Антенны, СВЧ / OC / Broadband microstrip antennas

Initially, the length L of the RM antenna is obtained as 27.6 cm for f L = 0.225 GHz by using (9.3) and (9.7) by choosing W = L . It has been shown in Section 9.2.3 that a broader BW is obtained when W is smaller than L without significantly affecting the lower resonant frequency. To keep the margin in f L , L is chosen as 28 cm and the width is chosen as 22 cm. These dimensions can be approximately obtained by using a frequencyscaling technique from the dimensions given in Section 9.2.3. Also, in this frequency range, an N-type connector is typically used instead of an SMA connector, which helps in supporting a large-size monopole antenna. Hence, the diameter of the probe is taken as 0.4 cm. For p = 0.7 cm, the theoretical input impedance and VSWR plots are shown in Figure 9.14. In the desired frequency range of 225–400 MHz, the VSWR is less than 2. The maximum variation of the pattern in the azimuthal direction is 2 dB at the higher frequency.

9.6.2 Monopole Antenna for 800–2,000 MHz

To cover the wireless local loop (WLL) and GSM communication frequency bands of 824–890 MHz and 890–960 MHz, respectively, and the proposed 1,710–1,880-MHz band, separate antennas are currently being used. A single planar monopole antenna can be designed to cover the entire frequency range. Since the BW is very large, a RM antenna will not meet the requirement, but a circular or elliptical monopole antenna can yield the BW as described in Sections 9.4 and 9.5, respectively.

Figure 9.14 Theoretical (a) input impedance and (b) VSWR plots of RM with L = 28 cm, W = 22 cm, and p = 0.7 cm.

For simplicity, CM is considered. The radius of the CM is obtained as 4 cm corresponding to f L = 800 MHz by using (9.16), (9.17), and (9.7). To keep the margin in f L , the radius is chosen as 5 cm. For the SMA connector of diameter 0.12 cm, p = 0.3 cm and the circular ground plane of diameter 25 cm, the measured input impedance and VSWR plots are shown in Figure 9.15. A VSWR ≤ 2 has been obtained in the frequency range of 710 MHz to 3,000 MHz.

9.7 Summary

A suspended microstrip patch in air fed at its periphery through a perpendicular ground plane becomes a planar monopole antenna, when the bottom ground plane is removed. Differently shaped planar monopole antennas, such as, rectangular, square, triangular, hexagonal, circular, and elliptical disc monopoles are described for very large VSWR BWs. Simple equations are given to predict the lower frequency corresponding to VSWR = 2 for these antennas. The square, rectangular, triangular, and hexagonal monopole antennas yield a lower BW than the circular and elliptical monopoles, but these BWs are large enough for many applications. Moreover, these antennas are attractive due to their ease of fabrication. Circular and elliptical disc monopoles yield multi-octave BW, which is extremely useful for broadband applications. An elliptical disc with an ellipticity ratio of 1.1 provides a maximum BW of more than 1:11 for VSWR ≤ 2. The radiation pattern at

Figure 9.15 Measured (a) input impedance and (b) VSWR plots of CM antenna with radius a = 5 cm and p = 0.3 cm.

various frequencies in the E- and H-planes of these antennas is similar to that of a cylindrical monopole antenna of equivalent height on a finite ground plane. Design examples of planar RM and CM antennas are presented using simple design equations to cover the 225–400 MHz and 800–2,000 MHz frequency bands, respectively.

References

[1]Honda, S., et al., ‘‘A Disc Monopole Antenna with 1:8 Impedance BW and Omnidirectional Radiation Pattern,’’ Proc. ISAP, Sapporo, Japan, 1992, pp. 1145–1148.

[2]Hammoud, M., P. Poey, and F. Colombel, ‘‘Matching the Input Impedance of a Broadband Disc Monopole,’’ Electronics Letters, Vol. 29, February 1993, pp. 406–407.

[3]Agarwall, N. P., G. Kumar, and K. P. Ray, ‘‘Wide-Band Planar Monopole Antennas,’’

IEEE Trans. Antennas Propagation, Vol. 46, No. 2, 1998, pp. 294–295.

[4]Ray, K. P., et al., ‘‘Broadband Planar Rectangular Monopole Antennas,’’ Microwave Optical Tech. Letters, Vol. 28, No. 1, 2001, pp. 55–59.

[5]Balanis, C. A., Antenna Theory Analysis and Design, New York: John Wiley & Sons, 1997.

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

[7]Ballantine, S., ‘‘High Quality Radio Broadcast Transmission and Reception,’’ Proc. IRE, Vol. 22, No. 5, 1935, pp. 564–629.

[8]Chang, D. C., C. W. Harrison, Jr., and E. A. Aroson, ‘‘Tubular Monopole of Arbitrary Dimensions: The Radiation Field,’’ IEEE Trans. Antennas Propagation, Vol. 17, May 1969, pp. 534–540.

380 Broadband Microstrip Antennas

because of high dielectric loss. However, for testing new designs, the antennas are fabricated on this substrate at higher frequencies also to reduce cost.

To make the size of the antenna compact in the UHF range and for many other specific applications, substrates with a high er are used. Some of the high er substrate materials are ferromagnetic, ceramic, and semiconductor substances. Ferrite and YIG are the main ferromagnetic materials [3]. The magnetic permeability ( mr ) of these substrates can be changed by varying the biasing magnetic field, which is utilized to tune the resonance frequency of the MSA. Besides having high er , these materials have high mr , leading to a further size reduction in MSAs. The most commonly used ceramic substrate is alumina, which has very low loss but which is brittle. The semiconductor materials are silicon and gallium arsenide (GaAs), which have a high er . Table A.1 lists the primary parameters of all these substrates.

The substrates of synthetic and composite materials are suitable for designing MSAs, but the cost of these substrates is high. New varieties of substrates are available with reasonably low tan d with prices as low as onefourth of the prices of the synthetic and composite substrates, allowing for reductions in the cost of the MSA. Table A.2 details these low-cost substrates [3, 4].

To sum up, the choice of the substrate is the first important step in the successful design of an MSA. Besides electrical and mechanical parameters, there are many other physical and chemical properties of the substrate, including flexibility, power-handling capability, chemical resistance, ruggedness, strain relief, bondability, and nonporousness. These factors also influence the decision of the selection of the substrate.

References

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

[2]Zurcher, J. F., and F. E. Gardiol, Broadband Patch Antennas, Norwood, MA: Artech House, 1995.

[3]Garg, R., et al., Microstrip Antenna Design Handbook, Norwood, MA: Artech House, 2001.

[4]Sainati, R. A., Microstrip Antennas for Wireless Applications, Norwood, MA: Artech House, 1996.

W ′ = W + (t /p) F1 + ln H4/ √(t /h )2 + (1/p )2/(W /t + 1.1)2 JG

(B.3)

Here, W is width of the patch or microstrip line, er is a dielectric constant of the substrate, h is the thickness of the substrate, and t is the thickness of the metallic patch conductor. The effective dielectric constant ee is given as

The above equations for obtaining ee have an accuracy of ±0.2% for

0.01 ≤ W /h ≤ 100 and er ≤ 50.

As the frequency of operation increases, the fields get more confined to the patch, thereby reducing the effective width. The frequency dependence of the parameters is taken into account by modifying expressions for ee and We . The frequency-dependent expressions for the effective dielectric constant ee ( f ) are given by

The frequency-dependent expression for the effective width We ( f ) is given by

B.2 Characteristics Impedance Z 0 (f ) of the Microstrip

Transmission Line