TABLE 1 Measured Peak Gain and Total Efficiency of the Proposed Antenna Frequency (GHz)
Peak Gain (dBi)
Total Efficiency (%)
2.3 2.5 2.7 5.2 5.3 5.5 5.7 5.8
2.59 2.33 2.09 2.72 2.25 2.88 3.49 3.15
71.58 74.90 68.49 56.24 64.67 73.34 86.32 86.33
principal plane cuts (x–z and y–z planes) at the center frequencies of the lower and upper resonant bands. The radiation patterns shown in Figure 7 represent the total field qamplitude ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi combining both the Ef and Ey components (¼ E/ 2 þ E/ 2 ). At both 2.5 GHz and 5.5 GHz, the measured radiation patterns approximate omni-directional patterns, which are suitable for a mobile environment. The measured peak gain and total radiation efficiency are given in Table 1. A peak gain value of 3.49 dBi was observed at 5.7 GHz. The total antenna efficiency was greater than 56% for all measured frequencies.
4. CONCLUSIONS
This letter describes the design of a miniaturized multiband antenna for USB dongle applications. A perturbed meander-line was used to implement the dual-resonant characteristic, and a tuning stub was employed to enhance the bandwidth performance. The fabricated antenna exhibits two wideband resonant frequencies supporting multiple wireless services such as WiBro, Bluetooth, WiMAX, S-DMB, and WLAN. In addition, omnidirectional radiation patterns can be obtained at all measured frequencies. Therefore, the proposed antenna is suitable for internal multiband antenna applications in compact wireless USB dongles.
REFERENCES 1. Available at: http://www.intel.com/technology/comms/wusb/. 2. M.-C. Huynh and W.L. Stutzman, A low-profile compact multi-resonant antenna for wideband and multi-band personal wireless applications, IEEE Int Symp Antennas Propag, Monterey, CA, vol. 2 (2004), 1879–1882. 3. Y.-X. Guo and H.S. Tan, New compact six-band internal antenna, IEEE Antennas Wireless Propag Lett 3 (2004), 295–297. 4. Z. Du, K. Gong, and J.S. Fu, A novel compact wide-band planar antenna for mobile handsets, IEEE Trans Antennas Propag 54 (2006), 613–619. 5. W.-S. Chen and K.-Y. Ku, A microstrip-fed monopole antenna for WLAN USB applications, Microwave J 51 (2008), 104–111. 6. P. Park and J. Choi, Internal multiband monopole antenna for wireless-USB dongle application, Microwave Opt Technol Lett 51 (2009), 1786–1788. 7. S.-W. Su, J.-H. Chou, and K.-L. Wong, Internal ultrawideband monopole antenna for wireless USB dongle applications, IEEE Trans Antennas Propag 55 (2007), 1180–1183. 8. Ansoft High Frequency Structure Simulator (HFSS) ver. 10, Available at: www.ansoft.com. 9. J.M. Gonzalez-Arbesu, S. Blanch, and J. Romeu, Are space-filling curves efficient small antennas? IEEE Antennas Wireless Propag Lett 2 (2003), 147–150. C 2010 Wiley Periodicals, Inc. V
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TEMPERATURE-INDEPENDENT VIBRATION SENSOR WITH A FIBER BRAGG GRATING Wenjun Zhou,1 Xinyong Dong,1 Changyu Shen,1 Chun-Liu Zhao,1 Chi Chiu Chan,2 and Ping Shum3 1 Institute of Optoelectronic Technology, China Jiliang University, Hangzhou 310018, China; Corresponding author:
[email protected] 2 School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 639798, Singapore 3 Network Technology Research Centre, Nanyang Technological University, Singapore 637553, Singapore Received 27 December 2009 ABSTRACT: A novel temperature-independent vibration sensor based on a fiber Bragg grating (FBG) is demonstrated. The FBG was glued in a slanted direction onto the lateral side of a right-angled triangle cantilever beam. Vertical vibration applied to the cantilever beam leads to a periodical variation of bending curvature along the beam length. As a result of beam bending, the FBG is chirped and its reflection bandwidth and optical power change with the deflection of the beam. Experimental results were compared with the data of vibration measurement of a conventional resistance strain gauge. Good agreement has been achieved. The sensor is temperature independent, owning to the temperature-independence nature of the reflection bandwidth and C 2010 Wiley Periodicals, Inc. Microwave optical power of the FBG. V Opt Technol Lett 52:2282–2285, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25429 Key words: vibration sensor; fiber Bragg grating; cantilever beam; temperature-independent measurement
1. INTRODUCTION
Fiber Bragg gratings (FBGs) have attracted considerable interests in various fiber-optic sensor implementations for the past two decades. The sensing principle is that the reflection wavelength of a FBG changes with the applied strain and/or temperature variation through their effects on the grating period and the refractive index of the fiber core. In addition to the well-known advantages of fiber-optic sensors such as the electrically passive operation, immunity to RFI and EMI, high sensitivity, compact size, and potentially low cost, FBG-based sensors have an inherent self-referencing capability and are easily multiplexed in a serial fashion along a single fiber. The success of FBG-based sensors on the measurement of strain has facilitated the modification of transducer designs that can deal with other measurands such as vibration [1–7]. Most of previously reported vibration sensors relied on the demodulation of reflection wavelength shift, but the wavelength shift is sensitive to temperature. Additional temperature compensation techniques are therefore necessary, which add to system cost and complexity. In this article, a novel temperature-independent vibration sensor based on a FBG is demonstrated. The FBG was glued in a slanted direction onto the lateral side of a right-angled triangle cantilever beam. Vertical vibration applied to the cantilever beam leads to a periodical variation of bending curvature along the beam length. As a result of beam bending, the FBG is chirped and its reflection bandwidth and optical power change with the deflection of the beam. Furthermore, the sensor is temperature insensitive, owning to the temperature-independence nature of the reflection bandwidth and optical power of the FBG.
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DOI 10.1002/mop
C ¼ 1 (the ideal condition) is assumed, the calculated value of A, based on related parameters of the experimental setup, is 0.94 nm/cm. When a vertical vibration is applied on the sensor setup, the deflection of the cantilever beam will be changed periodically. According to Eq. (1), the reflection bandwidth of the FBG will change periodically. Provided that a broadband light source with flat output power in terms of wavelength is used as the light source, the reflected optical power of the FBG will change periodically too. Therefore, intensity-demodulated measurement of vibration by monitoring the reflected optical power of the sensing FBG can be achieved. Based on previous studies [9, 10], a large linear response range of optical power versus beam deflection can be achieved based on the length and reflectivity of the used FBG. Figure 1 Schematic diagram and experimental system. [Color figure can be viewed in the online issue, which is available at www. interscience.wiley.com]
2. DESIGN AND PRINCIPLE
Figure 1 shows the schematic diagram of the proposed FBGbased vibration sensor. A 3-cm long FBG was glued in a slanted direction onto the lateral side of a right-angled triangle steel cantilever beam with length L ¼ 32 cm, width at the fixed end b0 ¼ 4.5 cm, thickness h ¼ 0.4 cm. The FBG was deeply written in a hydrogen-loaded photosensitive single-mode fiber with a scanning 244 nm UV laser beam using the phase-mask method. After fabrication, it was annealed at 100 C for 15 h. The achieved FBG has a high reflectivity better than 0.999 and a central Bragg wavelength of 1548.42 nm. After the FBG was glued onto the lateral surface of the cantilever beam, the angle (y) between the axis of the FBG and the neutral layer of the beam is 7.7 . The cantilever beam was fixed by a clamp. An eccentric gear with weight m ¼ 100 g was installed at the free end as a vibration generator. When the right-angled triangle cantilever beam is bent by applying a vertical vibration on the sensor setup, the strain along the length of grating has a uniform gradient because the cantilever is a uniform-strength beam [8]. In this case, half of the grating is under a varying tension, whereas the other half is under a varying compression. The strain on the neutral layer of the beam is zero; hence no change in wavelength happens to this segment of grating. If the center of the grating is located well to the neutral layer of the beam, the center wavelength of the grating may keep fixed during the vibration process. As a result, the Bragg reflection band of the FBG widens to the both sides from the center symmetrically. Based on the analysis previously reported by some of the authors in Ref. 8, the curvature of the cantilever beam is uniform along the length of the beam due to the right-angled triangle design. That ensures the equal variation in chirp rate along the FBG when the curvature of the beam is changed. The variation in the FBG’s bandwidth, Dkc, is related to the deflection, f, of the beam by [8] Dkc ¼ A f
In the experiment, a gain-flattened amplified spontaneous emission (ASE) source based on erbium-doped fiber was used as a light source; an optical spectrum analyzer (OSA) and a photodetector (PD) were used to measure the reflection bandwidth and the reflected optical power of FBG, respectively. The reflection spectra of the chirped FBG were measured when different static deflections were applied on the cantilever beam. Figure 2 shows the measured reflection spectra at different deflections of 0, 5, 10, 15, and 20 mm. The corresponding 3-dB bandwidths are 0.154, 0.447, 0.739, 1.030, 1.323 nm, and the measured maximum variation in center wavelength is very small, less than 0.04 nm. The reflected optical power and reflection bandwidth of the FBG versus the applied beam deflection are shown in Figure 3. It can be seen that bandwidth of the reflected spectrum increases linearly with beam deflection, as predicted in the theory analysis. Though the reflectivity of the FBG is reduced slightly with the increase of reflection bandwidth (as shown in Fig. 2), the reflected power changes linearly with the applied deflection in the tested range of 0–10 mm. The achieved sensitivity for optical power measurement is 0.264 mV/mm. Dynamic measurements were carried out by installing an eccentric gear on the free end of the beam as a vibration source. For comparison purpose, a resistance strain gauge was attached on the surface of the cantilever beam to monitor the variation through strain measurement. The reflected optical power of the FBG-based sensor and the electrical output signal of the resistance strain gauge were measured simultaneously. Because the FBG used in the experiment is a uniform period grating, the
(1)
where A % C(1 pe)kBcLgL2sin(2y) is a constant determined by the following parameters, C (0 < C < 1), a constant that represents the efficiency of the strain transfer from the beam to the grating; pe, the effective photoelastic constant (0.22) of the fiber material; kBc, the central wavelength of the grating; Lg, the length of the FBG; and L and y which were afore assigned. As
DOI 10.1002/mop
3. EXPERIMENTAL RESULTS AND DISCUSSION
Figure 2 Reflection spectra of the chirped FBG under various beam deflections. [Color figure can be viewed in the online issue, which is
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Figure 3 Reflected power and bandwidth of the FBG versus deflection of the beam
reflection bandwidth will increase either the deflection is positive or negative. To achieve linear response of the FBG, it is necessary to add a mass at the free end of the beam to make the FBG prechirped (or have an initial bandwidth). In this experiment, a mass with weight of 200 g was used to produce an original negative deflection of 5 mm. The output signals of FBG vibration sensor and the resistance strain gauge were recorded in a 0.5 second period and calculated by using a computer, as shown in Figure 4. The output traces of the two sensors agree very well. The calculated frequency spectra of the output signals of two sensors are shown in
Figure 5 Frequency spectra of the output of two sensors
Figure 5. The calculated vibration frequencies were 25.1 and 25.0 Hz for the FBG-based sensor and the resistance strain gauge, respectively. The difference is less than 0.5%. It is important to note that the sensitivity of the sensor can be easily adjusted by changing the following parameters, b0, L, h, and Lg. In practical vibration measurement, the eccentric gear is replaced of a mass to combine with the cantilever beam as a mass-spring model. So the weight of the mass is critical to the sensitivity of the vibration sensor. If the sensor is subjected to temperature variations, the Bragg wavelength will therefore shift as a result of the thermo-optic effect and the thermal expansion of the fiber. However, this wavelength shift will not affect the reflected optical power, because temperature variation doesn’t change bandwidth of the reflection spectra. Experimental test was carried out by putting the FBG-based vibration sensor, under a fixed deflection, into an oven and varying the temperature form 5 to 65 C. The measured results are shown in Figure 6. Small variations of 0.05 mV were
Figure 4
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Time domain traces of the output of two sensors
Figure 6
PD output versus temperature variation
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recorded in the output signal of the PD. This may related to the vibration induced by the fan in the oven.
4. CONCLUSION
A novel temperature-independent vibration sensor based on a FBG is demonstrated. The FBG was glued in a slanted direction onto the lateral side of a right-angled triangle cantilever beam. Vertical vibration applied to the cantilever beam led to a periodical variation of bending curvature of the beam. Reflection bandwidth and thus reflected optical power of the FBG changed with the bending curvature of the beam, so that vibration was measured by optical power measurement. Experimental result was compared with that from a conventional resistance strain gauge, and good agreement has been achieved. Furthermore, the sensor is temperature insensitive, owning to the temperature-independence nature of the reflection bandwidth and optical power of the FBG.
This work was supported by the National Basic Research Program of China (973 Program) under grant no. 2010CB327800, the National Natural Science Foundation of China under grant no. 60807021 and the Natural Science Foundation of Zhejiang Province China under grant no. R1080087. Author Wenjun Zhou appreciates Dr. Huaping Gong and Mr. Jixuan Chen for their helps in the experiment.
REFERENCES 1. T.A. Berkoff and A.D. Kersey, Experimental demonstration of a fiber Bragg grating accelerometer, IEEE Photonic Technol Lett 8 (1996), 1677–1679. 2. S. Theriault, K.O. Hill, F. Bilodean, D.C. Johnson, J. Albert, G. Drouin, and A. Beliveau, High-g accelerometer based on an infiber Bragg grating sensor, Opt Rev 4 (1997), 145–147. 3. J.M. Lopez-Higuera, M.A. Morante, and A. Cobo, Simple low-frequency optical fiber accelerometer with large rotating machine monitoring applications, J Lightwave Technol 15 (1997), 1120–1130. 4. M.D. Todd, G.A. Johnson, B.A. Althouse, and S.T. Vohra, Flexural beam-based fiber Bragg grating accelerometer, IEEE Photon Technol Lett 10 (1998), 1605–1607. 5. A. Mita and I. Yokoi, Fiber Bragg grating accelerometer for buildings and civil infrastructures, Proc SPIE 4330 (2001), 479–486. 6. S.R.K. Morikawa, A.S. Ribeiro, R.D. Regazzi, L.G.G. Valente, and A.M.B. Braga, Triaxial Bragg grating accelerometer, Opt Fiber Sens Conf Tech Dig 1 (2002), 95–98. 7. K.O. Lee, K.S. Chiang, and Z.H. Chen, Temperature-insensitive fiber-Bragg-grating-based vibration sensor, Opt Eng 40 (2001), 2582–2585. 8. X. Dong, P. Shum, N.Q. Ngo, C.C. Chan, J.H. Ng, and C.-L. Zhao, Largely tunable CFBG-based dispersion compensator with fixed center wavelength, Opt Express 11 (2003), 2970–2974. 9. X. Dong, X. Yang, C.-L. Zhao, L. Ding, P. Shum, and N.Q. Ngo, Novel temperature-insensitive displacement sensor using a fiber Bragg grating, Smart Mater, Smart Mater Struct 14 (2005), N7–N10. 10. X. Dong, P. Shum, X. Yang, M.F. Lim, and C.C. Chan, Bandwidth-tunable filter and spacing-tunable comb filter based on chirp tuning of CFBGs, Opt Commun 259 (2006), 645–648.
DOI 10.1002/mop
Hassan T. Chattha, Yi Huang, Yang Lu, and Xu Zhu Department of Electrical Engineering and Electronics, University of Liverpool, United Kingdom; Corresponding author:
[email protected] Received 4 January 2010 ABSTRACT: The planar inverted-F antenna (PIFA) is widely used in mobile and portable radio devices due to its excellent performance. However, it is not yet employed as an ultra-wideband antenna due to its perceived narrow band characteristics. This article introduces a new PIFA for frequencies from about 3 GHz to 10 GHz, with a fractional bandwidth of about 100%, which is ideal for emerging ultra-wideband (UWB) wireless applications. Simulated and measured results are C 2010 Wiley Periodicals, Inc. provided to verify the conclusion. V Microwave Opt Technol Lett 52:2285–2288, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ mop.25458 Key words: antennas; ultra wideband antennas; PIFA; planar antennas; broadband antennas
ACKNOWLEDGMENTS
C 2010 Wiley Periodicals, Inc. V
AN ULTRA-WIDEBAND PLANAR INVERTED-F ANTENNA
1. INTRODUCTION
The ultra-wideband (UWB) radio system as approved by the US Federal Communications Commission is defined as a device with a fractional bandwidth in excess of 20% or having an impedance bandwidth greater than 500 MHz with a carrier frequency in the range of 3.1–10.6 GHz [1, 2]. The inverted-F antenna is evolved from a quarter wavelength monopole antenna and is now widely used in mobile and portable systems due to its simple design, lightweight construction, low-cost, conformal nature, attractive radiation pattern, and reliable performance [3–6]. The planar inverted-F antenna (PIFA) is an extension of the wire inverted-F antenna in which the wire is replaced with a plate to increase the bandwidth. However, PIFA is still generally considered a narrow-band antenna although a significant amount of effort has been made to broaden its bandwidth [7–10]. There is a huge demand to incorporate this kind of antenna for emerging UWB applications. Recently, we have developed a PIFA antenna with the maximum fractional bandwidth around 65% in Ref. 10. This article presents a new UWB PIFA developed by applying three techniques: adjusting the width of feed, tuning the shorting plate, and adding a parasitic element. The approach of adding a parasitic element was used in producing resonances around 5 GHz in the case of wire type antennas [11, 12], but here it is shown that in case of the planar antenna, the resonance produced is around 8.5 GHz for similar dimensions. Similarly in conjunction with the other two techniques of changing the widths of the feed and shorting plates [10], we can make use of the parasitic element to produce the resonance around 8.5 GHz to further enhance the bandwidth. 2. ANTENNA CONFIGURATION
The configuration of the proposed PIFA is shown in Figure 1, which has employed three bandwidth broadening techniques. The radiating top plate has dimensions of W L, and the ground plane dimensions are Wg Lg. An air gap of thickness t ¼ 1.0 mm between the rectangular ground plane and the feed plate is employed to separate the ground plane and the feed. The antenna height is h and the space between the top plate and the substrate is filled with air (free space). In practice, a low dielectric material may be used to support the top plate. The shorting plate has dimensions of Ws (h þ t), the feed plate
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