5. S.-L. Jang, C.-Y. Lin, and C.-F. Lee, A low voltage 0.35um CMOSfrequency divider with the body injection technique, IEEE MicrowaveWireless Compon Lett, in press.

6. P. Wambacq and W. Sansen, Distortion analysis of analog integratedcircuits, Kluwer, Dordrecht, 1998.

7. S.-H. Lee, S.-L. Jang, C.-F. Lee, and M.-H. Juang, Wide locking rangedivide-by-4 injection locked frequency dividers, Microwave Opt Tech-nol Lett 49 (2007), 1533–1536.

© 2008 Wiley Periodicals, Inc.

PARAMETRIC ANALYSIS OFSIERPINSKI-LIKE FRACTAL PATCHANTENNA FOR COMPACT AND DUALBAND WLAN APPLICATIONS

Hui Li,1 Salman Khan,2 Jingxian Liu,1 and Sailing He1,3

1 State Key Laboratory of Modern Optical Instrumentation, Centre forOptics and Electromagnetic Research Zhejiang University, Hangzhou310058, China; Corresponding author: sunnyruby@hotmail.com2 Department of Physics, COMSATS Institute of InformationTechnology, Lahore, Pakistan3 Division of Electromagnetic Engineering, School of ElectricalEngineering, Royal Institute of Technology, S-100 44 Stockholm,Sweden

Received 26 April 2008

ABSTRACT: A Sierpinski-like fractal patch antenna with a slant stripin the first iteration is proposed in this article. The parameters of thiskind of antenna are analyzed and optimized for dual WLAN application.The added slant strip provides simple probe feeding for compactnessand also helps bandwidth enhancement due to multiple modes, whichcan be evident from current distribution. Measured return loss below�10 dB is from 2.38–2.45 GHz and 5.17–5.83 GHz, coinciding wellwith dual band WLAN standard. The boresight gain and radiation pat-tern in different planes are also presented. © 2008 Wiley Periodicals,Inc. Microwave Opt Technol Lett 51: 36–40, 2009; Published online inWiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23974

Key words: compact patch antenna; fractal; Sierpinski-like; dual bandWLAN

1. INTRODUCTION

Fractals are shape which displays self-similarity on differentscales while the dimensions consist of repeated geometricalstructure [1]. Fractal antennas can be designed in many shapes,such as Sierpinski carpet, Sierpinski gasket, Koch island,Minkoeski loop, and Tree-like structure [2– 4]. These antennashave been widely studied due to many advantages like multi-

band, wide bandwidth [5], compact size [6], and log-periodicalbehavior, which is because the construction process is a scaleddown version by a certain factor [4]. However, the removing ofsome metals from the patch makes probe feeding difficult, andthe microstrip feeding increases the overall dimension of theantenna.

The multiband characteristic of fractal antennas can be valuablefor selective dual band applications such as WLAN. According toIEEE a/b/g standard, the dual band WLAN occupies the frequencyband at 2.4–2.845 GHz, 5.15–5.35 GHz, and 5.725–5.825 GHz,simultaneously [7].

In this article, parametric analysis of modified Sierpinski-likefractal antenna is studied. Adding a slant strip within its firstiteration provides simple SMA connector to feed the antenna andalso reduces the overall size [8].The use of slant strip and thechoice of iterative size offer many advantages over the conven-tional fractal design, like compact size and selection of resonances.As a practical example, a prototype of the antenna optimized fordual band WLAN standard is fabricated, and the measured resultsare presented and discussed.

2. SCHEMATIC DESIGN

Schematic design of the Sierpinski-like carpet antenna along withiterative process and slant strip is presented in Figure 1.

The iterative process is based on the following rules [5]:

Nn � 8n (1)

Ln � �1

3�n

(2)

Dn � � limn3�

�lnNn

lnLn� � 1.89 (3)

Where Nn is the number of the black box, Ln is the ratio for thelength, and Dn is the capacity dimension.

A slant strip of width W helps to feed the antenna with SMAconnector and also reduces the overall dimension of the antenna[See Fig. 1(c)]. The simulation is performed using CST™ [9]based on finite integration technique and the metallization thick-ness for copper is 0.035 mm.

3. SIMULATION AND PARAMETRIC STUDIES

It can be seen in Figure 2 that the larger L1 (equivalent to increaseof strip length), the lower the first resonance (2.4 GHz), and theupper the second resonance frequency (5.5 GHz). Together withL1, the shape of the added strip also has some influence on the first

Figure 1 Sierpinski-like carpet patch antenna along with iterative process (left to right)

36 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 1, January 2009 DOI 10.1002/mop

resonance frequency of the Sierpinski-like antenna. As the widthof the strip (W) increases (see Fig. 3), the first resonant frequencyalso increases, whereas the second resonant frequency is almostconstant. This can potentially be used for selective resonance at 2.4GHz while keeping the second resonance (i.e. 5.5 GHz) un-changed. As a result of the strip, the current is forced to flow alongit, instead of only around the first iteration, which lengthens theelectrical length, and thus shifts the first resonance to lower fre-quency. It is concluded that the first resonance frequency mainlydepends on the size of the first iteration, and slightly on the widthof the strip. But in another aspect, the choice of W is moreimportant for 5.5 GHz band as it not only influences the bandwidthbut also the return loss level [See in Fig. 3(b)]. Substrate height canalso be adjusted for a further increase in bandwidth depending onthe requirement.

Finally, it can be seen from Figure 4 that the larger L2, thelower the resonance frequency of both bands, for it can increasethe electrical length around the second iteration.

The asymmetrical behavior of slant strip results in two distinctmodes at 5.5 GHz band, which form two resonance frequencies, in

turn helps to fulfill the bandwidth requirements of WLAN. Thetwo modes can be observed clearly in the surface current distri-bution in Figure 5.

To change the electrical length to a larger extent, a thirditeration (L3) is added [see Fig. 1(d)]. To achieve dual WLAN,more parameters are optimized. The influence of each parameter ofthe antenna is summarized in Table 1.

4. RESULTS AND DISCUSSION

A prototype is fabricated for a comparison between the simulatedand measured results and the optimized parameters are listed inTable 2. The measured and simulated results of return loss coin-cide very well, when compared with Figure 6. The bandwidth is2.38–2.45 GHz (centered at 2.41 GHz) at the first resonance, and5.17–5.83 GHz (centered at 5.3 GHz and 5.63 GHz, respectively)at the second resonance.

The far field patterns at 2.4 GHz and 5.5 GHz measured in ananechoic chamber are presented in Figure 7. The pattern of xy-plane is almost symmetric along 145°–325° line because of the

Figure 2 The behaviors of resonance with the variation of L1 (a) 2.4GHz (b) 5.5 GHz (L � 36 mm, L2 � 4 mm, and W � 1.5 mm). [Colorfigure can be viewed in the online issue, which is available at www.interscience.wiley.com]

Figure 3 The behaviors of resonance with the variation of W (a) 2.4 GHzand (b) 5.5 GHz, (L � 36 mm, L1 � 12 mm and L2 � 4 mm). [Color figurecan be viewed in the online issue, which is available at www.interscience.wiley.com]

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 1, January 2009 37

strip location. The discontinuity at xy-plane at 2.4 GHz (especiallyat 55°/225°) is partly due to the asymmetrical slant strip, and mightalso be due to the limitation of anechoic chamber. And the differ-ence of the pattern at xz-plane and yz-plane is due to the geomet-rical asymmetry of the Sierpinski-like structure and the feed point.In general, the measured results follow the trend of simulation. Themeasured gain is 3.82 dB and 3.48 dB at 2.4 GHz and 5.5 GHz,respectively.

5. CONCLUSION

In this article, a new Sierpinski-like fractal patch antenna with aslant strip within the first iteration is proposed for compact size andfrequency selective behavior. A parametric analysis of this pro-posed design is discussed in detail. And the selective resonancebehavior is further modified for practical dual band WLAN appli-cation. The added slant strip allows probe feeding, in turn compactantenna. Both the simulated and measured results are presented,describing a good behavior for WLAN from the input return lossgain and far field pattern view.

ACKNOWLEDGMENTS

The work is partially supported by the National Basic ResearchProgram (973) of China (No.2004CB719802) and the Scienceand Technology Department of Zhejiang Province (No.2005C31004), China. We are especially grateful to Prof. Fengand Mr. Jiangtian and Mr. Zhanghuan in Nanjing University forproviding measuring facilities. One of the authors (SalmanNaeem Khan) would also like to acknowledge the financialsupport provided by the Higher Education Commission (HEC)of Pakistan.

Figure 4 The behaviors of resonance with the variation of L2 (a) 2.4GHz and (b) 5.5 GHz. [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com]

Figure 5 Magnitude of surface current distribution at (a) 5.36 GHz and(b) 5.71 GHz

TABLE 1 Influence of Parameters on Frequencies

Parameter Name f1 � 2.4 GHz f2 � 5.5 GHz

Size of iteration1 (L1) 1 2(Decrease) 1(Increase)Size of iteration2 (L2) 1 2 2 littleSize of iteration3 (L3) 1 – Bandwidth 1Width of the strip (W) 1 1 –Height of substrate(H) 1 1 2Length of patch (L) 1 2 2

TABLE 2 Optimized Parameters List of the FabricatedPrototype

Parameter L L1 L2 L3 H �r W

Value 36mm 12mm 4mm 1.33mm 3mm 2.33 1.5mm

Figure 6 Comparison between the simulated and measured result of theSierpinski-like patch antenna. [Color figure can be viewed in the onlineissue, which is available at www.interscience.wiley.com]

38 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 1, January 2009 DOI 10.1002/mop

Figure 7 Comparison between the measured and simulated radiation pattern (a) xy-plane (H-plane) at 2.4 GHz, (b) xy-plane (H-plane) at 5.5 GHz, (c)xz-plane at 2.4 GHz, (d) xz-plane at 5.5 GHz, (e) yz-plane at 2.4 GHz, and (f) yz-plane at 5.5 GHz. [Color figure can be viewed in the online issue, whichis available at www.interscience.wiley.com]

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 1, January 2009 39

REFERENCES

1. B. Panoutsopoulos, Printed circuit fractal antennas, Consumer Electron-ics, ICCE, IEEE International Conference on June (2003), 438–442.

2. M. Kamal, A. Rahim, M.Z. Abidin, A. Aziz, and N. Abdullah, Micros-trip Sierpinski carpet antenna using transmission line feeding, APMC,Suzhou, China (2005).

3. C.P. Baliaada and J. Romeu, On the behavior of the Sierpinski Multi-band Fractal Antenna, IEEE Trans Antennas Propag 46 (1998), 517–519.

4. X. Liang and M.Y.W. Chia, Multiband characteristics of two fractalantennas, Microwave Opt Technol Lett 23 (1999), 242–244.

5. P.E. Mayes, Frequency-independent antenna and broad-band deriva-tives thereof, Proc IEEE 80 (1992), 1103–1123.

6. I.K. Kim, J.-G. Yook, and H.-K. Park, Fractal-shape small size micros-trip patch antenna, Microwave Opt Technol Lett 34 (2002), 15–17.

7. M.H. Ho and G.L. Chen, Reconfigured slot-ring antenna for 2.4/5.2GHz dual-band WLAN operations, Microwave Antennas Propag IET 1(2007), 712–717.

8. S. Wong and B.L. Ooi, Analysis and bandwidth enhancement of Sier-pinski carpet antenna, Microwave Opt Technol Lett 31 (2001), 13–18.

9. CST microwave studio, Reference manual, Wellesley Hills, MA.

© 2008 Wiley Periodicals, Inc.

HUMIDITY FIBER SENSORS BASED ONSUPERSTRUCTURE FIBER BRAGGGRATINGS COATED WITH POROUSSOL–GEL

Lung Ai,1 Wen-Fung Liu,2 Ming-Yue Fu,3 andTzu-Chiang Chen1

1 Department of Electrical Engineering, Chung Cheng Institute ofTechnology, National Defense University, Tahsi, Taoyuan 335,Taiwan, Republic of China; Corresponding author:lung.ai@msa.hinet.net2 Department of Electrical Engineering, Feng-Chia University,Taichung, Taiwan 407, Republic of China3 Department of Avionics Engineering, Air Force Academy,Gangshan, Kaohsiung, Taiwan 820, Republic of China

Received 26 April 2008

ABSTRACT: A fiber sensor based on an etched superstructure fiberBragg grating coated with the thin film of sol–gel is used for measuringthe relative humidity. The operation mechanism of this sensor is basedon that sol–gel materials can react with H2O molecules to result in itsreflective index change and then to cause both Bragg wavelength shift.The diameter of the superstructure fiber Bragg grating is etched downto 20 �m for obtaining the sensitivity of 0.014 nm/1%RH. © 2008Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 40–42, 2009;Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/mop.23973

Key words: fiber sensors; superstructure fiber Bragg grating; sol–gel;humidity

1. INTRODUCTION

An optic fiber sensor based on fiber Bragg gratings is regarded asa high potential sensing technique because of the fiber intrinsicadvantages such as small size, light weight, flexibility, immunity toelectromagnetic interference, anticorrosive property, long lifetime, etc. Fiber gratings can be divided into several different typesof gratings according to grating period and refractive index mod-ulation [1–3]. For the FBG fabrication, the most common methodis the phase-mask writing technique because of its simple estab-lishing and nice stability and repetition.

Humidity sensors are widely used in many practical sensingfields. There are many methods for achieving humidity sensors,such as capacitive humidity sensors based on dielectric changes ofthin films upon water-vapor uptake, resistive humidity sensors byusing transducer air humidity into an impedance change, andhygrometric humidity sensors by transduction from air via themechanical domain. There exists also a special miniaturized hu-midity sensor with the integrated design to be operated in anindirect mode [4–8].

In this article, we propose to use a superstructure fiber Bragggrating (SFBG) coated with sol–gel thin-film to detect relativehumidity. Its operating mechanism is based on the reaction ofsensing materials (sol–gel) with water molecules to result in theindex change of the sensing materials to cause the Bragg wave-length shift [9–12].

2. OPERATION PRINCIPLES

SFBG is fabricated by long-periodically modulated exposure of afiber Bragg grating to combine the optical characteristics of boththe FBG and the LPG. For mode coupling in an SFBG, by thephase-matching condition the forward propagating core-mode iscoupled to numerous backward core modes at a set of serieswavelengths with several narrow-band loss dips in the transmis-sion spectrum [13, 14]. Therefore, the cladding effective indexvariation will induce the coupling change from the forward core-mode into different backward core modes to cause the wavelengthshift. This phenomenon can be used for designing a fiber sensor bymeans of an etched SFBG coated with sol–gel materials [15],which are composed of oxide and silicon materials with a meshedporous structure formed by hydrolysis, condensation reaction, andpolymerization. The diameter of the solid particle of sol–gel isfrom one to several hundreds nm. The porous mesh structure isformed in solution by means of the combination of sub-micronholes and the chain molecules larger than 1 �m.

Figure 1 schematically depicts a fiber grating sensing head usedfor sensing the relative humidity. The sensor head, composed of anetched SFBG and the sensing material of sol–gel thin-film, can beplaced in the programmable humidity chamber for measuring thegrating wavelength shift in different levels of relative humidity.The sensing operating mechanism is that the H2O molecules areinteracted with the holes of sol–gel to cause the index change andthen to result in the grating wavelength shift and grating reflectiveintensity variation. From the basic grating equation and at thetemperature of 27°C, the relative humidity will result in the gratingwavelength shift of ��B which can be derived as

��B�n� � ��B

�n� ��neff

neff� � ��B

�n)

neff� a � �RH� (1)

where ��(n) is the grating wavelength shift of n-mode signal; neff

is the effective index, �neff is the variation of effective index, theparameter of a is the related-humidity coefficients, and �RH

Figure 1 The configuration of fiber sensing head

40 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 1, January 2009 DOI 10.1002/mop