Research Article | Open Access
Kazuki Ide, Satoshi Ijiguchi, Takeshi Fukusako, "Gain Enhancement of Low-Profile, Electrically Small Capacitive Feed Antennas Using Stacked Meander Lines", International Journal of Antennas and Propagation, vol. 2010, Article ID 606717, 8 pages, 2010. https://doi.org/10.1155/2010/606717
Gain Enhancement of Low-Profile, Electrically Small Capacitive Feed Antennas Using Stacked Meander Lines
The present paper describes the gain enhancement of a small and low-profile linear antenna with capacitive feed (C-feed) using three metallic layers. The antenna has very small leakage current on the outer conductor of the coaxial cable and can easily control the imaginary part of the input impedance. The gain of the stacked three-layer meander line antenna, with the meander line in the middle layer being opposite to that of the other two layers, has increased by around 7 dB compared to the single layered C-feed antenna. The antenna gain is discussed based on simulated and measured results, which demonstrates that the antenna has successfully achieved the acceptable impedance and sufficient gain for mobile terminals and RFID tags.
Small [1–3], low-profile antennas [4–6] with back reflectors have been widely studied in recent years. The antenna which is close to the back conductor can reduce the electrical effects from the backing material when the antenna is installed on IC chips, human body, or any metallic or lossy materials. Using a folded line such as meander lines [7–9] and incorporating capacitances [10, 11] are effective ways to make electrically small antennas to operate at the serial resonance frequency. A small, low-profile antenna having an electrically small and low-profile structure that uses a capacitive coupling with a back conductor and that can be operated at the serial frequency has been presented in . However, the coupling terminal with small area easily generates leakage current on the feeding coaxial cable unless a ferrite choke on the cable is used. The leakage current existing on the outer surface of a coaxial cable causes drastic changes in antenna characteristics [13, 14] when the feed is unbalanced. Moreover, typical electrically small and low-profile antennas have disadvantages of low radiation efficiency , and it is difficult to attain impedance matching characteristics . The problems of the leakage current and the impedance matching can be improved by a capacitive feed (C-feed) technique in .
This paper proposes the techniques to enhance the antenna gain of the C-feed mender line antenna (CFMA) shown in . Additional meander line layers are stacked on the CFMA to enhance the antenna gain. The gain of the two-layered CFMA (2L-CFMA) with opposite meander lines has increased by around 6 dB compared to the CFMA. Furthermore, a three-layered CFMA (3L-CFMA) with a reversed meander line to that of the adjacent layers incorporated at the feed plate can enhance the gain by around 7 dB as compared to the CFMA.
2. Low-Profile, Electrically Small Meander Antenna Using a Capacitive Feed Structure
A design of electrically small and low-profile linear antenna with a reflector has been reported in [12, 17]. The antenna yields capacitive impedance at low frequency and approximates the parallel resonant frequency as the serial resonant frequency. Figures 1(a) and 1(b) show the top and bird’s-eye view of dismantled CFMA. The antenna has a height (the distance between the antenna and the back conductor) of 2 mm (0.008λ0 0.25λ0, λ0: wavelength at resonance frequency) and uses RT/Duroid 5880 substrate with a permittivity () of 2.2 and dielectric loss (tan) of 0.001. The substrate dimension is fixed at 22.5 mm × 14 mm (0.091λ0 × 0.057λ0) and satisfies the condition of electrically small antennas (ka 0.338 0.5 (k: wave number and a: radius of a sphere surrounding the antenna) . The meander line has a width (Wm) of 1 mm and spaced (Wd) at 0.5 mm with the adjacent lines. A metallic feed plate is installed in between the meander line and the back conductor. The back conductor, which acts as the ground plane, is 22.5 mm × 14 mm. The feed plate has a length (fl) of 14 mm and width (fw) of 2 mm. The imaginary part of the input impedance of the antenna is controlled primarily by varying the fl and fw, substrate thicknesses Th1 and Th2, and the length of the extended meander line (ml), as shown in Figure 1(c). The imaginary part of the input impedance can be independently controlled using these parameters as shown in Figure 2. This corresponds to the variation in capacitors in the equivalent circuit shown in Figure 1(d). In addition, the antenna generates very small leakage current on the outer conductor surface of the connected coaxial cable. The effect of touching the SMA connector with the human hand is also investigated, and the results are shown in Figure 3. The antenna shows stable S11 characteristics even if the SMA connector is touched with the hand. However, the antenna shows low antenna gain of around 10 dBi. Therefore, novel structures are presented in the next section in order to enhance the antenna gain.
(a) Top view
(b) Dismantled structure
(c) Extended meander line
(d) Equivalent circuit
3. Enhancement of Gain
3.1. Two-Layer Stacked Structure
A meander line is stacked on the presented CFMA to enhance the gain. So, the structure has two layers of meander lines. Two kinds of stacked structures are proposed. In Figures 4(a) and 4(c), the two meander lines of the antenna are in the same direction (2L-CFMA1). But in Figures 4(b) and 4(d), the second meander line is reversed and placed on the first layer (2L-CFMA2). The 2L-CFMA2 has regions where the meander lines of the two layers are not overlapped as shown in Figure 4(d). Since an additional meander line layer is used in this stacked structure, the height of these antennas is increased by 0.8 mm and is shown in Figures 4(e) and 4(f).
(a) CFMA stacked a meander line in the same direction (2L-CFMA1)
(b) CFMA stacked a meander line in the opposite direction (2L-CFMA2)
(c) Expanded figure of 2L-CFMA1
(d) Expanded figure of 2L-CFMA2
(e) Dismantled structure for 2L-CFMA1
(f) Dismantled structure for 2L-CFMA2
3.2. Simulated Results
The simulated results for the input characteristics of these antennas are shown in Figure 5. In Figure 5(a), the resonance frequency is shifted to higher frequency by stacking a meander line. This shift in the frequency is due to the capacitance incorporated by the stacked meander line layer. Moreover, the resonance frequency of 2L-CFMA2 is shifted more to the higher frequency than the 2L-CFMA1, because of the reduced capacitance between the two meander layers due to the reduction of the overlapped metal area. Figures 5(b)–5(d) show the Smith chart expressions of the input impedance and radiation impedance for CFMA, 2L-CFMA1, and 2L-CFMA2. The radiation resistance shown here of the CFMA is lower than that of the other two structures. In Figure 5(b), the self-resonance condition is satisfied at around 1.252 GHz (serial resonance) and 1.260 GHz (parallel resonance) for CFMA. Similarly, this condition is satisfied at 1.502 and 1.510 GHz for 2L-CFMA1 and 1.695 GHZ and 1.705 GHz for 2L-CFMA2. The resonance frequencies of input impedance are different from the self-resonance frequencies. This is probably due to the skin depth of the metallic loss.
(a) S11 characteristics
(b) Smith chart expression for CFMA
(c) Smith chart expression for 2L-CFMA1
(d) Smith chart expression for 2L-CFMA2
Figure 6 shows the simulated antenna gain of these antennas. The antenna characteristics at the resonance frequency are presented in Table 1. The gain of the CFMA is enhanced by stacking the meander line. The gain of the 2L-CFMA2 has increased by around 6 dB as compared to the CFMA. This is due to the electric field generated at the left and right edges of the structure where the meander lines of the two layers are not overlapped as shown in Figure 7. This region behaves like slot antennas and contributes to higher gain.
3.3. Three-Layer Stacked Structure
Based on the previous section, an additional meander line, which is opposite to that of the adjacent layer, is installed with the same height of the feed plate at the feed plate layer of the 2L-CFMA2 to enhance the antenna gain as shown in Figure 8. Therefore, the structure, three-layered CFMA (3L-CFMA), has three layers of meander line although the height of the antenna is the same as that of the 2L-CFMA. The simulated results of input characteristics and the antenna gain are shown in Figures 9 and 10, respectively. The resonance frequency of 3L-CFMA is shifted more to the higher frequency than the 2L-CFMA2 by installing a meander line. A small resonance occurs at around 1.55 GHz where the antenna gain has minimum. This may be due to the capacitance between the two meander lines in the same direction and the inductance offered by the two meander lines. In Figure 9(b), the Smith chart expression shows the input and radiation impedance. The radiation resistance shown here is higher than 2L-CFMA1 and 2. Furthermore, the self-resonance condition is satisfied at 1.727 GHz and 1.735 GHz. These frequencies are higher than those of the other three structures and closer to the serial and parallel resonance frequencies of the input impedance. These differences show that the effect of the metallic loss is smaller than that in other structures. The gain of the 3L-CFMA has increased by around 7 dB as compared to the CFMA. The antenna characteristics at the resonance frequency are presented in Table 2.
(a) S11 characteristics
(b) Smith chart expression for 3L-CFMA
3.4. Measured Results
The 3L-CFMA (22.5 mm × 14 mm (0.130λ0 × 0.081λ0) with a thickness of 2.8 mm (0.016) and ) is fabricated and measured. Figure 11 shows the fabricated 3L-CFMA. The simulated and measured results of the S11 characteristics of the antennas are shown in Figure 12. It could be noticed that measured resonance frequency is shifted to the lower frequency when compared to the simulated results. Figure 13 shows the radiation patterns of the antenna. The simulated maximum gain is 2.76 dBi, and the measured gain is 3.81 dBi. Table 3 shows the simulated and measured antennas characteristics at the resonance frequency. The differences between simulated and measured results are due to fabrication error and slight leakage current. Also, the adhesive used to stack the meander line layers greatly influences the performance of the antenna because the electric field is concentrated at this region. This causes the shift in the resonance frequency and the reduction in the antenna gain. The 3L-CFMA can enhance the gain by around 7 dB as compared to the CFMA in measurement. Although the antenna has the same radiation pattern as a dipole antenna, the front-back ratio can be improved by using larger back conductor.
The techniques to enhance the gain of a low-profile, electrically small meander antenna using a capacitive feed structure with stacked layers have been presented. The antenna gain of 9.8 dBi in the CFMA structure can be enhanced by a two-layered stacked meander line antenna with opposite meander lines (2L-CFMA2). The gain of 2L-CFMA2 has increased to 3.81 dBi. Finally, a three-layered meander line antenna (3L-CFMA), which has the same height as that of the 2L-CFMA, could achieve a gain of 2.76 dBi, which is around 7 dB higher as compared to the CFMA. These antennas can find their application in RFIDs and mobile terminals.
The authors would like to acknowledge the Japan Science and Technology Agency (JST) for its financial support with the Collaborative Development of Innovative Seeds Program. They would also thank Mr. Kazuki Iwata, Technologist of Kumamoto University, for his technical supports.
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Copyright © 2010 Kazuki Ide et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.