Patch antennas, printed on a circuit board, are suitable for integration within a communication device since they occupy a very small volume within the system, while at the same time decreasing the final product fabrication cost. The use of this kind of printed patch antenna permits concealing the antenna within the system, so that possible damage caused by a protruding antenna can be avoided. A number of researchers have studied the improvements relating to the narrow bandwidth of patch antennas. In this article, a solution to the problems of narrow bandwidth and frequency control is offered. A bandwidth of 22.5 percent was required for this antenna. It was difficult to find the required resonance frequency relative to the electrical size of the antenna. Antennas have been designed with a very small size relative to the wavelength at the operating frequency. A modified planar T-type microstrip antenna is proposed as a viable solution, where the resonance frequency is controlled by adjusting the length of the stub. The proposed dual-T antenna uses two stubs, which are fed by a CPW transmission line. Experimental results show that the frequency tuning depends on the lengths and gap of the dual stubs. The frequency shift is determined mainly by calculating the gap between stubs and the length of the upper stub. Several designs were investigated experimentally and, within these, the characteristics of the input impedance and radiation patterns were analyzed. This article details the design of the IMT2000 band antenna and the construction and analysis of its prototypes. The impedance bandwidth obtained was approximately 470 MHz, as determined from a return loss of –10 dB.
Antenna Design and Description
The design of CPW-fed patch antennas has received a lot of attention recently. This is because a CPW-fed patch antenna has the advantage of offering a significant bandwidth, while being easy to integrate within monolithic microwave integrated circuits (MMIC) and low temperature co-fired ceramic circuits (LTCC). A few attempts have been made to increase the bandwidth of CPW-fed patch antennas. The geometry of the proposed CPW-fed dual-T microstrip patch antenna is shown in Figure 1. Table 1 shows the parameters used in the design of the antenna. The antenna was printed on an FR4 substrate with a thickness h = 1.6 mm and a relative permittivity ?r = 4.2, with a length of l/4 to l/3 and a width of 6 mm. The antenna was excited by a CPW feed line, whose impedance is controlled by the width of the strip and the gap between the strip and ground. The gap between the two stubs was set at G1 = 4 mm. The spacing between the lower stub and the edge of the ground plane was G2 = 1.5 mm. The length of the upper and lower stubs were chosen as L1 = 14 mm and L2 = 10 mm, respectively. The dual-T microstrip patch antenna was fabricated and analyzed with various design parameters. As a first step, the input impedance was obtained as a function of the width and gap of the CPW-feed line. The L1, L2, G1 and G2 parameters were then varied and their effect on the characteristics of the antennas was investigated. Figure 2 shows the simulated surface current distribution at 1.8 GHz. The current distribution at the edge of the dual stub is zero. Figure 3 shows the simulated return loss versus frequency for different lengths of the upper stub L1. The value of L1 was varied from 8 to 20 mm. When L1 was increased from 8 to 20 mm, the fundamental resonance was shifted to a lower frequency. However, when L1 increased beyond 20 mm, there was a poor response in the fundamental resonance. A large frequency shift occurred in the fundamental resonance when changing the parameter L1. The values for L2, G1 and G2 were fixed at 10, 6 and 1.5 mm, respectively. Figure 4 shows the resonating frequency and bandwidth versus the length of the upper stub. It shows that the fundamental resonant frequency varies inversely with the length L1. Figure 5 shows the simulated return loss versus frequency as a function of the length of the lower stub of the dual-T antenna L2, varying from 6 to 14 mm. When L2 is increased from 6 to 14 mm, the fundamental resonant frequency does not shift, but the return loss changes. Figure 6 shows the return loss when G1 is varied from 2 to 8 mm. The center frequency of the antenna changed appreciably. When G1 was increased from 2 to 8 mm, the fundamental resonance shifted to a low frequency. Here, the values of L1, L2 and G2 were fixed at 14, 10 and 1.5 mm, respectively. Figure 7 shows the resonance frequency and bandwidth versus the gap between the dual stubs. The resonance frequency varies inversely with the length of the gap G1. Figure 8 shows the return loss simulated for a change in the gap G2 between 0.5 and 2.5 mm while keeping L1 = 14 mm, L2 = 10 mm and G1 = 4 mm. The resonance frequency does not show much change. Figure 9 shows a photograph of the fabricated antenna. Figure 10 shows a comparison between the measured and simulated return losses of the antenna. The simulated and measured data of the proposed antenna are approximately the same. Some errors occurred due to phase differences between the two stubs as a result of poor manufacturing in the laboratory. Figure 11 shows typical measured omni-directional radiation patterns of the antenna at three frequencies, plotted along the E-plane (y-z plane) and H-plane (x-z plane).
Conclusion
CPW-fed dual-T antennas, with a widened tuning stub for broadband operation and controlled frequency, were designed and successfully fabricated. The efficient proposed antenna was achieved simply by tuning the gap between the stubs and the length of the upper stub. The return loss and radiation patterns were simulated by using the FDTD method and were measured using a vector network analyzer, model 37325A, and an anechoic chamber Stargate-32A. By properly choosing the gap between stubs and the length of the upper stub, the center frequency can be shifted by approximately 700 MHz. An antenna impedance bandwidth of 450 MHz can be obtained within the IMT2000 band.
References
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