1384 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013

Dual-Polarization Cylindrical Long-Slot Array(CLSA) Antenna Integrated With Compact

Broadband Baluns and Slot Impedance TransformersJennifer Rayno, Student Member, IEEE, Nuri Celik, Member, IEEE, and Magdy F. Iskander, Life Fellow, IEEE

Abstract—In this letter, a realistic method for feeding thedual-polarized cylindrical long-slot array (CLSA) antenna ispresented. A compact lumped-element balun with 2.5:1 band-width is designed, fabricated, measured, and used for feeding thedipole-mode, while a compact microstrip coupled-line impedancetransformer is used for feeding the slot mode of the antenna.Simulation results of these feeds are compared to experimentalmeasurements, and the overall performance of the dual-polarizedCLSA antenna with these feeds is also simulated. Simulationresults agree with measured data, and it is shown that this an-tenna has the capability of achieving four polarization modes[vertical, horizontal, left-handed circular polarization (LHCP),right-handed circular polarization (RHCP)]. The gain over225–450 MHz ranges from 1.42 to 3.15 dBi for the dipolemode, 0.44 to 2.24 dBi for the slot mode, 1.76 to 2.70 dBicfor the LHCP mode, and 1.80 to 2.68 dBic for the RHCPmode.

Index Terms—Broadband antenna, connected array, long-slotarray, lumped-element balun.

I. INTRODUCTION

U LTRAWIDEBAND communication systems are criticalto the future of commercial and military communica-

tions, as they enable access to many if not all narrowbandsystems using a single cost-effective antenna and often provideadditional enhanced performance. The cylindrical long-slotarray (CLSA) antenna [1] with vertical polarization was de-veloped to address this need, and when integrated with anultrawideband (40:1) hybrid EBG/ferrite ground plane [2],achieved an omnidirectional radiation pattern that was ver-ified experimentally. For size considerations, however, theCLSA antenna was designed in multiple different bands withvarying achievable bandwidths. Specifically, 4:1 bandwidthwas achieved for the lower-frequency design (150–600 MHz)and over 9:1 bandwidth was achieved for the higher-frequency(0.7–6.5 GHz) design. These bandwidths were specified by thesponsor, and the need for multiple designs was necessary toreduce the antenna size in the higher frequency band. In these

Manuscript received July 19, 2013; revised September 12, 2013; acceptedSeptember 24, 2013. Date of publication October 10, 2013; date of current ver-sion October 31, 2013. This work was supported by the Antennas & SpectrumAnalysis Division, Space & Terrestrial Communication Directorate, CERDECunder Contract No. IIP-0934091.The authors are with the Hawaii Center for Advanced Communications

(HCAC), University of Hawaii at Manoa, Honolulu, HI 96822 USA (e-mail:jrayno@hawaii.edu; celik@hawaii.edu; magdy.iskander@gmail.com).Color versions of one or more of the figures in this letter are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LAWP.2013.2284595

Fig. 1. CLSA antenna: (a) vertically polarized prototype with slot impedancetransformer feeds [3], [5]; (b) with dual-polarization and ideal lumped portfeeds [6].

designs, the slots of the CLSA antenna are periodically fed withspacing less than at the highest frequency of operation,and the feed ports have an impedance of about 150 each.For feeding these ports with 50- coaxial lines, a compactcoupled-line impedance transformer [3], [4] was developed.A prototype vertically polarized/slot-fed CLSA antenna cov-ering 225–450 MHz was fabricated with these impedancetransformers to experimentally verify the performance [5] [seeFig. 1(a)].With the successful design and testing of the vertically

polarized/slot fed CLSA antenna, it is desired to further im-prove the design and develop a dual-polarization [6] version.To achieve dual-polarization, the strips forming the longslots are segmented to create a connected dipole array [seeFig. 1(b)]. Dipoles require balanced feeding (as opposed tothe unbalanced slot feeds), so a broadband balun is necessaryto transform from the 50- unbalanced feed to the requiredbalanced feed for the dipole-mode. Also, a connected dipolearray has a higher impedance than a typical dipole (on the orderof [7]), hence a broadband balun that alsoprovides impedance transformation is desired.In this letter, a realistic method for feeding the dipole-mode

of the dual-polarized CLSA antenna covering 225–450 MHzis presented. Specifically, a compact lumped-element balun

1536-1225 © 2013 IEEE

RAYNO et al.: DUAL-POLARIZATION CLSA ANTENNA WITH COMPACT BROADBAND BALUNS AND SLOT IMPEDANCE TRANSFORMERS 1385

Fig. 2. Modification of the dual-polarization LSA antenna unit cell to makespace for the dipole balun.

with 2.5:1 bandwidth is designed, fabricated, and measured.The overall dual-polarization CLSA is then simulated whenintegrated with both the broadband balun dipole-mode feedsand the previously developed impedance transformer slot-modefeeds. Section II describes the design of the dipole-mode feed,while in Section III we discuss the overall CLSA antenna sim-ulation results when the dual-polarization feeds are integratedin the design. Section IV includes overall design observationsand concluding remarks.

II. DESIGN OF CLSA ANTENNA DIPOLE-MODE FEED

A dual-polarization CLSA antenna with ideal lumped portfeeds was simulated in [6] using Ansoft HFSS, and the feedgap at each dipole was too small to accommodate a realisticbalun feed [see Fig. 1(b)]. To this end, the dipole ends at the gapare tapered , the feed point is offset, and a parametric studywas conducted to optimize the design including the minimumpossible dimension of the feed gap. Fig. 2 shows the modifiedunit cell used to study the effects of these variables. The dimen-sions used for the metallic strips and slots of the unit cell arethe same as in the CLSA antenna prototype (strip width cmand slot width cm) [5]. The simulation goals were to min-imize the reactive input impedance, make space for the balun,and reduce the effect on the slot-mode of the antenna.While stillusing a lumped port excitation [see Fig. 2(a)], the taper anglewas varied from 0 to 30 , the feed point offset was varied from0.06 (near the top of the dipole) to 3.5 cm (at the center of thedipole), and the feed gap was varied from 1 to 10 mm.The final configuration that best met the goals was ,

offset cm, and gap mm. In this configuration, theinput impedance is at the center fre-quency of 337.5 MHz. As a comparison, for a design withoutthe tapering and using a center feed location offset

cm with gap mm, the input impedance is. As the taper angle is limited, these two cases

have nearly identical slot-mode radiation performance.Balanced microstrip feedlines are designed [see Fig. 2(b)] to

match the final dipole input impedance, thus the individual lineimpedance is . Finally, the balancedfeedlines are integrated with the unit cell [see Fig. 2(c)], re-sulting in the following line impedances:at 225 MHz, at 337.5 MHz, and

at 450 MHz. The total input impedance for thedipole mode is thus . An impedance transformationof about 1.7:1 along each of the balanced lines (from a 50-line) would provide optimal matching. However, even withoutan impedance transformation, an acceptable return loss of about11.7 dB can be obtained. For the final unit-cell model with

Fig. 3. HFSS model of lumped-element balun and fabricated balun.

TABLE IVALUES OF L AND C USED IN THE BALUN TO ACHIEVE VARIOUSBANDWIDTHS (BW) AND IMPEDANCE TRANSFORMATIONS (K)

integrated balanced lines, the return loss for the slot mode re-mained 15 dB over 225–450 MHz for a 150- lumped portimpedance, showing negligible effects due to modifications.The balun needs to be compact, planar, and broadband, and

as such, a lumped-element second-order lattice circuit [8] is se-lected. The circuit is a mix of high-pass (HP) and low-pass (LP)filters along each path, with the two output ports sharing acommon ground to achieve 180 phase difference, and thecenter frequency for the HP and LP filters slightly offset toachieve equal power magnitude over a wide frequency range.The initial balun design was done using Agilent ADS and

the table from [8], using a 50- input port impedance and acenter frequency of 337.5MHz for achieving the 2:1 impedancetransformation from the unbalanced line to each bal-anced one over 3:1 bandwidth. Then, the actual RF microstripcircuit was designed using HFSS, and the closest commerciallyavailable inductors and capacitors components were selected forfabrication (see Fig. 3). The overall circuit was reoptimized inHFSS using only commercially available components. Table Ishows inductor and capacitor values necessary to achieve var-ious bandwidths (BWs) and impedance transformations ( ) over225–450 MHz. The first column of values shows the final balundesign component values ( nH, pF,

nH, pF) compared to the ideal componentvalues calculated using the table from [8]. The effect of usingthese new and values, which was verified by both simula-tion and measurement, is that the bandwidth is now 2.5:1 andthere is no impedance transformation. Variable capacitors andinductors could be used to overcome this, but at the expense ofincreased cost and complexity.

1386 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013

Fig. 4. Comparison of simulated and measured performance of balun:(a) input port return loss; (b) output ports transmission phase and phasedifference ; (c) output ports transmission magnitude.

An optimally performing balun will achieve 3 dB mag-nitude and (equal power splitting) with 180 phasedifference between output ports, while having amatched input port. Fig. 4 shows a comparison of the simulatedand measured performance of the balun in Fig. 3, with thedesired performance being achieved over about 190–520 MHz.The balun is matched with a return loss 10 dB for180–600 MHz, the output port phase difference is about 180for 190–560 MHz, and the transmission magnitude is close to3 dB for 190–520 MHz. Thus, the desired 225–450-MHz

range is covered.

III. DUAL-POLARIZATION CLSA ANTENNA WITHDUAL-POLARIZATION FEEDING

The dual-polarization CLSA antenna with realistic feedingmethods was created by integrating the broadband balun andthe previously developed slot impedance transformer with theantenna. Fig. 5 shows the HFSS model of this antenna with 40dipole baluns, 30 slot impedance transformers, and a groundplane consisting of a 5-mm-thick layer of ferrite-tile with PECbacking. The ground plane is a simplified version of the one in[2], and it is an absorber in the design frequency range. Eachof the baluns and impedance transformers is fed by an SMAcoaxial line with all of the cables oriented perpendicular to theground plane [all cables are fed from the center of the antenna,

Fig. 5. HFSS model CLSA antenna with realistic feeds (40 dipole baluns and30 slot impedance transformers) and ferrite-tile with PEC backing: (a) frontview; (b) top view showing center coaxial cables (also includes fictitious cylin-drical surfaces that are used to analyze the fields).

Fig. 6. CLSA 3-D radiation patterns for the four polarization modes.

TABLE IIGAIN OF CLSA ANTENNA FOR THE FOUR POLARIZATION MODES

to reduce interference with the antenna performance as seen inFig. 5(b)]. When only the slots are excited (slot mode), ver-tical polarization results, whereas when only the dipoles are ex-cited (dipole mode), horizontal polarization results. When bothmodes are excited simultaneously in phase, left-handed circularpolarization is achieved (LHCP mode), whereas exciting bothmodes with 180 phase difference generates right-handed cir-cular polarization (RHCP mode).The impedance matching of this antenna is evaluated using

the overall return loss (ORL) [1], which is calculated using

where is the input power and is the accepted power.The ORL for the dipole mode ranges from 6.13 to 14.26 dB,the slot mode ranges from 10.60 to 14.30 dB, the LHCPmode ranges from 7.66 to 13.05 dB, and the RHCP moderanges from 7.76 to 13.06 dB. Omnidirectional radiationpatterns are achieved for all four modes of operations at all fre-quencies (see Fig. 6), and the corresponding gain values areshown in Table II. Considering half of the power is automati-cally lost due to the absorbing ground plane (used just to verifyproof of concept) and the small size of the antenna, the gainachieved is considered reasonable. The gain ranges from 1.42to 3.15 dBi for the dipole mode, 0.44 to 2.24 dBi forthe slot mode, 1.76 to 2.70 dBic for the LHCP mode, and1.80 to 2.68 dBic for the RHCP mode. The legacy antennas

RAYNO et al.: DUAL-POLARIZATION CLSA ANTENNA WITH COMPACT BROADBAND BALUNS AND SLOT IMPEDANCE TRANSFORMERS 1387

Fig. 7. Illustration of the electric field at 337.5 MHz on cylindrical surface 3[see Fig. 5(b)] surrounding the antenna for the dipole mode and slot mode.

Fig. 8. Axial ratio of the LHCP mode of CLSA antenna with realistic feeds.

(RAM1254, RAM16870, and 4310-A UHF) in commercial op-eration in the same band have return loss ranging from 5.1to 9.5 dB, and vertically polarized gain ranging from 1.85 to2 dBi. With a properly designed nonabsorbing ground plane

(e.g., using metamaterial perfect magnetic conductor structureinstead of the ferrite backing), the gain could be significantly in-creased, and preliminary efforts to design such a ground planeutilizing genetic programming are underway [9]. Excellent po-larization purity is achieved for the two linear orthogonal polar-izations. The maximum cross polarization for the slot mode is37.54, 52.01, and 34.77 dBi for 225, 337.5, and 450 MHz,

respectively, whereas for the dipole mode it is 35.95, 39.20,and 32.14 dBi.To analyze the mechanism by which the antenna achieves

omnidirectional radiation with the null along the axis of the an-tenna ( -axis) for all four polarization modes, the electric fieldis analyzed on three cylindrical surfaces [see Fig. 5(b)] sur-rounding the antenna, with surface 1 being on the surface ofthe antenna, surface 2 spaced 5 cm from the antenna, and sur-face 3 spaced 10 cm from the antenna. All three surfaces arein the near field of the antenna, but the electric field orientationdid not change much from surface 2 to surface 3. Fig. 7, there-fore, shows the configuration of the electric field on surface 3 forboth linear modes at 337.5MHz. The electric field for the dipolemode is oriented along the surface of the dipole, as expected forhorizontal polarization, and the electric field is vertically ori-ented for the slot mode. Finally, the axial ratio for the LHCP

mode at different frequencies is shown in Fig. 8, with similarresults achieved for the RHCP mode. A near constant value isachieved around the azimuth ( -plane), and close-to-constantvalues are achieved around the elevation, except near the nullsof the pattern. Thus, fairly consistent performance is achievedfor the two circular polarization modes, and with design opti-mization the axial ratio could be improved.

IV. CONCLUSION

An ultrawideband, omnidirectional, cylindrical long-slotarray antenna with realistic feeding methods and four po-larization modes was developed. The antenna operates inthe 225–450-MHz band and provides vertical, horizontal,left-handed circular, and right-handed circular polarization.For the dipole-mode feeds, a compact lumped-element mi-crostrip broadband balun was designed and fabricated. For theslot-mode feeds, a previously developed compact coupled-linemicrostrip impedance transformer was used. Good overallreturn loss and gain are achieved with the CLSA antenna for allfour modes of operation when the realistic feeds are integrated.Future and ongoing work includes further improvement in thebalun design for compactness, improving the axial ratio, andthe design of a broadband dual-polarization artificial magneticconductor ground plane to increase the gain.

ACKNOWLEDGMENT

The authors would like to thank J. Griffith, G. C. Huang, andJ. Pascual for help with the balun fabrication and measurements,and the reviewers for their constructive suggestions that not onlycontributed to the clarity of the contribution but also enhancedit.

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