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Keywords:

  • microstrip antennas;
  • bandwidth widening;
  • FDTD method

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Theory
  5. 3. Results and Discussion
  6. 4. Design Guide
  7. 5. Conclusion
  8. References
  9. Supporting Information

[1] In this paper, the finite difference time domain method is employed to analyze the T-probe-proximity-fed rectangular patch antennas. With a supporting foam substrate of thickness around 0.1 λ0, the patch antenna has shown a bandwidth (SWR ≤ 2) of 40% and an average gain of 7 dBi. Numerical results for the input impedance are presented and compared with the measurements. Good agreement between the computed and measured results is obtained. The effects of geometric parameters on the characteristics of the T-probe patch antenna are extensively studied. For design purposes, the variations of the input impedance at resonance with different geometric parameters are plotted on Smith charts, and the peak values for the resonant resistance are summarized in tables.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Theory
  5. 3. Results and Discussion
  6. 4. Design Guide
  7. 5. Conclusion
  8. References
  9. Supporting Information

[2] Microstrip antennas offer the attractive features of low profile, lightweight, ease of fabrication, conformity to the supporting surface and compatibility with microwave integrated circuits (MICs). If the expense of materials and fabrication is not prohibitive, they can also be low in cost. However, probably the most serious limitation of this technology is the narrow bandwidth of the basic element. The conventional microstrip patch element typically has an impedance bandwidth of only a few percent.

[3] For this reason, much of the large volume of research and development in the area of microstrip antennas in the last two decades has been devoted to various techniques for wideband operation of microstrip antennas. Typically, there are several techniques for increasing the bandwidth of microstrip antennas, including using multiple resonators [Lee et al., 1987; Wood, 1980], using lossy materials [Wong and Lin, 1997] and using a thick substrate with an impedance matching network [Hall, 1987; Huynh and Lee, 1995; Luk et al., 1998]. The multiple-resonator configuration may create problems in the design and manufacturing stages along with a considerable increase in height and area. Moreover, good radiation efficiency is practically desirable, so lossless bandwidth enhancement methods are generally preferable to adding loss. Techniques for widening the bandwidth of a microstrip antenna in the use of a thick substrate are of interest due to its possible single-layered structure. Among the several popular feeding techniques, coaxial feed is the most efficient and popular method for the electrically thick substrate patch antenna. However, the probe inductance limits the impedance bandwidth less than 10%. This probe inductance can be compensated in several ways [Hall, 1987; Huynh and Lee, 1995; Luk et al., 1998]. More recently, a novel feeding approach with the utilization of a T-shaped probe has been proposed [Mak et al., 2000]. This design avoids drilling, soldering and slot etching of the patch and yields 40% impedance bandwidth. The T-probe incorporated with the patch introduces a capacitance suppressing some of the inductance introduced by the feed probe due to thick substrate, and another resonance near the resonance of the patch antenna can be created.

[4] In this paper, the T-probe patch antenna is studied using the finite difference time domain (FDTD) method [Yee, 1966]. The Berenger's perfectly matched layer (PML) [Berenger, 1994] is used as the absorbing boundary condition (ABC). Numerical results for the input impedance are presented and compared with the measurements. Good agreement between the computed and measured results is obtained. The effects of geometric parameters on the characteristics of the T-probe patch antenna are extensively studied. In order to provide design information, the variations of input impedance at resonance with different parameters are plotted on Smith charts, and the peak values for the resonant resistance are summarized in tables. The T-probe patch antenna is simple in structure and is a good candidate of base-station antennas for various mobile phone systems and other wireless communication systems.

2. Theory

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Theory
  5. 3. Results and Discussion
  6. 4. Design Guide
  7. 5. Conclusion
  8. References
  9. Supporting Information

[5] The T-probe patch antennas are analyzed based on a FDTD code developed in house [Guo et al., 2001]. Detailed modeling of the coaxial and T-shaped probes is straightforward in the FDTD method. A simplified feed model using the thin-wire approximation technique [Jensen and Rahmat-Samii, 1994] can be used in the simulation. In this work, the excitation for the coaxially fed antenna is performed using a gap voltage model in which a voltage is introduced in one cell of the coaxial center conductor. The input impedance was computed using the voltage and current at the feed point of the antenna. The voltage was computed from the radial electric field across the feed line, and the current was computed from the line integral of the magnetic fields around the base of the probe according to Ampere's law. The input impedance of the antenna is determined from the ratio of the Fourier transform of the incident voltage wave and the Fourier transform of the input current wave.

3. Results and Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Theory
  5. 3. Results and Discussion
  6. 4. Design Guide
  7. 5. Conclusion
  8. References
  9. Supporting Information

3.1. Validation

[6] The antenna as shown in Figure 1 has a rectangular patch separated from the infinite ground plane by a foam layer of dielectric constant close to unity. The patch is proximity fed by a T-shaped coaxial probe, and it is excited in TM01 mode. It has the following parameters: Wx = 30 mm, Wy = 25 mm, Th = 9 mm, Tv = 5.8 mm, D = 1 mm, and H = 6.6 mm (∼0.1 λ0, where λ0 is the free-space wavelength corresponding to the resonant frequency, 4.5 GHz, of the patch antenna). This is the same antenna that was studied by Mak et al. [2000]. Therefore, the numerical results using the FDTD method can be verified by comparison with measurements published by Mak et al. [2000]. Figure 2 shows the computed input impedance of the antenna as compared with the measurement results given by Mak et al. [2000], which has shown a bandwidth (SWR ≤2) of 40% and an average gain of 7 dBi. Good agreement between theory and experiment is obtained. From Figure 2, an error less than about 5% is observed between the computed and measured resonant resistance, which is acceptable in engineering designs. The resonant resistance is much of interest for broadband patch antenna designs. The discrepancy between the measured and computed results is mainly due to the infinite ground plane assumption used in the calculation. We prefer to generate the design data based on infinite ground plane assumption, as in practice, an individual design may have a different finite ground plane size.

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Figure 1. Geometry of the patch antenna with a T-probe feed.

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Figure 2. Input impedance versus frequency. Wx = 30 mm, Wy = 25 mm, Th = 9 mm, Tv = 5.8 mm, H = 6.6 mm and D = 1 mm.

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3.2. Parametric Study

[7] In the following, the variation of the antenna characteristics with the antenna parameters is numerically investigated using our FDTD code.

[8] The variation of the input impedance with the horizontal length Th of the T-probe is illustrated in Figure 3. It can be seen that the impedance loop moves toward the upper right part in the Smith chart, which indicates that both the input resistance and reactance increase with Th. Next, the effect of the vertical length Tv of the T-probe is studied. The input impedance for different Tv with the same dielectric thickness H is plotted in Figure 4. From this figure, we can see that as Tv increases, the input resistance increases, and the input reactance becomes more inductive. The effect of changing the displacement D of the probe from the patch is studied as well. Figure 5 shows the input impedance at different values of D. From Figure 5, it is interesting to see that the loop first moves toward the upper right and then toward the lower left in the Smith chart when D increases. The variation of the input impedance with patch width Wx is shown in Figure 6. It can be observed that the loop becomes larger with decreasing Wx. The parameter Wy determines the resonant frequency of the patch.

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Figure 3. Input impedance versus frequency for different Th. Wx = 30 mm, Wy = 25 mm, Tv = 5.8 mm, H = 6.6 mm and D = 1 mm.

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Figure 4. Input impedance versus frequency for different Tv. Wx = 30 mm, Wy = 25 mm, Th = 9 mm, H = 6.6 mm and D = 1 mm.

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Figure 5. Input impedance versus frequency for different D. Wx = 30 mm, Wy = 25 mm, Th = 9 mm, Tv = 5.8 mm, and H = 6.6 mm.

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Figure 6. Input impedance versus frequency for different Wx. Wy = 25 mm, Th = 9 mm, Tv = 5.8 mm, H = 6.6 mm and D = 1 mm.

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[9] From all of the aspects we consider, it is found that the input resistance and reactance are sensitive to the parameters Th, Tv and Wx. For design purposes, the input impedance under different values of Th and Tv at the resonant frequency 4.5 GHz are plotted in Smith charts as shown in Figures 7, 8, and 9 with Wx = 0.225 λ0, 0.45 λ0, and 0.675 λ0, respectively.

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Figure 7. Smith chart of the resonant input impedance under different Th and Tv with Wx = 0.225 λ0.

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Figure 8. Smith chart of the resonant input impedance under different Th and Tv with Wx = 0.45 λ0.

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Figure 9. Smith chart of the resonant input impedance under different Th and Tv with Wx = 0.675 λ0.

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4. Design Guide

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Theory
  5. 3. Results and Discussion
  6. 4. Design Guide
  7. 5. Conclusion
  8. References
  9. Supporting Information

[10] In this section, the design guide for a wideband patch antenna using the T-probe technique is provided. For a particular operation frequency range centered at f0, we can easily design a wideband patch antenna using the proposed T-shaped probe feeding technique. Referring to Figure 1, we can easily decide the air/foam substrate thickness H ≈ 0.1 λ0 and the patch resonant length Wy ≈ 0.375 λ0. After that, we will present how to choose other parameters of the T-probe-fed patch antenna.

[11] We need know the antenna SWR level for the required bandwidth. Let that value be S. At resonance, the patch input impedance has a peak value. Let its value be R0. Pues and Van de Capelle [1989] show that when connected to a transmission line with characteristic impedance, Z0, the maximum bandwidth occurs when T = (S + 1/S)/2 with T = R0/Z0. To realize the maximum bandwidth with the given SWR level S, the patch needs have a peak resonance resistance R0 = T × Z0. This, of course, introduces a mismatch with the feed line, whose impedance is Z0. The introduced mismatch means that the match is not good at resonance but is better for frequencies away from resonance. After the resonant resistance R0 is fixed, we can refer to Tables 13, summarized from the achieved design curves in Figures 79 to choose other key parameters Th, Tv and Wx of the T-probe and the patch.

Table 1. Peak Resonant Resistance R0 (Ω) With Wx = 0.225 λ0
Th0)Tv0)
0.060.06750.0750.08250.09
0.125877102138200
0.156181108148215
0.186286113156225
0.216391115159231
Table 2. Peak Resonant Resistance R0 (Ω) With Wx = 0.45 λ0
Th0)Tv0)
0.060.06750.0750.08250.09
0.1234445878113
0.1536476284122
0.1837506690129
0.2139526994134
Table 3. Peak Resonant Resistance R0 (Ω) With Wx = 0.675 λ0
Th0)Tv0)
0.060.06750.0750.08250.09
0.122735476393
0.1528385068100
0.1830405373106
0.2131415576111

5. Conclusion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Theory
  5. 3. Results and Discussion
  6. 4. Design Guide
  7. 5. Conclusion
  8. References
  9. Supporting Information

[12] A numerical study of the rectangular patch antenna with a T-probe feed has been presented. The approach is based on the FDTD method with PML. Numerical results for input impedance are presented. The effects of various parameters on the characteristics of the T-probe patch antenna are studied. From the simulation, we find that its input impedance is sensitive to Th, Tv and Wx. To provide design information, the values for the input impedance under different Th, Tv and Wx at the resonant frequency are plotted in Smith charts, and the peak values for the resonant resistance are summarized in tables. The T-probe patch antenna is simple in structure and is a good candidate for base-station antennas for various mobile phone systems and other wireless communication systems.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Theory
  5. 3. Results and Discussion
  6. 4. Design Guide
  7. 5. Conclusion
  8. References
  9. Supporting Information
  • Berenger, J. P., A perfectly matched layer for the absorption of electromagnetic waves, J. Comput. Phys., 114(2), 185200, 1994.
  • Guo, Y. X., C. L. Mak, K. M. Luk, and K. F. Lee, Analysis and design of L-probe proximity fed patch antennas, IEEE Trans. Antennas Propag., AP-49, 145149, 2001.
  • Hall, P. S., Probe compensation in thick microstrip patches, Electron. Lett., 23, 606607, 1987.
  • Huynh, T., and K. F. Lee, Single-layer single-patch wideband microstrip antenna, Electron. Lett., 31, 13101312, 1995.
  • Jensen, M. A., and Y. Rahmat-Samii, Performance analysis of antennas for hand-held transceivers using FDTD, IEEE Trans. Antennas Propag., AP-42, 11061113, 1994.
  • Lee, R. Q., K. F. Lee, and J. Bobinchak, Characteristics of a two-layer electro-magnetically coupled rectangular patch antenna, Electron. Lett., 23, 10701072, 1987.
  • Luk, K. M., C. L. Mak, Y. L. Chow, and K. F. Lee, Broadband microstrip patch antenna, Electron. Lett., 34, 14421443, 1998.
  • Mak, C. L., K. F. Lee, and K. M. Luk, Broadband patch antenna with a T-shaped probe, IEE Proc. Microwaves Antennas Propag., 147(2), 7376, 2000.
  • Pues, H. F., and A. R. Van de Capelle, An impedance matching technique for increasing the bandwidth of microstrip antennas, IEEE Trans. Antennas Propag., AP-37, 13451354, 1989.
  • Wong, K. L., and Y. F. Lin, Small broadband rectangular microstrip antenna with chip-resistor loading, Electron. Lett., 33, 15931594, 1997.
  • Wood, C., Improved bandwidth of microstrip antennas using parasitic elements, IEE Proc. Part H, Microwaves Antennas Propag., 127(3), 231234, 1980.
  • Yee, K. S., Numerical solution of initial boundary value problems involving Maxwell's equations in isotropic media, IEEE Trans. Antennas Propag., AP-14, 302307, 1966.

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Theory
  5. 3. Results and Discussion
  6. 4. Design Guide
  7. 5. Conclusion
  8. References
  9. Supporting Information
FilenameFormatSizeDescription
rds4818-sup-0001-tab01.txtplain text document0KTab-delimited Table 1.
rds4818-sup-0002-tab02.txtplain text document0KTab-delimited Table 2.
rds4818-sup-0003-tab03.txtplain text document0KTab-delimited Table 3.

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