The conventional printed wide-slot antenna has an operating bandwidth on the order of 10 to 20 percent.1,2 Because modern wireless applications often require broadband operation, some printed wide-slot antennas for broadband operation have been reported.3-6 A square slot antenna,3 with a fork-shaped microstrip feed structure, was investigated for broadband operation. The fork-shaped microstrip feed structure was applied to a printed round corner rectangular wide-slot antenna4 and a broadband design was achieved. A semicircular slot antenna5 with a protruded small rectangular slot was excited by a 50 ? microstrip. Recently, an isosceles triangular slot antenna6 with a small rectangular slot for broadband operation was proposed, and has shown an impedance bandwidth of 77 percent for a 2:1 VSWR. However, the impedance bandwidth of this antenna design is still not enough to cover most wireless applications. It is important to enhance the impedance bandwidth of microstrip-fed wide-slot antennas.
In this article, a novel design of a microstrip-fed, printed rectangular slot antenna, with a small trapezoidal slot tuning for wireless applications, is proposed. The radiation characteristics of such a design are also investigated. The proposed antenna can be easily excited by a 50 ? microstrip printed on an FR-4 dielectric substrate, and good impedance matching can be obtained for operation at frequencies within the wireless communications system bands. A comparison of the proposed design with a corresponding isosceles triangular slot antenna6 is also given.
Antenna Design
Figure 1 shows the geometry of the novel broadband rectangular slot antenna printed on a dielectric substrate. In this study, the dielectric substrate material is FR-4 with a thickness h = 1.6 mm and relative permittivity ?r= 4.4. For design convenience, the proposed antenna is fed by a 50 ? microstrip, printed on the dielectric substrate. The microstrip, with a width Wf = 3.0 mm, is placed on the centerline of the rectangular slot (y axis). In order to achieve broadband operation, a small trapezoidal slot is placed on the feed side of the rectangular slot. The primary slot is the rectangular slot, which has a horizontal width of Rw and a vertical length of Rl. The small trapezoidal slot has a top width Tt, a bottom width Tb and a vertical height Th. The two slots are etched in the ground plane that is on the opposite side of the dielectric substrate. The design parameters of the proposed antenna can easily be determined. The perimeter of the polygonal slot at the lowest operating frequency required is given by
The perimeter of the polygonal slot can be determined to be approximately two guided wavelengths in the slotline at the lowest operating frequency. Then, by fine tuning the small trapezoidal slot and adjusting the length of the 50 ? microstrip feed-line, a new resonant mode can be excited in the proximity of the fundamental resonant mode and good impedance matching over a broad frequency range can be obtained.
Experimental Results and Discussion
The proposed antennas were simulated with the High Frequency Structure Simulator (HFSS) from Ansoft and the prototypes were fabricated and measured with a HP-8720ES network analyzer. The first parameter under design was the size of the rectangular slot. For the designs shown in Table 1, the ratio of Rl to Rw is approximately 1:1.5. To achieve the requirements of wireless applications, the perimeter of the polygonal slot, Lperimeter, was chosen to be approximately 180 mm, which corresponds to approximately 2?g at 1.83 GHz (?g is the guided wavelength at the lowest frequency of operation). Table 2 shows Lperimeter and bandwidth of all the antennas studied. Figure 2 shows the measured and simulated return loss results of the proposed antenna 1 with an impedance bandwidth of 104 percent for VSWR = 2. The impedance bandwidth obtained with the present design is approximately five times that of a conventional printed wide-slot antenna. From Tables 1 and 2, for antenna 1 with the widest bandwidth, the ratio of Tb to Rw is about 1:4 and the ratio of Tt to Tb is about 1:8. The optimal microstrip feed-line length Ls was chosen to achieve a good impedance match for the constructed prototype. By observing the influence of various parameters on the antenna performance, it was found that the dominant factor in the proposed antenna designs for wireless communications applications is the perimeter of the polygonal slot in terms of ?g. From that numerical experiment, ?g can be calculated as
where the er,eff is determined by
Then, the lowest frequency (fL) relative to a half of the Lperimeter is formulated by
where C0 is the speed of light in free space.
For the reasons mentioned above, antenna 1 is the optimum design. Its radiation characteristics have been measured in the STUT Anechoic Chamber. Figures 3 and 4 show the measured radiation patterns in the x-z plane and y-z planes, respectively, at F = 1.90, 2.45, 3.10, 4.70, 5.15 and 5.75 GHz . Antenna 1 is suitable for GSM (1900 to 1990 MHz), PCS (1900 to 1990 MHz), IMT-2000 (1920 to 2170 MHz), Bluetooth (2400 to 2484 MHz), IEEE 802.16a/e, IEEE 802.11b/g (2400 to 2484 MHz), IEEE 802.15.3a (UWB), PHS (1905 to 1915 MHz), PACS (1930 to 1990 MHz), UMTS (Regular 1, 2, 3), IEEE 802.11a (5150 MHz) and HIPERLAN /1 /2 (5150 MHz). It is also noted that, for antenna 1, the radiation patterns in the x-z plane and y-z plane are good, which makes the proposed antenna suitable for practical applications. The measured peak antenna gain for antenna 1 is also presented in Figure 5. It shows a peak gain of approximately 5.6 dBi; the gain variation is observed to be less than 1.6 dB.
A comparison of the proposed design with a corresponding isosceles triangular slot antenna6 is given in Table 3. It shows a small gain variation for the proposed antenna design compared to that of the isosceles triangular slot antenna. In addition, the 104 percent impedance bandwidth of the proposed antenna is larger than that of the isosceles triangular slot antenna.
Conclusion
A microstrip-fed, printed rectangular slot antenna with a small trapezoidal slot for broadband operation has been implemented. Experimental results show that the impedance bandwidth of a printed rectangular slot antenna can be significantly improved by selecting the proper dimensions of the small trapezoidal slot and the perimeter of the polygonal slot. The results, obtained in this study, show that the impedance bandwidth for the proposed antenna is approximately 104 percent (1.83 to 5.78 GHz) for a 2:1 VSWR. In addition, the proposed antenna also shows a compact structure and a simple feeding structure, compared to a corresponding printed wide-slot antenna. The design of this proposed antenna, with broadband operation, is suitable for most wireless applications.
Acknowledgment
The partial financial funding of this study by the National Science Council, Taiwan, Republic of China, under contract number NSC92-2213-E-218-038, is gratefully acknowledged.
References
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2. S.M. Shum, K.F. Tong, X. Zhang and K.M. Luk, “FDTD Modeling of a Microstrip-line-fed Wide-slot Antenna,” Microwave and Optical Technology Letters, Vol. 10, 1995, pp. 118–120.
3. H.L. Lee, H.J. Lee, J.G. Yook and H.K. Park, “Broadband Planar Antenna Having Round Corner Rectangular Wide Slot,” 2002 IEEE Antennas and Propagation International Symposium Digest, Vol. 2, pp. 460–463.
4. J.Y. Sze and K.L. Wong, “Bandwidth Enhancement of a Microstrip-line-fed Printed Wide-slot Antenna,” IEEE Transactions on Antennas and Propagation, Vol. 49, 2001, pp. 1020–1024.
5. W.S. Chen, C.C. Hung and K.L. Wong, “A Novel Microstrip-line-fed Printed Semicircular Slot Antenna for Broadband Operation,” Microwave and Optical Technology Letters, Vol. 26, No. 4, August 2000, pp. 237–239.
6. W.S. Chen and F.M. Hsieh, “A Broadband Design for a Printed Isosceles Triangular Slot Antenna for Wireless Communications,” Microwave Journal, Vol. 48, No. 7, July 2005, pp. 98–112.
