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Industry News / Antennas

A Wideband and High Gain Microstrip Four-slot Antenna Array

A numerical simulation and an experimental implementation of a cross-shaped, microstripline-fed, printed, four-element slot antenna array are presented in this article. The proposed antenna, with a relative permittivity of 4.3 and a thickness of 1.0 mm...

January 1, 2003
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Since microwave communications equipment is low profile and light weight, low profile and light weight antennas are essential. A microstrip antenna has been developed to meet this requirement. Microstrip antennas offer a conformal structure, low cost and ease of integration with solid-state devices, and are low profile and lightweight. However, microstrip antennas have a narrow bandwidth of approximately 10 to 20 percent. In the last decade, many researchers have studied bandwidth widening techniques for microstrip antennas.1-3


A popular method uses parasitic patches, either in another layer (stacked geometry)4 or in the same layer (coplanar geometry). However, the stacked geometry has the disadvantage of increasing the thickness of the antenna, and the coplanar geometry has the disadvantage of increasing the antenna area.5 In a conventional microstripline-fed slot antenna, a narrow rectangular slot is cut in the ground plane, and the slot is excited by a microstrip feedline with a short6 or an open circuit7 termination. With this feed configuration, a good impedance match has been achieved for a narrow slot, and an impedance bandwidth of approximately 20 percent has been obtained.6

However, as the width of the slot increases, the radiation resistance of the slot antenna increases proportionately. This, in turn, reduces the impedance bandwidth of the antenna even though the size of the slot is larger.8 Shum, et al.,9 demonstrated the possibility of increasing the bandwidth of a wide slot antenna by terminating the open end of the feedline within the width of the slot, but substantial bandwidth improvement was not achieved. The feed structures of the conventional transverse slot antenna are either center oriented6 or are offset.10-11 The center-feed has a larger value of radiation impedance than an offset feed. This means that the impedance bandwidth of a center-feed is less than for an offset feed.

Recently, a T-shaped microstrip-fed ground plane slot has been proposed,12 and an impedance bandwidth of 39.6 percent has been obtained. This type of feed has permitted an improved impedance matching for a 16 mm wide slot.

In this article, the characteristics of one- and four-element slot antennas with cross-shaped feedlines are studied. A cross-shaped microstrip feedline is proposed to match the input impedance for narrow as well as wide slot antennas. When the cross-shaped feedline is used, the bandwidth can be extended proportionally to the slot width. In this case, the proposed antenna leads to an impedance match over a wide frequency band. The excited slot antenna with a cross-shaped feedline has been analyzed using the FDTD method and the return losses have been calculated by transforming the time-domain results into the frequency domain. The optimal design of a cross-shaped, microstripline-fed, four-element slot antenna for a broad bandwidth using the FDTD method is reported. A bandwidth wider than for the conventional feed,6-11 or T-shaped feed,12 has been obtained. The proposed antenna was fabricated and measured from these results.

The measured bandwidth of the four-element slot antenna array is 1.4467 to 2.5979 GHz, which is approximately 57 percent for S11≤ -10 dB, and the cross-polarization level is less than -25 dB. The measured peak gain of the four-element slot antenna array is approximately 7 dBi in its usable frequency range.

FDTD Method and Numerical Results

The FDTD method is formulated by discretizing Maxwell's curl equations over a finite volume and approximating the derivatives with central difference approximations. These FDTD approximate equations contain second-order errors in both space and time steps, according to Yee's notation.13 In the analysis of the microstrip slot antenna design,13 the Mur's absorbing boundary condition14 was applied.

Once the calculated value in the time-domain is obtained by the FDTD method15 its value in the frequency domain can be calculated through a Fourier transform.

The structure of a one-element microstrip slot antenna is shown in Figure 1 and the structure of a four-element microstrip slot antenna array is shown in Figure 2. The relative permittivity of the substrate is 4.3 and the thickness of the substrate is 1.0 mm. Ls is the slot length, Ws is the slot width, ld is the horizontal component length of the T feed, lu is the length of the vertical open stub, offset is the distance between the slot center and the center of the horizontal component of the feedline, and Wf is the width of the feedline. To correctly analyze the antenna, Δx and Δz are chosen so that an integral number of nodes fit the feedline and slot exactly. Δz is chosen so that an integral number of nodes fit the thickness h of the substrate exactly.

The spatial step sizes used are Δx = 0.333 mm, Δy = 0.25 mm and Δz = 0.50 mm. Then h = 2 Δz, ls = 189Δx, Ws = 128Δy, ld = 93Δx, lu = 40Δy and the offset = 32Δy. To calculate the far-field pattern, 20 free-space mesh cells are added to the top and bottom of the substrate. The total mesh dimensions of the one-element antenna are 360Δx x 400Δy x 62Δz and those of the four-element antenna are 1051Δx x 480Δy x 62 Δz. The antenna is excited by a Gaussian pulse just underneath the dielectric interface. In order to calculate the input S-parameters, a standard technique of time gating the signal on the microstrip line to separate the incident and reflected waveforms is used. The S-parameters are obtained by taking the ratio of the Fourier transforms of these waveforms.

Figure 3 shows the comparison of the calculated SWR of the one-element and four-element microstrip slot antennas. The usable frequency (less than -10 dB return loss) of the one-element microstrip slot antenna is from 1.46 to 2.73 GHz with a bandwidth of 1.27 GHz. The usable frequency of the four-element microstrip slot antenna array is from 1.43 to 2.56 GHz (1.13 GHz bandwidth). The bandwidth of the four-element antenna is 0.14 GHz less than for the one-element antenna.

Figure 4 shows the calculated SWR of the four-element microstrip slot array antenna as a function of offset length. When the offset is 5, 8 and 11 mm, the bandwidth is approximately 0.3, 1.13 and 1.10 GHz, respectively.

Figure 5 shows the calculated SWR of the four-element microstrip slot antenna array as a function of the length of the horizontal component of the T feedline (ld), while all the other parameters are set to their fundamental values (1s = 63 mm, Ws = 32 mm, offset = 8 mm, ld = 10 mm, h = 1.0 mm). When ld decreases, the bandwidth change is noticeable.

Figure 6 shows the calculated SWR of the four-element microstrip slot antenna array as a function of the length of the vertical component of the feedline (lu), with all other parameters set to their fundamental values (1s = 63 mm, Ws = 32 mm, 1d = 31 mm, offset = 8 mm, h = 1.0 mm). When lu decreases, the bandwidth narrows.

Experimental Results

The proposed antenna was fabricated using an FR-4 substrate (εr = 4.3, h = 1.0 mm) and the ground plane size of the four-element microstrip slot antenna array is 350 mm x 120 mm. The measurements were made on an HP8510 network analyzer.

In Figure 7, the measured return losses of the four-element antenna array are shown. The measured bandwidth of the antenna is 1.4467 to 2.5979 GHz, which is approximately 57 percent (S11 ≤-10 dB) at the center frequency of 2.02 GHz.

In this case the measured results exhibit a three-resonance characteristic, which can be contrasted with that of a conventional6-11 or T-shaped microstripline-fed structure.12 The measured bandwidth (1.152 GHz) is wider than the simulated one (1.13 GHz).

Figure 8 presents the experimental co-polarization and cross-polarization radiation patterns in the x-z plane at f = 1.89 GHz, after calibration, using a horn antenna. The radiation pattern was measured. The measured beamwidth of the four-element microstrip slot antenna array is approximately 86° and the cross-polarization level is less than -25 dB.

The measured gain versus frequency for the one-element slot antenna and that for the four-element slot antenna is given in Figure 9. The measured gain of the one-element antenna is greater than approximately 3 dBi in the usable frequency range. The measured gain of the four-element antenna array is approximately 7 dBi. Relatively high gain characteristics were obtained in the usable frequency range.

Conclusion

In this article, the characteristics of a wideband, cross-shaped, microstrip-fed, four-element slot antenna array was investigated using the FDTD method. The measured bandwidth of the four-element microstrip slot antenna array is from 1.4467 to 2.5979 GHz, which is approximately 57 percent, and the cross-polarization level is less than -25 dB.

Since this antenna is wideband, low profile and lightweight, it may find applications in PCS, IMT-2000, mobile communication, satellite communication and wideband communication systems.

References

1. S.H. David, "A Survey of Broadband Microstrip Patch Antennas," Microwave Journal, Vol. 39, No. 9, September 1996, pp. 60-84.
2. A. Sangiovanni, J.Y. Dauvignac and C. Pichot, "Embedded Dielectric Antenna for Bandwidth Enhancement," Electronics Letters, Vol. 33, No. 25, December 1997, pp. 2090-2091.
3. Z.F. Liu, P.S. Kooi, L.W. Li, M.S. Leong and T.S. Yeo, "A Method for Designing Broadband Microstrip Antennas in Multilayered Planar Structures," Vol. 47, No. 9, September 1999, pp. 1416-1420.
4. B.L. Ooi and C.L. Lee, "Broadband Air-filled Stacked U-slot Antenna," Electronics Letters, 1999, Vol. 35, No. 7, April 1999, pp. 51-52.
5. T.M. Au, K.F. Tong and K.M. Luk, "Analysis of Offset Dual-patch Microstrip Antennas," IEEE Proceeding on Microwave and Antennas Propagation, Vol. 141, No. 6, June 1994, pp. 523-526.
6. Y. Yoshimura, "A Microstrip Slot Antenna," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-20, November 1972, pp. 760-762.
7. D.M. Pozar, "Reciprocity Method of Analysis for Printed Slot and Slot-coupled Microstrip Antennas," IEEE Transactions Antennas and Propagation, Vol. AP-34, December 1986, pp. 1439-1446.
8. M. Kahrizi, T.K. Sarkar and Z.H. Maricevic, "Analysis of a Wide Radiating Slot in the Ground Plane of a Microstrip Line," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-41, January 1993, pp. 29-37.
9. S.M. Shum, K.F. Tong, X. Zhang and K.M. Luk, "FDTD Modeling of a Microstrip-line-fed Wide-slot Antenna," Microwave Optical Technology Letters, Vol. 10, October 1995, pp. 118-120.
10. B.N. Das and K.V.S.V.R. Prasad, "Impedance of a Transverse Slot in the Ground Plane of an Offset Strip," IEEE Transactions on Antennas and Propagation, Vol. AP-32, No. 11, November 1994, pp. 1245-1248.
11. Y. Qian, S.I. Iwata and E. Yamashita, "Optimal Design of an Offset-fed, Twin-slot Antenna Element for Millimeter-wave Imaging Arrays," IEEE Microwave and Guided Wave Letters, Vol. 4, No. 7, July 1994, pp. 232-234.
12. Y.W. Jang, and J.C.H.S. Shin, "A Large Bandwidth T-shaped Microstrip-fed Ground Plane Slot Antenna," Microwave Journal, Vol. 45, No. 1, January 2002, pp. 92-103.
13. K.S. Kunz and R.J. Luebbers, The Finite-difference Time-domain Method for Electromagnetics, CRC Press Inc., 1993, pp. 11-26.
14. G. Mur, "Absorbing Boundary Conditions for the Finite-difference Approximation of the Time-domain Electromagnetic Field Equations," IEEE Transactions Electromagnetic Compatibility, Vol. EMC-23, November 1981, pp. 377-382.
15. D.M. Sheen, S.M. Ali, M.D. Abouzahra and J.A. Kong, "Application of the Three-dimensional Finite-difference Time-domain Method to the Analysis of Planar Microstrip Circuits," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-38, No. 7, July 1990, pp. 849-857.

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