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Multiband Fractal Antenna for Wireless Communication Systems for Emergency Management a
b
c
L. Lizzi , R. Azaro , G. Oliveri & A. Massa
d
a
Department of Information Engineering and Computer Science, University of Trento, Via Sommarive 14, Trento 38050, Italy b
Department of Information Engineering and Computer Science, University of Trento, Via Sommarive 14, Trento 38050, Italy c
Department of Information Engineering and Computer Science, University of Trento, Via Sommarive 14, Trento 38050, Italy d
Department of Information Engineering and Computer Science, University of Trento, Via Sommarive 14, Trento 38050, Italy;, Email:
[email protected] Published online: 13 Apr 2012.
To cite this article: L. Lizzi , R. Azaro , G. Oliveri & A. Massa (2012) Multiband Fractal Antenna for Wireless Communication Systems for Emergency Management, Journal of Electromagnetic Waves and Applications, 26:1, 1-11, DOI: 10.1163/156939312798954865 To link to this article: http://dx.doi.org/10.1163/156939312798954865
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J. of Electromagn. Waves and Appl., Vol. 26, 1–11, 2012
MULTIBAND FRACTAL ANTENNA FOR WIRELESS COMMUNICATION SYSTEMS FOR EMERGENCY MANAGEMENT
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L. Lizzi, R. Azaro, G. Oliveri, and A. Massa* Department of Information Engineering and Computer Science, University of Trento, Via Sommarive 14, Trento 38050, Italy Abstract—This paper is aimed at proposing a multiband fractal antenna suitable for wireless communication systems devoted to emergency management. The synthesis of the antenna is carried out by means of an iterative procedure based on a Particle Swarm Optimization (PSO) to identify the optimal descriptors of a perturbed Sierpinski geometry. The result is a miniaturized antenna exhibiting a good impedance matching within the Wi-Fi/WiMAX and Public Safety bands. The results from numerical validations and experimental tests are reported to demonstrate the effectiveness of the synthesized antenna as well as the reliability of the synthesis process. 1. INTRODUCTION Improving public safety is a key challenge of today’s world. In the last years, the scientific community has paid great attention to this topic focusing on the development of efficient systems for emergency management and disaster recovery [1]. A fundamental element of an emergency system is a reliable communication infrastructure allowing rescue operators to be connected and able to support the collection and the elaboration of data coming from monitoring devices distributed in the area of the disaster [2]. Usually, wireless networks are used because of their rapid deployment and their capabilities to give connectivity to heterogeneous services operated with different mobile devices such as laptops, PDAs, and smart-phones [3]. Towards this end, a suitable radiating structure is required to support multiple standards. Microstrip antennas are commonly used because of their advantages such as low profile, light weight, and easy fabrication. To Received 18 October 2011, Accepted 18 November 2011, Scheduled 24 November 2011 * Corresponding author: Andrea Massa (
[email protected]).
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obtain a multiband behavior, different techniques have been proposed over the years. In [4–9], slots have been used to generate extra resonances controlled by varying the slot geometry. Differently, the multiband behavior has been obtained in [10–14] by extending the geometry of a reference shape adding strips or stubs. Alternative methods rely on the use of frequency-selective surfaces (FSS) [15, 16] or parasitic elements [17, 18]. Although effective for yielding multipleband operability, the use of additional geometric elements complicates the antenna structures making the fabrication process more difficult or more expensive. Fractal antennas naturally exhibit a multiband behavior thanks to the self-similarity property [19–26]. The underlying idea is that, since small regions of the fractal geometries are copies of the whole structure on a reduced scale, a similar electromagnetic behavior at different frequencies can be expected. However, the fixed relationships among working resonances of standard fractal-shaped antennas make them unsuitable for many practical applications. To break such a constraint and to exploit fractal shapes for practical applications, solutions based on the perturbation of fractal geometries have been proposed. Effective design examples using different fractal geometries (e.g., Koch, Sierpinski, and Hilbert fractals) can be found in [27–31]. In this paper, a multiband antenna based on the Sierpinski fractal shape is proposed for wireless communication systems for emergency management. The antenna operates in the frequency range 2.4– 2.7 GHz to enable Wi-Fi (2.400–2.484 GHz) and WiMAX (2.500– 2.690 GHz) services as well as the use of the Public Safety band from 4.940 to 4.990 GHz relocated by the Federal Communication Commission (FCC) to serve first responders during major incidents [3]. The Wi-Fi and WiMAX standards have been chosen because of the high data-rates suitable for multimedia services. In addition, the WiMAX technology shows good properties in terms of coverage and user mobility. Moreover, it integrates a priority management system able to differently allocate the network resources to favor critical applications in emergency conditions [2]. The antenna is synthesized with an iterative procedure based on the Particle Swarm Optimization (PSO) described in [32] to identify a structure exhibiting a good impedance matching in the frequency bands of interest and having small dimensions. The outline of the paper is as follows. The geometry of the synthesized radiator is described in Section 2 where some indications on the design process are given. The performances of the antenna prototype are then discussed in Section 3. Eventually, some remarks are reported in Section 4.
Multiband fractal antenna for wireless communication systems
3
r2
r3
φs a1 r1
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z b1
b2
x
y
a2
Figure 1. Antenna geometry. 2. ANTENNA DESIGN According to the approach presented in [32], the antenna synthesis has been carried out by iteratively modifying the descriptors of the antenna geometry in Figure 1 to identify a geometrical configuration fulfilling the project requirements. The reference geometry has been chosen equal to a Sierpinski fractal shape with two fractal iterations to yield a two resonances behaviour [19]. More specifically, the whole dimension of the antenna is equal to a1 × a2 , while a microstrip line of size b1 × b2 has been used to feed the antenna. The reference fractal shape has been perturbed by varying the parameters r1 , r2 , and r3 , which determine the position and the dimension of the inner triangular slot, and the value of φs that defines the slant of the antenna geometry from the vertical axis passing from the center of the feed section. In order to enable the operability in the frequency bands B1 = {2.40, 2.70} GHz and B2 = {4.94, 4.99} GHz, while miniaturizing the radiating structure, the antenna has been required to exhibit a good impedance matching over the working frequencies and to have a maximum linear extension equal/smaller than that of a standard monopole antenna operating at the lowest frequency fmin = 2.4 GHz a1 ≤ Lmax
a2 ≤ Lmax
(1)
λmax = 31.25 mm, λmax being the wavelength at fmin . where Lmax = 4 As for the impedance matching, such a condition has been properly
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formulated in terms of a VSWR constraint as follows
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VSWR(f ) ≤ 2,
f ∈ B1 ∪ B2
(2)
The PSO-based optimization procedure has been applied according to the guidelines in [33]. More specifically, the following parameter setup has been adopted: a swarm of R = 8 trial solutions, a maximum number of iterations equal to K = 200, and the convergence threshold for the minimization set to η = 10−5 . The entire optimization process took approximately two hours on a standard PC (Intel Core i5 at 3.2 GHz with 4 GB of memory). After the optimization, the descriptors of the antenna have assumed the values in Table 1. As it can be noticed, the synthesized antenna not only fits the dimension constraints (a1 = 23.5 mm, a2 = 15.5 mm), but it also allows a size reduction of about 25% compared to a standard quarter-wave resonant monopole (a1 = 0.19λmax ). 3. NUMERICAL AND EXPERIMENTAL VALIDATION In order to assess the efficiency and the effectiveness of the proposed design solution in fitting the electrical and radiation requirements, the performances of the optimized antenna have been numerically and experimentally evaluated. Towards this end, the prototype shown in Figure 2 has been printed on an Arlon dielectric substrate characterized by r = 3.38 and tan δ = 0.0025. Moreover, the experimental Table 1. Geometrical descriptors.
a1 23.5
Geometry Descriptors [mm] a2 b1 b2 r1 r2 r3 15.5 4.0 5.0 15.1 1.9 6.6
Figure 2. Antenna prototype.
[deg] φs −12.3
5
measurements have been collected by mounting the antenna prototype on a metallic ground plane and feeding it with a 50 Ω coaxial cable. More in detail, by using a standard SMA connector, the outer shield of the coaxial cable was connected to ground plane while the inner core was connected to the center of the feed section at the bottom side of the antenna prototype. The ground plane was a square metallic plate of side 600 mm, corresponding to 5λ1 , λ1 being the wavelength at the center frequency of the lower band of interest, f1 = 2.55 GHz. As far as the electric features are concerned, simulated and measured VSWR values from 2 GHz up to 6 GHz are compared in Figure 3. By setting the operating bands to the frequency ranges where VSWR ≤ 2, it turns out that the antenna has an evident dualband behavior. From the simulated data, one can notice that the project requirements in terms of impedance matching are fully satisfied between 2.27 GHz and 2.87 GHz and from 4.89 GHz up to 5.11 GHz. This is confirmed by measurements as shown in Figure 3 and further pointed out in Table 2, despite some slight discrepancies mainly caused by the soldering effects and small imperfections in prototyping process. Moreover, it is worth to observe the presence of a stop-band region between the two operating bands, where the VSWR values increase up to 8.6 in order to avoid the transmission/reception over unintended frequencies. The good impedance matching exhibited by the proposed antenna is also confirmed by the behavior of the input impedance over the frequency shown in Figure 4. Indeed, the imaginary part of the antenna input impedance is negligible and its real part is close to 50 Ω in the B1 and B2 operative bands. In order to analyze the relation between the antenna geometry and its dual-band behavior, the currents flowing on the metallic 9
150
Re (Z in ) Im (Z in )
Simulate d Measured
8
100
Input Impedance
7
VSWR
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Multiband fractal antenna for wireless communication systems
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50 0 -50
2 1
2
2.5
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3.5
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Frequency [GHz]
Figure 3. Simulated and measured VSWR values.
-100
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Frequency [GHz]
Figure 4. Simulated antenna input impedance.
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Table 2. Electrical performances.
Requested Simulated Measured
Wi-Fi/WiMAX Band 2.40–2.70 GHz 2.27–2.87 GHz 2.19–2.79 GHz
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Surface Current [dBA/ m]
Surface Current [dB A/ m]
+36 +25 +14 +3 −8
+36 +25 +14 +3 −8
(a)
Figure 5. 4.965 GHz.
Public Safety Band 4.94–4.99 GHz 4.89–5.11 GHz 4.80–5.02 GHz
(b)
Surface current at (a) f1 = 2.55 GHz and (b) f2 =
surfaces of the synthesized structure have been simulated at f1 = 2.55 GHz and f2 = 4.965 GHz (i.e., the center frequencies of the two bands of interest). As expected (Figure 5), the current distributions considerably differ. At f1 = 2.55 GHz [Figure 5(a)], the radiating area is the whole metallic surface. The currents flow from the feeding point along the edges of the metallic shape up to the top of the structure where the minimum values are visible. The antenna works like a classic quarter-wave monopole since the total length of the left slanted side, λ1 (i.e., 0.26λ1 = 31.1 mm). When where the current concentrates, is 4 f2 = 4.965 GHz [Figure 5(b)], the active region is the triangular slot where the current mainly concentrates. Since the total length of the slot is about 0.46λ2 = 27.3 mm (λ2 being the wavelength corresponding to f2 ), the antenna acts as a half-wavelength structure. The antenna radiating mechanism just explained defines also the range of variation of the ratio between the two resonances (“frequency ratio”). Indeed, setting the antenna overall dimensions (i.e., the first resonance) and the position of the two bottom corners of the slot (i.e.,
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the radiation efficiency), the second resonance depends only on the variation of the position of the third corner of the slot along the top side of the antenna. The minimum and the maximum slot perimeters that can be achieved are 26.7 mm (obtained for r2 = 5.2 mm) and 33.0 mm (obtained for r2 = 14.3 mm), respectively. Accordingly, the second antenna resonance varies in 4.55–5.62 GHz, resulting in a range of the frequency ratio equal to 1.78–2.20. As for the radiation properties, Figures 6 and 7 give an overview of the antenna behaviour. More specifically, Figure 6(a) shows the three-dimensional total gain radiation pattern of the optimized antenna at f1 = 2.55 GHz. As expected from the surface current analysis, the antenna behaves as a standard quarter-wave monopole with an omnidirectional behavior along the H-plane, while a minimum appears in the z-direction. When the operating frequency increases to f2 = 4.965 GHz [Figure 6(b)], the antenna radiates differently and it does not exhibit anymore the null along the vertical direction, but it mostly radiates along the x-axis because of the modification of the active region (i.e., the slanted sides vs. the triangular slot). Although the radiation pattern in the H-plane turns out to be slightly compressed along the y-direction, it does not present any minima, still making the antenna suitable for communication applications. For the sake of completeness, the simulated patterns have been validated against experimental data as shown in Figure 7. As it can be observed, there is a good agreement between simulation and measurements besides some small oscillations due the non-ideality of the antenna prototype as well as the measurement environment.
+7 +2 −3 −8 −13
z
+7 +2 −3 −8 − 13
Gain[dBi]
z
Gain[dBi ]
y
x
y x
(a)
(b)
Figure 6. Three-dimensional radiation pattern at (a) f1 = 2.55 GHz and (b) f2 = 4.965 GHz.
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60
0
10 0 -10 -20 -30
30
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60
10 0 -10 -20 -30
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210 180
Gain [dBi]
10 0 -10 -20 -30
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300
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(a)
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Gain [dBi]
330
120
210 180
(c)
270
240
150
Simulated Measured
210 180
(d)
Figure 7. Simulated and measured radiation patterns. (a) (b) Hplane and (c) (d) E-plane. Working frequency. (a) (c) f1 = 2.55 GHz and (b) (d) f2 = 4.965 GHz. 4. CONCLUSIONS In this paper, a multiband fractal antenna suitable for wireless communication systems for emergency management has been presented. The antenna enables connectivity at Wi-Fi and WiMAX standards as well as in the Public Safety band at 4.9 GHz. Numerical and experimental results have confirmed the efficiency of the prototype in terms of impedance matching and radiation properties within the range of frequencies of interest. Moreover, a non-negligible reduction of about 25% with respect to a standard quarter-wave monopole operating at the lowest frequency has been yielded.
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