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. L_s is the slot length, W_s is the slot width, l_d is the horizontal component length of the T feed, l_u 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 W_f 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, l_s = 189Δx, W_s = 128Δy, l_d = 93Δx, l_u = 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 (l_d), while all the other parameters are set to their fundamental values (1_s = 63 mm, Ws = 32 mm, offset = 8 mm, l_d = 10 mm, h = 1.0 mm). When l_d 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 (l_u), with all other parameters set to their fundamental values (1_s = 63 mm, W_s = 32 mm, 1_d = 31 mm, offset = 8 mm, h = 1.0 mm). When l_u decreases, the bandwidth narrows.

Experimental Results

The proposed antenna was fabricated using an FR-4 substrate (ε_r = 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 conventional^6-11 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

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