Circularly polarized omnidirectional millimeter wave monopole with

gular slot antenna with tuning pad for UWB applications, Electron Lett 41 (2005), 1095–1097. 4. R. Chair, A.A. Kishk, and K.F. Lee, Ultrawide-band coplanar waveguide-fed rectangular slot antenna, IEEE Antennas Wireless Propag Lett 3 (2004), 227–229. 5. T.A. Denidni and M.A. Habib, Broadband printed CPW-fed circular slot antenna, Electron Lett 42 (2006), 135–136. © 2007 Wiley Periodicals, Inc.

CIRCULARLY POLARIZED OMNIDIRECTIONAL MILLIMETER WAVE MONOPOLE WITH PARASITIC STRIP ELEMENTS Figure 4 Measured antenna gain for the fabricated antenna. —measured results; – – – simulated results

permittivity of 2.65) with its size 70 ⫻ 57 mm2. Its dimension parameters are listed in Table I. 3. RESULTS AND DISCUSSION

To examine the performance in terms of impedance bandwidth, the return loss for the fabricated antenna was measured using an HP8510C network analyzer. The simulated and measured return losses of the antenna are shown in Figure 2. It can be seen that the investigated antenna can approach an impedance bandwidth from 2.0 to over 12 GHz for S11 ⱕ ⫺10 dB, and a notch centered at 5.76 GHz (about 260 MHz higher than that of the simulated one) is also observed. The difference between simulations and measurements is mainly due to the substrate errors (including its thickness and relative permittivity) and fabrication tolerances. The measured radiation patterns in the E-plane and H-plane at 3.0, 5.0, 7.0, 9.0, and 10.5 GHz are illustrated in Figure 3. Notice that the proposed antenna has almost bi-directional patterns, especially at lower frequencies, in the E-plane, and omni-directional patterns in the H-plane at the lower or higher frequencies. Meanwhile, the measured antenna gain, given in Figure 4, shows the antenna has a gain of about 3.8 – 6.8 dBi across the UWB frequency band. A sharp notch, centered at 5.76 GHz, is also observed with the rejection level approximately 10 dB. 4. CONCLUSION

A planar elliptical slot antenna has been investigated. The proposed antenna has a compact size of 70 ⫻ 57 mm2 because of employing FG-CPW structures. To co-exist with the WLAN systems, a V-shaped slot is introduced. The antenna offers UWB performance with frequency band-rejection characteristic and suitable radiation patterns, which is met for UWB applications. ACKNOWLEDGMENT

This work was supported by the Research Grant Council of Hong Kong SAR under grant no. CityU 121905. REFERENCES 1. Y. Yoshimura, A microstripline slot antenna, IEEE Trans Microwave Theory Tech 20 (1972), 760 –762. 2. W.J. Lui, C.H. Cheng, Y. Cheng, and H. Zhu, Frequency notched ultra-wideband microstrip slot antenna with fractal tuning stub, Electron Lett 41 (2005), 294 –296. 3. D.C. Chang, J.C. Liu, and M.Y. Liu, Improved U-shaped stub rectan-

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J.M. Fernańdez, J.L. Masa-Campos, and M. Sierra-Pe´rez Department of Signals, Systems and Radio-communications, Polytechnic University of Madrid, 28040 Madrid, Spain Received 27 July 2006 ABSTRACT: A circularly polarized omnidirectional millimeter wave monopole with parasitic strip elements for a system of arrival signal direction detection in millimeter band is presented. As a consequence of the radiation pattern requirements, the proposed antenna consists of a conical skirt monopole with a polarizer radome. The polarizer converts a wave from linear to circular polarization. The proposed antenna has the advantage to be robust, low cost, and easy to fabricate with conventional materials and printed circuit technology. The study presents simulation and experimental results obtained with an antenna prototype that validate the design. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 664 – 668, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22237 Key words: circular polarization; arrival signal direction detection system; omnidirectional antenna; millimeter antenna 1. INTRODUCTION

Circularly polarized omnidirectional millimeter wave antennas have been widely studied for state of the art communication applications [1–3]. An omnidirectional radiation pattern is commonly required for the antenna of systems with detection of arrival signal direction. In addition, the use of circular polarization avoids the effect of multipath reflection of waves caused by the ground surface [4]. Circularly polarized antennas usually require a feeding network to radiate the appropriate polarization and a complex arrangement of the radiating elements. No circularly polarized omnidirectional antennas with a simple structure have been developed; one of the main reason for this is the difficulty of matching the antenna to a coaxial cable [5]. This work differs from previously published contributions because, in this case, a conical skirt monopole set around with parasitic strip elements is presented. Measurements with a prototype and simulation are compared and they validated the proposed antenna. In this study, a circularly polarized omnidirectional antenna consisting of a conical skirt monopole is proposed. Four pairs of tilted parasitic elements are set around the monopole to obtain circular polarization. The antenna has a simple mechanical structure and no feeding network such as a 90° hybrid phase shifter, a split-sheath balun, or a ␭/4 shift line is used to achieve the circular polarization. The main advantages of the proposed antenna are robustly, low cost and simple manufacturing process with conven-

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 3, March 2007

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Figure 1 Configuration of the conical skirt monopole with polarizer. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com]

tional materials and printed circuit technology. In this study, the design process of the proposed antenna is described. Simulation and comparative measured results of a prototype antenna are presented. Good agreement and satisfactory behavior are achieved in radiation pattern, polarization, and return losses. 2. ANTENNA STRUCTURE

The required radiation pattern of the antenna is omnidirectional in the azimuthal plane. In elevation plane, a ⫺10° to ⫹30° (zenith 0°) coverage must be satisfied. A ⫺6 dB axial ratio specification is required in the entire operation frequency band (36.7–37 GHz), and in the radiation pattern coverage. The general structure of the antenna, which consists of two different parts, is shown in Figure 1 and the antenna prototype is presented in Figure 2(a). The first one includes the radiating element (vertical monopole antenna of length ␭/4; ␭ is the freespace wavelength) made of the semirigid coaxial cable EZ118-TP as shown in Figure 2(b). The omnidirectional radiation pattern is obtained because of the monopole azimuth radiation property. By adding a “skirt” in cone-shaped metal, the radiation over and under the horizon is improved and the presence of the cylindrical slot of ␭/2, as shown in Figure 1, allows suppressing the surface currents of the radiating element. Starting from the idea of the typical ␭/4 monopole above a ground plane, the conical skirt monopole is introduced by tilting the plane downward to allow more radiation toward the sky and toward the ground [4]. The ⫺10° to ⫹30° coverage is achieved with this strategy. The dimensions of these frequency band antennas are critical, so the skirt in cone-shaped metal has been designed to reduce the critical aspects of tolerance manufacturing process at this frequency band.

Figure 2 Antenna prototype without polarizer: (a) Conical skirt monopole; (b) Radiating element. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com]

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The second part of the antenna is the polarizer, which is implemented in cylindrical form, as shown in Figure 3, to obtain the circular polarization. The cylindrical polarizer consists of two layers with four rows of ⫹45° tilted parasitic elements, as presented in Figures 3(b) and 3(c), set around the vertical conical skirt monopole [Fig. 3(d)]. The four rows of tilted parasitic strip elements are needed to fulfill the required radiation pattern and circular polarization purity. The polarizer consists of two printedcircuit sheets with etched-copper parasitic elements. The printedcircuit sheets are PTFE Taconic TLY-5 of 0.13 mm thickness and a dielectric constant ␧r ⫽ 2.2. All the parasitic elements in the sheets are uniformly spaced approximately ␭/2 (␭ is the free-space wavelength at the operating frequency) apart. The length of the elements L is 0.61␭ and the width of each element W is around ␭/13.5. The accurate spacing between the two sheets is achieved using a low-loss polyfoam of the desired thickness. The dielectric foam spacer between the two sheets has a dielectric constant of ␧r ⫽ 1.05 and a thickness of d ⫽ ␭/4 ⫽ 2.65 mm, so the spacing between the two sheets have a spatial phase difference of 90°. The ⫹45° tilted parasitic elements present a ␭/4 ⫽ 2.65 mm offset in the horizontal plane from one sheet to the other, as shown in Figure 5 (a). The parasitic strip elements are used to generate the omnidirectional radiation pattern with low ripples in the azimuthal plane [6]. The polarizer converts a wave from linear to circular polarization, when the wave propagates through the polarizer at normal incidence [7, 8]. The radiation characteristic of circular polarization depends on the parameters of the antenna, so the length L and the width W of each parasitic element and the spacing S between the two sheets must be appropriately adjusted. The center of the

Figure 3 Complete antenna prototype with polarizer: (a) Dielectric foam spacer: central part; (b) Inner part and first sheet of the polarizer; (c) Outer part and second sheet of the polarizer; (d) Experimental prototype. [Color figure can be viewed in the online issue, which is available at www. interscience.wiley.com]


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TABLE 1

Dimensions of the Parasitic Strip Elements for the Polarizer PTFE Taconic TLY-5 Sheet Thickness (mm)

Parasitic Elements Dimensions (mm)

Sheets First sheet (inner) Second sheet (outer)

Spacing Between Parasitic Elements (S)

Length (L)

Width (W)

Thickness (mm)

Dielectric Constant (␧r)

4 4

5 5

0.6 0.6

0.13 0.13

2.2 2.2

two layers of tilted parasitic elements is placed at the same height than the center of the radiating element. Because of the two layers of tilted parasitic elements, the spacing of about a quarter wavelength between the monopole and the parasitic strips is not required. The dimensions of two layers of four rows of ⫹45° tilted parasitic strips are summarized in Table 1. The polarizer has been designed to operate from 36.7–37 GHz. 3. PRINCIPLE OF GENERATION OF CIRCULARLY POLARIZED WAVES

It is well known that a pair of 45° tilted dipole antennas with a ␭/4 spacing, in orthogonal crossed disposition, radiates circular polarization in both directions of the array axe. Each dipole must be fed with equal amplitude and phase current [1]. The principle of the proposed antenna is based on this above cited antenna. A model antenna for explaining the basic idea consists of a vertical dipole and a conformal array of ⫹45° tilted parasitic strip elements set on the same axis as shown in Figure 4. The spacing between the dipole and the parasitic elements is set to R ⫽ ␭/4 [2]. Since the radiation field from the vertical dipole can be divided into two orthogonally crossed electric fields (parallel and perpendicular components to the parasitic strip elements), each tilted element is excited by the parallel component of the field. Because of the ␭/4 distance between the dipole and the parasitic strips, the reflection of the perpendicular component is cancelled. The 90° phase difference between the two radiated field components is achieved by the interaction between the parasitic strips and

Foam Spacer Thickness (mm)

Dielectric Constant (␧r)

2.65

1.05

the parallel component. In this case, the distance R ⫽ ␭/4 of the spacing between the vertical dipole and the parasitic elements is very important for the generation of a good circular polarization purity. In our case, a ⫺6 dB axial ratio of right hand circular polarization (RHCP) specification is required in the azimuth and elevation radiation pattern. The parasitic strip elements define the polarization properties. The principle of operation to convert linear to circular polarization is summarized in Figures 5(a) and 5(b). The radiation field Ei from the vertical monopole can be divided into two orthogonally crossed electric fields (E储 and E⬜); each tilted element is excited by the parallel component of the field (E储). The components E⬜ and E储 are in phase when they impinge at the polarizer. When the wave passes through the polarizer, a phase shift of 90° T⬙ between the two transmitted components E⬜ and E㛳T⬙ is produced. Furthermore, a circularly polarized wave emerges after the two layers of parasitic strips. The distance d between the first and the second sheet of parasitic strip elements and its length are adjusted to cancel the total reflection of the first (E㛳R⬘) and second sheet (E㛳R⬙). This distance also optimizes the transmission of the component E储 through the polarizer (E㛳T⬘ and E㛳T⬙). It is well known that the component E⬜ passes through the polarizer without any reflection because of the perpendicular component of the excited tilted T⬙ element field, which results in E⬜ as shown in Figure 5(b). ThereT⬙ fore, the two radiated field components E⬜ and E㛳T⬙ with equal amplitude and a 90° phase difference generate the circular polarization. The circular polarization purity depends on the length and width of the parasitic strip elements and the distance d between the T⬙ first and the second sheet that generate both radiated field E⬜ and T⬙ E㛳 components with a 90° phase difference between them. The advantage of this polarizer in comparison with that in Ref. 2 is that the distance of the spacing between the monopole and the parasitic strip elements is not dependent for the generation of the circular polarization. 4. ANTENNA RESULTS

Figure 4 elements

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Dipole antenna and a conformal array of 45° tilted parasitic

In this section, the simulation antenna results are presented, as well as the measurements of a manufactured prototype antenna. As we can observe in Figure 6, the simulated and the experimental reflection coefficients show a wide band response, the S11-parameter of the antenna is under ⫺18 dB in the entire operating frequency band. For elevation plane radiation pattern, the following criterion is established: The direction of the horizon is located at a ␪ of 90° (␪ ⫺90°), with the inferior angles to 90° (superior to ⫺90°) steering toward the sky and angles over 90° (under ⫺90°) steering toward the ground (Fig. 1). The simulated and experimental radiation patterns in the elevation plane are presented in Figure 7. As shown in Figure 8, the axial ratio is based on the angle in the vertical plane. The coverage


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Figure 6 Antenna reflection coefficient. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com]

in elevation for fulfilled the requirements of circular polarization would occur between ⫺30° and ⫹30° around the horizon. Likewise, for angles over ⫹30° a noticeably linear polarization component is observed, whereas for angles below ⫺30° there is no coverage because of signal lack. The results show the beneficial effect of the polarizer. The polarizer allows a higher coverage of the circularly polarized omnidirectional millimeter wave antenna. Figure 9 shows the simulated and measured radiation patterns in the azimuthal plane. Both results show an omnidirectional (360°) radiation pattern in the horizontal plane of the proposed conical skirt monopole antenna with polarizer. The maximum measured ripple in the copolar RHCP component is 1.5 dB. The conical skirt monopole antenna without the polarizer, as shown in Figure 2, has a linear polarization. Therefore, when the polarizer is added to the conical skirt monopole antenna, the circular polarization is achieved. An axial ratio (AR) ⬍⫺6 dB in the elevation and azimuthal plane specification is obtained, as we can observe in Figures 8 and 10. So, the obtained polarization satisfies the axial ratio requirements. The average antenna axial ratio in the omnidirectional plane is ⫺2.8 dB, with ripples of 1.5 dB. Measurements are seen in reasonable agreement with the simulation results. The measured directivity of the antenna is 3.2 dB in the operating frequency band. The required directivity is ⬎2 dB, so the specification is fulfilled.

Figure 5 Circularly polarized antenna description scheme: (a) front view; (b) profile view. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com] Figure 7 Compared simulated and measured radiation pattern in the elevation plane. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com]

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Figure 8 Axial ratio in the elevation plane. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com]

5. CONCLUSION

In this study, a circularly polarized omnidirectional millimeter wave antenna for a detection of arrival signals direction system is presented. The antenna consists of a vertical conical skirt monopole and two sheets of four rows of ⫹45° tilted parasitic strips and a foam spacer of ␭/4 between the two sheets. The simulation and experimental results show reasonable agreement. A low ripple omnidirectional radiation pattern in the azimuthal plane is achieved due to the parasitic strip elements. The manufactured prototype antenna shows successful result in circular polarization purity with this polarizer. The behavior in the extreme of the frequency band is the same as that in the central frequency. The simple structures of the antenna, as well as the robust and simple manufacturing process, recommend the use of this antenna in the signal detection system. ACKNOWLEDGMENTS

The simulations presented in this study have been realized using CST Microwave Studio version 5.0 under a co-operation agreement between Computer Simulation Technology (CST) and Universidad Politećnica de Madrid. This work has been developed with the support of INDRA S.A.

Figure 10 Axial ratio in the azimuthal plane. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley. com]

REFERENCES 1. G.H. Brown and O.M. Woodward, Circularly polarized omnidirectional antenna, RCA Rev 8 (1947), 259 –269. 2. K. Sakaguchi and N. Hasebe, A circularly polarized omnidirectional antenna, In: Proceedings of the Eighth International Conference on Antennas and Propagation, Heriot-Watt University, Edinburgh, UK, March 30 –April 2, 1993, Vol. 1, pp. 447– 480. 3. R.S. Elliott, Antenna theory and design, Prentice-Hall, Englewood Cliffs, NJ, 1981, Section 1.18, pp. 53–56. 4. D.S. Lerner, A wave polarization converter for circular polarization, IEEE Trans Antennas Propag AP-13 (January 1965), 3–7. 5. C.A. Balanis, Antenna theory, analysis and design, Wiley, New York, 1997, Chapter 9.6, pp. 462– 464. 6. J.M. Fernańdez and M. Sierra-Pe´rez, A circularly polarized omnidirectional millimeter wave antenna, Presented at the Joint COST 284 URSI Meeting, Proceedings of the 6th COST 284, Session 6C, Barcelona, Spain, September 8 –10, 2004. 7. L. Young, L.A. Robinson, and C.A. Hacking, Meander-line polarizer, IEEE Trans Antennas Propagat AP-21 (May 1973), 376 –378. 8. R.S. Chu and K.M. Lee, Analytical model of a multilayered meanderline polarizer plate with normal and oblique plane-wave incidence, IEEE Trans Antennas Propagat AP-35 (June 1987), 652– 661. © 2007 Wiley Periodicals, Inc.

MINIATURIZED OPTICAL FIBER BRAGG GRATING SENSOR INTERROGATOR BASED ON ECHELLE DIFFRACTIVE GRATINGS Gaozhi Xiao,1 Fengguo Sun,1 Zhiyi Zhang,1 Zhenguo Lu,1 Jiaren Liu,1 Fang Wu,2 Nezih Mrad,3 and Jacques Albert4 1 Photonic Systems Group, Institute for Microstructural Science, National Research Council, M-50, 1200 Montreál Road, Ottawa, ON K1A 0R6, Canada 2 MetroPhotonics Inc., Ottawa, ON K1C 7J2, Canada 3 Air Vehicles Research Section, Defence R&D Canada, Department of National Defence, National Defence Headqurters, Ottawa, ON K1A 0R6, Canada 4 Department of Electronics, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada Received 27 July 2006 Figure 9 Compared simulated and measured radiation pattern in the azimuthal plane. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com]

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ABSTRACT: An Optical fiber Bragg grating sensor interrogation device based on monolithically integrated echelle diffractive grating and detector arrays on an InP chip is designed and prototyped. The device


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