paper on antennas

1-4244-1355-9/07/$25.00 @2007 IEEE

International Conference on Intelligent and Advanced Systems 2007

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A Hexagonal Fractal Antenna for Multiband Application

N.A. Saidatul, A.A.H. Azremi, P.J Soh School of Computer and Communication Engineering,

University of Malaysia Perlis. P.O Box 77, d/a Pejabat Pos Besar,

01007 Kangar Perlis, Malaysia.

Abstract - This paper describes the concept of a new fractal multi-band antenna based on the hexagon shape. Three iterations of the hexagonal fractal multi-band antenna arranged are examined. With this structure it is possible to configure the multi-band frequency and radiation pattern to obtain directional patterns with high directivity and gain. The ADS software was used to design and analyze the antenna array for applications at range 0.5-10GHz. Fabrication of the antenna is done by using wet-etching method, on FR-4 dielectric.

Keywords - fractals element; multi-band frequency; hexagonal multi-band antenna; radiation pattern; high directiviy; gain.

I. INTRODUCTION

The explosive growth in the wireless industry has renewed interest in multi-band antennas. The most recent multi-band antenna development is the incorporation of fractal geometry into radiators and the Sierpinski gasket antenna is a prime example. Since the Sierpinski gasket has proven itself to be an excellent multi-band antenna, other multi-band antennas can similarly be constructed using fractal geometry [1, 2].

Fractal antenna's response differs obviously from traditional antenna designs. It is capable of operating optimally at many different frequency ranges simultaneously [3]. This makes the fractal antenna an excellent design for broadband applications. Due to the self-similar property of fractal antennas, microstrip fractal antennas demonstrate higher bandwidths than conventional microstrip antennas [4]. They are also considered in multi-frequency antenna designs.

By connecting fractal shaped antennas, wideband coverage can be achieved [5]. Key benefits of fractal antennas are reduced size and compactness. Furthermore, the fractal antennas allow controlling of characteristics such as location of frequency bands, radiation pattern and entire bandwidth owing to feeding technique and antenna geometry variations [6]. This letter examines the input characteristics of one such fractal design based on the hexagon and evaluates its suitability for multi-band usage [7].

II. HEXAGONAL FRACTAL ANTENNA DESIGN

The Hexagonal Fractal antenna was design to achieve multi-band resonance. To achieve this goal, it is useful to design the antenna to operate at a set of frequency bands commonly used by wireless devices. The band of interested is 2 GHz and above. This antenna design was divided into three stages. The first stage is operating at 2 GHz frequency. From the first iteration comes to second iteration and from second iteration, develop to third iteration.

The hexagonal fractal is constructed by reducing a hexagon generator shape to one third its former sizes, and grouping six smaller hexagons together. This log periodic frequency behavior is the direct result of constructing the hexagonal fractal with a scale factor of one half [8]. The triangles within the hexagonal fractal antenna are interconnected to maintain conductivity and to preserve electrical self-similarity [9].

To design a Hexagonal Fractal FSS antenna, the following parameters is been used.

• Loss Tangent, tan = 0.019 • Dielectric constant, r = 4.7 • Substrate height , h = 1.6 mm

Circular equation was used to calculate the actual radius, a of circular patch antenna to match at 2GHz.

a = F / {[1 + (2h / r F][ln ( F / 2h) + 1.7726]} 1/2 (1)

where

F = 8.791 x 109 (2)

fr r

= 2.0275

a = F { 1 + 2h/ rF [ln ( F/2h) + 1.7726]}½

= 1.804 cm @ 18 mm

Optimizations for the design are done to get the desired frequency. The first three hexagonal fractal iterations are displayed in Figure 1.


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Figure 1: First three iterations of the hexagonal fractal.

The hexagonal fractal antenna is implemented in the corner fed configuration with overlapping vertices to preserve the electrical self-similarity paramount in multi band design. When viewed from the corner input, it is apparent the hexagonal fractal presents similar scale lengths of one third and, therefore, it can generate resonant frequencies with a power of three [10].

A ground plane of size 54.53 mm x 80.1 mm was printed on the opposite side of the substrate from the fractal pattern, as is required in microstrip patch antenna design. Generally, larger ground planes allow better radiation performance, but a limit must be decided for the final size of the antenna allowed. In addition, larger areas of PCB board will cost more to produce.

III. RESULT

The Return Loss, S11 of the Hexagonal Fractal Antenna simulated using ADS software. Different iteration will produce a different amount of Return Loss, dB and bandwidth. The data results are shown in Table 1.

TABLE 1SIMULATED RESULT FOR 3-ITERATION HEXAGONAL FRACTAL ANTENNA

Type of Iteration Resonant Frequency, (GHz)

Return Loss, (dB)

BW (%)

1st Iteration 2.00 -12.80 51.5 1.31 -27.65 0.29 2.33 -12.12 8.58 2nd Iteration 3.35 -19.55 25.07 1.23 -23.34 52.84 2.32 -13.34 16.38 3rd Iteration 3.63 -17.95 17.93

The Hexagonal fractal Antenna produced a high return loss compared to the Sierpinski Carpet Fractal Antenna. The higher return loss is at -37.65 dB for 9.763 GHz. This is also better when compared against the simulation result. This is due to the fact that during measurement, the readings taken are the best. Even there is an averaging of a few measurement readings; this still could not help in lowering down the return loss value, closer to the simulated values. Bandwidth generated, on the other hand, is slightly smaller than the simulated results.

Figure 2: Return Loss, S11 for Hexagonal Fractal Antenna

The antenna input reflection (referred to a 50 impedance) has been measured with network analyzer (E8363B, 10MHz – 40GHz). Comparison of S11 theoretical and experimental result is shown in Figure 2.

Table 2 shows the result of simulation and measurement. Although fabricated antennas have shown a small percentage of frequency shift from the design frequency, radiation performance of this antenna exhibited are acceptable.

TABLE 2SUMMARY OF THE SIMULATION AND MEASUREMENT RESULT OF S11

HEXAGONAL FRACTAL ANTENNA

Typical simulation far-field radiation pattern at every operating frequency band are shown in Figure 3. The simulated Hexagonal Fractal antenna produced a larger E-plane than the H-plane. In most of the readings, the main lobe was not to be found at the center ( = 0o), but are either slightly shifted to the right or left. On the E-plane, the patterns of azimuth (x-y) plane are symmetrical. The patterns on the H-plane are almost omni-directional, thus, extremely suitable for applications in mobile communication devices.

No Rep Model Return Loss (dB)

Resonant Freq (GHz)

BW (%)

Freq Shift (%)

Simulation -23.34 1.23 52.54 1Meas -10.43 1.27 2.30

3.30

Simulation -13.34 2.32 16.38 2 Meas -15.97 2.29 2.19 1.53

Simulation -17.95 3.21 2.188 -11.72 3.065 17.93 3

Meas -12.71 3.255 3.07

1) 4.70

2) 1.38 Simulation -17.00 5.490 13.11 4

Meas -18.48 5.210 2.88 5.48

Simulation -34.80 7.350 8.80 5Meas -32.17 6.870 17.77

7.06

6 Meas -13.53 8.060 6.29 - 7 Meas -22.50 8.670 5.760 -

Simulation -15.76 9.370 10.60 8 Meas -37.65 9.763 3.07 4.02


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E-plane, f = 1.23GHz H-plane, f = 1.23GHz

E-plane, f = 2.32HZ H-plane, f = 2.32GHz





Figure 3: Simulation radiation pattern of Hexagonal Fractal Antenna. ( co-polarization, cross-polarization)

A log-periodic antenna (LPA) has been used to measure the radiation patterns. Transmitting antenna with a limited size produces a spherical surface wave rather than plane wave resulting in an error. The measured far field and simulation radiation pattern for 1.23 GHz hexagonal antenna are shown as in Figure 4.

(a) (b)

(c) (d)

Figure 4: Simulated and measurement radiation pattern. (__ simulation, ---measurement). (a) E-plane co-polarization, f = 1.93 GHz. (b) H-plane co polarization, f = 1.93GHz. (c) E-plane cross-polarization, f = 1.93 GHz. (d) H-plane cross polarization, f = 1.93GHz.

Table 3 summarizes the simulated and measurement radiation pattern parameters with the Ratio of Wave Level, and value of each frequency’s isolation level.


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TABLE 3COMPARISON RADIATION PATTERN PARAMETERS SIMULATED AND

MEASUREMENT FOR 1.23GHZ

Data Max E-

co (dB)

Max E-cross (dB)

Ratio of Wave Level,

(k)

Isolation (dB)

Simulation -0.40 -21.1 0.02 20.7 Measurement -53.7 -70.1 0.82 16.4

Data Max H-cross

Max H-co

Ratio of Wave Level,

(k)

Isolation (dB)

Simulation 0.00 -33.3 0.00 33.3 Measurement -57.9 -86.98 0.67 29.1

Table 4 shows the simulation result for gain, efficiency and directivity of Hexagonal Fractal Antenna for their each frequency.

TABLE 4SIMULATION GAIN, DIRECTIVITY, AND EFFICIENCY OF HEXAGONAL FRACTAL

ANTENNA

Frequency (GHz) Gain (dB) Directivity (dB) Efficiency (dB) 1.230 1.3248 3.651 4.837 2.219 3.6276 5.758 20.888 3.625 4.884 2.159 10.545 5.557 5.554 7.048 39.145 7.438 5.944 6.647 39.510 9.444 6.563 8.191 53.758

IV. CONCLUSION

The hexagonal fractal antenna is observed to possess multi-band behavior similar to the Sierpinski gasket antenna. This new fractal antenna allows flexibility in matching multiband operations in which larger frequency separation is required. It is possible with this fractal to shape and update the radiation pattern according to the environment in real time, with the aid of control algorithms, making it steerable. The simulated results have shown a good radiation structure, which has high directivity and gain, when compared to a simple patch antenna. The return loss measurements show an excellent dip and suitable bandwidth. The directivity and the gain are directly proportional to the number of fed array elements.

REFERENCES

[1]. Tian Tehong, Zhou Zheng, “A novel multiband antenna: fractal antenna,” Communication Technology Proceeding 2003. ICCT, 2003, International Conference, Volume 2, 9-11 April 2003, pp. 1907-1919.

[2]. Yao Na, Shi Xiao-wei, “Analysis of the multiband behavior on Sierpinski carpet fractal antennas,” Microwave Conference Proceedings, 2005. APMC 2005, Asia Pasific Conference Proceedings, vol 4,4-7 Dec. 2005, pp. 4.

[3] Song, N.S. Chin, K.L. Liang, D.B.B. Anyi, M. “Design of Broadband Dual-Frequency Microstrip Patch Antenna with Modified Sierpinski Fractal Geometry,” Communication systems, 2006. ICCS 2006. 10th IEEE Singapore International Conference, pp. 1-6, Oct. 2006.

[4] Rahmani, Maryam, Tavakoli, Ahad, Amindavar, Hamidreza, A lireza, Moghaddamjoo, “Analysis of Microstrip Fractal Antennas By Means of RWG MOM and Wavelet Transformation,” Antennas and Propagation Conference, 2007. Loughborough, pp:185 -188, 2-3 April 2007.

[5] Song, N.S. Chin, K.L. Liang, D.B.B. Anyi, “Design of Broadband Dual- Frequency Microstrip Patch Antenna with Modified Sierpinski Fractal Geometry,” Communication systems, 2006. ICCS 2006. 10th IEEE Singapore International Conference, pp. 1-6, Oct. 2006.

[6] Krupenin, S.V.; Kolesov, V.V.; Potapov, A.A.; Petrova, N.G., “The Irregular-Shaped Fractal Antennas for Ultra Wideband Radio Systems,” Ultrawideband and Ultrashort Impulse Signals, The Third International Conference, pp. 323 – 325, Sept. 2006.

[7] P.W. Tang and P.F Wahid; “Hexagonal Fractal for Multi-band Antenna”: IEEE antennas and wireless propagation letters, VOL. 3, No.111, 2004.

[8] C. Puente, J. Romeu, R. Pous, and A. Cardama, ‘‘On the behavior of the Sierpinski multiband fractal antenna,’’ IEEE Trans. Antennas Propagat., vol. 46, pp. 517---524, Apr. 1998.

[9] H. A. Wheeler; “Small Antennas”; IEEE Transactions on Antenna and Propagation, Vol. AP-23, No.4; July 1975.

[10] D. H. Wemer, R. 1. Haupt, and P. L. Wemer, “Fractal Antenna Engineering : The Theory and Design of Fractal Antenna Arrays,” IEEE Antennas and Propagation Magazine, 41, 5, October 1999,pp. 37-59.

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