LS1 � 13.783 nH, LS2 � 6.892 nH, LS3 � 5.627 nH, RS � 75 �.

And the corresponding matching and isolation characteristics areshown in Figure 7(a), whereas the transmission characteristics areshown in Figure 7(b). From the aforementioned results, it can beobserved that the simulated results of power dividing ratios areidentical with the desired 3 dB in three examples. Therefore, theclosed-form design method has been verified.

4.2. Design Layouts of Unequal Power DividersIn this subsection, three types of design layouts for proposedunequal power dividers are presented in Figure 8. The surfacemount devices (SMD0805) of the components should be used inthe layouts. Based on different power dividing ratios in the prac-tical applications, the corresponding unequal power dividers canbe fabricated easily by simple welding, and the S-parameters canbe measured by vector network analyzers.

5. CONCLUSIONS

In this article, the unequal Wilkinson power dividers using Pi-network and T-network including lumped-elements are proposed.Then, based on analytical results of Pi-network and T-networktransformers, the closed-form design method for the proposedunequal power dividers is obtained. To reduce the number oflumped-elements, a simplified unequal power divider is presented

with closed-from design equations. Furthermore, the frequencycharacteristics of three examples are simulated according to thecalculated design parameters results, and the simulated resultsindicate that these power dividers can fulfill the ideal ports match-ing, ideal isolation and unequal power division at the desiredfrequency. Finally, the circuit layouts are given to fabricate theproposed power dividers conveniently.

ACKNOWLEDGMENTS

The authors express their gratitude to the financial support of NationalHigh Technology Research and Development Program of China (863Program, No. 2008AA01Z211), National Natural Science Foundationof China (No.60736002), and Project of Guangdong Province Edu-cation Ministry Demonstration Base of Combining Production,Teaching and Research (No.2007B090200012).

REFERENCES

1. E. Wilkinson, An N-way hybrid power divider, IRE Trans MicrowaveTheory Tech MTT-8 (1960), 116–118.

2. F. Noriega and P.J. Gonzalez, Designing LC Wilkinson Power splitters,RF interconnects/interfaces (www.rfdesign.com), (2002), pp. 18–24.

3. H. Hayashi, T. Nakagawa, and K. Uehara et al, Miniaturized broadbandlumped-element in-phase power dividers, IEICE Trans Electron E90-C6 (2007), 1222–1227.

4. D.M. Pozar, Microwave engineering, 3rd ed., Wiley, New York, 2005,Chapter 7.

© 2009 Wiley Periodicals, Inc.

ULTRA-LIGHTWEIGHT DUAL-POLARIZED X-BAND ARRAY ANTENNAFOR AIRBORNE WEATHER RADARAPPLICATIONS

Yu-Jiun Ren1 and Yan Zhang2

1 Intelligent Automation, Inc., Rockville, MD 20855; Correspondingauthor: yren@i-a-i.com2 School of Electrical and Computer Engineering, University ofOklahoma, Norman, OK 73019

Received 12 September 2008

ABSTRACT: A dual-polarized X-band array antenna for advanced air-borne weather hazard monitoring radar applications is presented. Thislow-profile array is composed of 8 � 8 circular patch antenna elementswith a hybrid feed-line network providing dual-polarization function andhigh isolation (�40 dB) between H- and V-ports. The antenna operationbandwidth is from 9.9 to 10 GHz, and has a radiation gain of 19 dBi andcross-polarization of 31 dB. The array can be easily placed on the aircraftfuselage because of its ultra-thin thickness (0.13 mm), ultra-lightweight, andconformal structure, and it has potential applications in radar surveillance,remote sensing, and wireless communications. © 2009 Wiley Periodicals,Inc. Microwave Opt Technol Lett 51: 1324–1326, 2009; Published onlinein Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24323

Key words: antenna; microstrip; airborne; radar; UAV

1. INTRODUCTION

Airborne radars have the capabilities of high-resolution, real-time,and in-situ hazard monitoring in a localized air-space. In general,the dual-polarized operation can provide more useful informationabout target features, enhance the isolation between transmitter/receiver, and double the capacity of the communication channels

Figure 8 The circuit layouts for the proposed power dividers: (a), Thelayout for Figure 3(a); (b), The layout for Figure 3(b); (c), The layout forFigure 4. [Color figure can be viewed in the online issue, which is availableat www.interscience.wiley.com]

1324 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 5, May 2009 DOI 10.1002/mop

by means of the frequency reuse [1, 2]. Even though the polari-metric antennas have been used in ground-based radar systems [3],unique characteristics of airborne weather radars should be con-sidered. For example, as the ground-based systems use C/S band,airborne radars tend to use X-band or higher frequencies. Inaddition, the size, power, and installation requirements pose spe-cial challenges to dual-polarized airborne radar antenna design.Moreover, using dual-polarized array antenna for multifunctionalaviation hazard monitoring has not been reported.

Although many dual-polarized antennas have been studied, notall of them are good candidates for the airborne array designbecause of their complex structure and special feed-line network.Many of them are bulky and heavy, hence not suitable for airborneapplications. On the other hand, the isolation is one importantparameter to be considered in the dual-polarized array design.

Most reported dual-polarized arrays achieve at least 20-dB isola-tion [4–6]. In this article, an ultra-lightweight dual-polarized arrayfor airborne sensor is demonstrated. To reduce the array volumeand weight, an ultra-thin substrate is chosen so the antenna can beeasily installed on the aircraft fuselage or inside the aircraft. Ahybrid planar feed-line network is used to improve the isolationbetween ports and the two (copolarized and cross-polarized) pat-terns. The characteristics of this array are discussed. The measuredreturn losses, radiation gains, and array patterns are also presented.

2. ANTENNA DESIGN

The geometry of the proposed array is shown in Figure 1. Thearray has a dimension of 20.4 � 22.4 (W � L) cm2 and is printedon RT/Duroid 5880 substrate of 0.127 mm (�5 mil) thickness (h),whose substrate relative dielectric constant (�r) is 2.2. This arrayantenna consists of 8 � 8 circular patch elements on the front sideand the ground plane on the backside. The radius of the circularpatch at the dominant TM110 mode can be calculated using theequation in [7]. The calculated initial radius R (radius) of the9.95-GHz (f0) patch is 5.85 mm and the optimized value used inthe final design is 5.96 mm. The space between adjacent elementcenters is 10.1 mm that equal to 0.49 �g (�g is guided-wavelength).

To produce lower sidelobe and improve decoupling betweenthe copolarized and cross-polarized patterns, a tapered amplitudedistribution is implemented for the array. The 8 � 8-element arrayis divided into 16 subarrays with different magnitude of excitation,this is achieved by using the transmission line T-junction powerdividers with different power ratios, as shown in Figure 2. Eachsubarray consists of 2 � 2 elements and the series-fed resonanttype array structure is used, which could reduce the total arrayspace and have less microstrip line loss than that of the purelycorporate-fed type arrays. Besides choosing feed points of thepatch, the port isolation also depends on the quality factor of thepatch. Increasing the substrate thickness decreases the isolation sousing a thin substrate in this design improves the quality ofisolation.

Figure 1 Geometry of the X-band dual-polarized array antenna

Figure 2 Tapered amplitude (in dB) on each subarray

Figure 3 Measured E-plane radiation pattern of Port 1

Figure 4 Measured H-plane radiation pattern of Port 1

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 5, May 2009 1325

3. MEASUREMENT RESULTS

The measured operation band of this dual-polarized is 9.9–9.97GHz. The center frequency of both polarizations is at around 9.95GHz. The isolation between two ports (S21 � S12) is between�38.5 and �40 dB. These results are considered excellent for adual-polarized array of that the feed-lines for both polarizationspresent at the same layer. Measured normalized radiation patternsare shown in Figures 3 to 6, which demonstrate good symmetry.Cross-polarization levels of Port 1 in both E- and H-planes are lessthan �38 dB, while those of Port 2 are less than �31 dB. On eachplane, the 3-dB beamwidth is between 8 to 10°, which is close tothat of a uniform linear array.

The measured peak sidelobe levels are around �15 dB for mostcases. The possible reason of the increased sidelobe levels is dueto the power divider, since its realization with different and/orhigher power division ratio becomes critical. Besides the imper-fection of the power dividers, the mutual coupling between theantenna elements and the coupling due to the transmission lines ofthe feed-line network can disturb the power distribution and thusincrease the sidelobe levels. The measured array gains on eachplane are summarized in Table 1, which shows a very stableperformance over the entire operating frequency band, and themaximum measured gain occurs at 9.95 GHz (19.4 dBi at Port 1E-plane).

4. CONCLUSION

Motivated by the great potentials of polarimetric processing fornext generation airborne weather hazard monitoring radar, a dual-polarization X-band array antenna with an ultra-thin structure isdeveloped. Because of its compact size and light weight, theantenna can be installed on the airframe or inside the aircraft. Thearray design uses a hybrid-fed configuration with a tapered ampli-tude excitation to save space and improve isolation and cross-polarization between the ports. The measured antenna patternsverified the excellent performance of the array design and fabri-cation. Using such subarrays as building blocks, larger dual-

polarized arrays can be assembled to achieve high gain and sharpbeamwidth.

ACKNOWLEDGMENT

This work was supported by the NASA-Langley Research Center(Grant# NNX07AN15A). The authors thank Dr. Robert Neece,Mr. Steven Harrach, and other NASA researchers for their com-ments and guidance.

REFERENCES

1. S. Gao and A. Sambell, Dual-polarized broad-band microstrip antennasfed by proximity coupling, IEEE Trans Antenn Propag 53 (2005),526–530.

2. X. Qu, S. Zhong, Y. Zhang, and W. Wang, Design of an S/X dual-banddual-polarised microstrip antenna array for SAR applications, IET Mi-crowave Antenn Propag 1 (2007), 513–517.

3. V.N. Bringi, G. Huang, V. Chandrasekar, and E. Gorgucci, A method-ology for estimating the parameters of a gamma raindrop size distribu-tion model from polarimetric radar data: Application to a squall-lineevent from the TRMM/Brazil campaign, J Atmos Ocean Technol 4(2002), 464–478.

4. B. Lindmark, S. Lundgren, J. Sanford, and C. Beckman, Dual-polarizedarray for signal-processing applications in wireless communications,IEEE Trans Antenn Propag 46 (1998), 758–763.

5. A Parfitt and N. Nikolic, A dual-polarised wideband planar array forX-band synthetic aperture radar, IEEE Antennas and Propagation So-ciety International Symposium, Boston, 2001, pp. 464–467.

6. H. Wong, K. Lau, and K. Luk, Design of dual-polarized L-probe patchantenna arrays with high isolation, IEEE Trans Antenn Propag 52(2004), 45–52.

7. C. Balanis, Antenna Theory, 2nd ed., Wiley, New York, 2002.

© 2009 Wiley Periodicals, Inc.

COMPACT COPLANAR DESIGN FORHARMONIC SUPPRESSION INMICROSTRIP ANTENNA

Shaoqiu Xiao, Bing-Zhong Wang, Li Jiang, and Shanshan GaoInstitute of Applied Physics, University of Electronic Science andTechnology of China, Chengdu 610054, People’s Republic of China;Corresponding author: xiaoshaoqiu@uestc.edu.cn

Received 21 August 2008

ABSTRACT: In this letter, the compact coplanar designs of the mi-crostrip antenna are performed to suppress the harmonic radiations.The antennas with the second-order harmonic suppression as well as thesecond- and the third-order harmonic suppressions are designed, re-spectively. The results indicate that good harmonic suppression perfor-mance is achieved without increasing remarkably additional circuit areaand decreasing antenna performances. © 2009 Wiley Periodicals, Inc.Microwave Opt Technol Lett 51: 1326–1329, 2009; Published online inWiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24317

Figure 5 Measured E-plane radiation pattern of Port 2

Figure 6 Measured H-plane radiation pattern of Port 2

TABLE 1 Summary of the Radiation Gains (in dBi) on EachPlane

Frequency (GHz)

Port 1 Port 2

E-plane H-plane E-plane H-plane

9.9 19.1 18.6 18.9 18.59.925 20.1 19.3 19.7 19.09.95 19.4 18.6 18.7 18.5

1326 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 5, May 2009 DOI 10.1002/mop