There is a requirement for low profile and lightweight antennas for microwave communications equipment. According to these requirements a microstrip antenna is best suited. Microstrip antennas have a conformal structure, are low cost and easy to integrate with solid-state devices, as well as low profile and lightweight. However, microstrip antennas have a narrow bandwidth, which is approximately 10 to 20 percent. In the last decade, many researchers have studied bandwidth widening techniques for microstrip antennas.1–3 A popular method is the use of 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 lateral size of the antenna.5 In a conventional microstrip line-fed slot antenna, a narrow rectangular slot is cut in the ground plane and the slot is excited by a microstrip feed line with a short6 or an open7 termination. With this feed configuration, a good impedance match has been achieved with a narrow slot, and a bandwidth of approximately 20 percent has been obtained.6 However, as the width of the slot increases, the radiation resistance of the slot antenna also increases proportionately. This, in turn, reduces the impedance bandwidth of the antenna, even though the size of the slot is larger.8 Shuum, et al.9 showed the possibility of increasing the bandwidth of a wide slot antenna by terminating the open end of the feed line within the width of the slot, although substantial bandwidth improvement has not been achieved. The conventional feeding structures of conventional transverse slot antennas are center feeding6 and offset feeding.10–11 The center feed has a larger value of radiation impedance than an offset feed. It means that the bandwidth of a center feed antenna is less than for an offset fed antenna.
In this article, the characteristics of one- and eight-element slot antennas with T-shaped feed lines have been studied. A T-shaped microstrip feed line is proposed to match the input impedance of narrow as well as wide slot antennas. When a cross-shaped feed line is used, the bandwidth can be extended in proportion to the slot width. In this case, the proposed antenna leads to good impedance matching over a wide frequency band. The slot antenna, excited by a T-shaped feed line, has been analyzed using the FDTD method. The return loss is obtained by transforming the time domain results into the frequency domain. The optimal design of a cross-shaped microstrip line-fed eight-element slot antenna for a broad bandwidth, using the FDTD method, is reported. A bandwidth wider than for the conventional feed6-11 is obtained. From these results, the proposed antenna was fabricated and measured. The measured bandwidth of an eight-element slot antenna array is 1.3933 to 2.5989 GHz, which is approximately 60.3 percent for S11 ≤ –10 dB. The obtained 10 dB return loss bandwidth covers the DCS (1.71 to 1.88 GHz), PCS (1.85 to 1.99 GHz), IMT-2000 (1.90 to 2.20 GHz), UMTS (1.92 to 2.17 GHz) and WLL (2.40 to 2.48 GHz) operations. The measured peak gain of the eight-element slot antenna array is approximately 10.9 dBi. This antenna is an attractive candidate for wireless, satellite and mobile communication applications.
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 finite-difference time-domain approximate equations contain the second-order error in both the space and time steps. According to Yee’s notation12 in the FDTD cell, the space point is (i∆x, j∆y and k∆z), the time increment is n∆t and the arbitrary function is represented by F (i∆x, j∆y, k∆z, n∆t). In the analysis of the microstrip slot antenna design,12 the Mur’s absorbing boundary condition13 has been applied.
As the calculated values in the time domain, which are calculated by the FDTD method,14 are Fourier-transformed, the response values in the frequency domain can be calculated.
The geometrical structure of a one-element microstrip slot antenna is shown in Figure 1; the geometrical structure of an eight-element microstrip slot antenna array is shown in Figure 2. The geometry of the λg/4-transformer and T-shaped microstrip feed line is shown in Figure 3. This antenna was designed by using a T-shaped microstrip line in the slot.
The relative permittivity of the substrate is εr = 4.3 and the thickness of the substrate is 1.0 mm. Ls is slot length, Ws is the slot width, ld is the horizontal component length of the T-feed, Wgs is the length of the interval between the slot center and the center of the horizontal component of the feed line, and Wf is the width of the feed line. To correctly analyze the antenna, ∆y and ∆x are chosen so that an integral number of nodes fit the feed line and the slot exactly. ∆z is chosen so that an integral number of nodes fit the thickness h of the substrate exactly.
The spatial step size used is ∆x = 0.25 mm, ∆y = 0.25 mm and ∆z = 0.50 mm. The thickness of the substrate (h) is 2∆z, the length of the slot (Ls) is 252∆x, the width of the slot (Ws) is 96∆y, the length of the horizontal component of the feed line (ld) is 152∆x and Wgs is 18∆y. The total mesh dimensions of the one-element antenna are 400∆x × 400∆y × 62∆z; those of the eight-element slot antenna are 1460∆x × 640∆y × 62 ∆z. In order to calculate the input S-parameter, the standard technique of time gating the signal on the microstrip line to separate the incident and reflected waveforms is used. The S-parameter is obtained from the ratio of the Fourier transforms of these waveforms.
Figure 4 shows a comparison of the calculated return losses of one- and eight-element microstrip slot antennas. The usable frequency range of the one-element microstrip slot antenna below –10 dB return loss is from 1.64 to 2.24 GHz and the bandwidth is 0.6 GHz. As shown, the usable frequency of the eight-element microstrip slot antenna array is from 1.41 to 2.61 GHz (bandwidth = 1.20 GHz). The bandwidth of the eight-element antenna is 0.6 GHz larger than for the one-element antenna. Figure 5 shows the calculated return loss of an eight-element microstrip slot antenna array as a function of varying offset distances.
Figure 6 shows the calculated return losses of an eight-element microstrip slot antenna array as a function of the horizontal component of the feed line length (ld). All the other parameters are set to their fundamental values (Ls = 63 mm, Ws = 24 mm, Wgs = 4.5 mm, εr = 4.3, h = 1.0 mm). The bandwidth is sensitive to changes in ld.
Figure 7 shows the Ez-field distribution of the eight-element microstrip slot antenna array just underneath the dielectric interface at 1000 time steps.
The proposed antenna was fabricated using an FR-4 substrate (εr = 4.3, h = 1.0 mm) and the ground plane size of the eight-element microstrip slot antenna array is 365 mm × 160 mm. The measurements were made with an HP8510 network analyzer.
The measured return losses of an eight-element microstrip slot antenna array are shown in Figure 8. The measured bandwidth of the antenna is 1.3933 to 2.5989 GHz, which is approximately 60.3 percent (S11 ≤ –10 dB) at the center frequency of 2.0 GHz. In this case, the measured results exhibit a three-resonance characteristic, which can be contrasted with that of a conventional antenna.6–11 The obtained 10 dB return loss bandwidth covers all of the DCS (1.71 to 1.88 GHz), PCS (1.85 to 1.99 GHz), IMT-2000 (1.90 to 2.20 GHz), UMTS (1.92 to 2.17 GHz) and WLL (2.40 to 2.48 GHz) operating frequencies.
Figure 9 presents the experimental E-plane radiation pattern at 2 GHz. After calibration using a horn antenna, the radiation pattern was measured. The measured beam width of the eight-element microstrip slot antenna array is approximately 50°. Figure 10 presents the experimental H-plane radiation pattern at 2 GHz. The measured beam width of the eight-element microstrip slot antenna array is approximately 24°.
The measured gain versus frequency for the one-element slot antenna and the eight-element slot antenna are given in Figure 11. The measured peak gain of the eight-element antenna is approximately 10.9 dBi within in the usable frequency range. The measured peak gain of the eight-element slot antenna array with T-shaped feed lines (10.9 dBi) is approximately 7.6 dBi higher at F = 2.6 GHz than for a one-element slot antenna array with a T-shaped feed line (3.3 dBi). High gain characteristics have been obtained in the usable frequency range.
In this article, the characteristics of a wide-band T-shaped microstrip-fed eight-element slot antenna array have been investigated using the FDTD method. The measured bandwidth of the eight-element microstrip slot antenna array is 1.3933 to 2.5989 GHz, which is approximately 60.3 percent. The 10 dB return loss bandwidth obtained covers the DCS (1.71 to 1.88 GHz), PCS (1.85 to 1.99 GHz), IMT-2000 (1.90 to 2.20 GHz), UMTS (1.92 to 2.17 GHz) and WLL (2.40 to 2.48 GHz) operations. The measured peak gain of the eight-element slot antenna array is approximately 10.9 dBi.
Since this antenna is wide-band, low profile, lightweight and has a high gain characteristic, it may find applications in PCS, IMT-2000, WLL, mobile communications, satellite communication and wide-band communication systems.
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