letters, Vol. 21, September 1985,

pp.936–937.

[29]Daniel, A. E., and G. Kumar, ‘‘Tunable Dual and Triple Frequency Stub Loaded

Rectangular Microstrip Patch Antenna,’’ IEEE AP-S Int. Symp. Digest, 1995, pp. 2140–2143.