Effect of the Gap Feeding on the Multi-band Small Antenna Using a Branch Structure

Transcription

1 Progress In Electromagnetics Research Symposium, Hangzhou, China, March 24-28, 28 8 Effect of the Gap Feeding on the Multi-band Small Antenna Using a Branch Structure Hyengcheul Choi, Hojeong Kim, Sinhyung Jeon, and Hyeongdong Kim Department of Electrical and Computer Engineering, Hanyang University Haengdang-dong, Seongdong-gu, Seoul, Korea Abstract This research investigates about the effect an inter-coupling capacitance at small antenna using a branch structure. From the analysis by using an equivalent transmission line model and the comparison with three chip antennas consisted of meandered pattern, it is derived that the large inter-coupling capacitance occurred between two radiating elements leads a very narrow bandwidth. To solve this problem, the branch structure antenna with one gap feeding is proposed in this paper, and this technique is found to be useful for the design of multi-band small antenna.. INTRODUCTION In the past several years there has been increasing interest in the design of small multi-band antenna, and many antenna types are developed []. Planar Inverted F-Antenna (PIFA) among them is frequently used for a mobile handset because it has small volume and multi-band operations [2]. However, PIFA is not suitable to be installed to the inside of thin mobile handset because it must be installed high from the ground plane of a mobile phone to achieve a broad bandwidth []. For this reason, the dual-band small chip antennas were developed in [4]. Because the resonant frequency of these developed antennas is ISM band, higher than PCS band, the chip antenna to operate at GSM, DCS, and PCS bands is studied at []. Comparing antenna in [2] with it in [], the size of chip antenna should be more smaller to be mounted on the thin mobile handset. However, to design small chip antenna for multi-band is very hard work because the extreme size reduction of monopole antenna makes very narrow bandwidth [6] and strong inter-coupling capacitance. Since the narrow bandwidth occurred by size reduction is an avoidable phenomenon [], this paper concentrate only on the effect of the inter-coupling capacitance between radiating elements, and the gap feeding is introduced to alleviate it. To examine the inter-coupling capacitance, the multi-band chip antenna having a branch structure is proposed in this paper, and the transmission line model is used as the equivalent model of an antenna. From the circuit analysis and simulation results of chip antennas, it is derived that the bandwidth of an antenna becomes the narrower by the higher value of the inter-coupling capacitance. Also, gap feeding is proposed to decrease the effect of the inter-coupling capacitance, and its effect is proven by comparing two antennas; One is the branch structure antenna having gap feeding, and the other is the branch structure antenna having direct connect feeding. 2. THEORY The branch structure is utilized to achieve a multi-band or a wide bandwidth at the antenna design [8]. When the branch structure is used at small volume antenna, the inter-coupling capacitance become very large. To study about the effect of the inter-coupling capacitance, the antennas and the inter-coupling capacitance at Fig. (a) convert into the open lossless transmission line model at Fig. (b). This paper assumes that two antennas have no coincident resonant frequencies, a low operation band and high operation band are assigned to a long antenna and a short antenna, respectively, and antenna loss is neglected to focus only the effect of inter-coupling capacitance. The inter-coupling capacitance in a resonator is utilized to decrease the resonant frequency []. Although this technique is very useful in the design of the filter having very narrow bandwidth, it is not suitable for the design of the broadband antenna. Because of the neglect of antenna loss, Eq. (): j(y cos θ ωc inter sin θ) Z in = X in = () Y (Y sin θ 2ωC inter ( cos θ)) expresses the input reactance of Fig. (b), and Fig. 2 based on Eq. () shows the input reactance as function of frequency when the value of the inter-coupling capacitor is different.

2 82 PIERS Proceedings, Hangzhou, China, March 24-28, 28 Figure : Illustration of small antenna using branch structure. (a) Geometry of small antenna using branch structure. (b) Transmission line model of a branch structure antenna with at inter-coupling capacitance. Figure 2: Input reactance with two different inter-coupling capacitors. From Fig. 2, it is shown that the larger value of the inter-coupling capacitance leads the steeper slope of input impedance at fundamental resonant frequency. Therefore, a narrow bandwidth appears in condition that many antenna patterns are near each other in a fixed small volume. Because it is impossible to reduce significantly the inter-coupling capacitance in a fixed volume, this paper proposes the gap feeding method to decrease the value of capacitance connected with a low frequency antenna. As known well, the value of the total capacitor decreases when two Figure : Illustration of small antenna using branch structure with one gap feeding. (a) Geometry of small antenna using branch structure with one gap feeding. (b) Transmission line model of a branch structure antenna with at inter-coupling one capacitor with gap feeding.

3 Progress In Electromagnetics Research Symposium, Hangzhou, China, March 24-28, capacitors are connected in series. Fig. shows the geometry and the equivalent circuit of the branch structure antenna with gap feeding, respectively. In next chapter, branch structure antennas using meandered patterns will be examined to find the effect of inter-coupling capacitance. Furthermore, the gap feeding antenna is suggested to alleviate narrow bandwidth by the inter-coupling capacitance between radiating elements.. EXPERIMENTS Figure 4 shows three antennas for low frequency band, high frequency band, and dual band, separately. Fig. shows the of each antenna in Fig. 4 when these are installed to 4 8 mm 2 ground plane. From Fig., it is derived that the inter-coupling capacitance between two meander patterns makes the fundamental resonant bandwidth extremely narrow. To solve this problem, the antenna using gap feeding mentioned in Chapter 2 is proposed as shown in Fig. 6. According to theory in Chapter 2, the bandwidth of the fundamental frequency is broaden by connecting gap feeding with the short antenna in series as illustrated in Fig.. Figure 4: Illustration of the branch structure antenna using two meandered patterns. Meander antenna at top layer Meander atenna at bottom layer Meander antenna using branch structure Figure : of each antenna in Fig. 4. Figure 6: Illustration of the branch structure antenna using two meandered patterns with one gap feeding. 4. RESULTS The measured graph and radiation pattern of the proposed antenna are illustrated as shown in Fig. 8 and Fig., respectively. From these results, it is shown that the gap feeding is efficient method in small antenna design using branch structure.

4 822 PIERS Proceedings, Hangzhou, China, March 24-28, 28 Meander antenna at top layer Meander antenna with one gap feeding at bottom layer Meander antenna using branch structure with one gap feeding Simulation data Measured data Figure : of each antenna in Fig. 6. Figure 8: Simulation and measure of the proposed antenna H-Plane E-plane E2-plane 24 2 H-Plane E-plane E2-plane Figure : Measured gain of the proposed antenna on three radiation planes: H-plane, E-plane, E2-plane, (a) at low resonant frequency, (b) at high resonant frequency.. CONCLUSIONS To find the effect of inter-coupling capacitance, the small antenna using branch structure is converted into transmission line and a shunt capacitor, and meander antenna using branch structure is designed and manufactured. From the results of theses analysis and measured data, it is derived that the inter-coupling capacitance makes the antenna bandwidth very narrow. Therefore, this paper proposes the gap feeding method to solve this problem. As shown in simulation and measured, the gap feeding is considered as the efficient solution for the multi-band antenna using a branch structure in a fixed small volume. ACKNOWLEDGMENT This research was supported by the MIC (Ministry of Information and Communication), Korea, under the ITRC (Information Technology Research Center) support program supervised by the IITA (Institute of Information Technology Advancement), (IITA-26-C-62-). REFERENCES. Wong, K. L., Planar Antennas for Wireless Communications, Wiley-Interscience, Hoboken, New Jersey, 2.

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