Pencil-Beam Full-Space Scanning 2D CRLHLeaky-Wave Antenna Array

Hoang V. Nguyen∗†, S. Abielmona†, A. Rennings‡ and C. Caloz†

†Poly-Grames Research Center, Ecole Polytechnique de Montreal2500 Chemin Polytechnique, Montreal, QC H3T 1J4, Canada

‡IMST GmbH, Carl-Friedrich-Gauss-Str. 2, D-47475 Kamp-Lintfort, Germany

Abstract— A novel pencil-beam, full-space scanning 2D com-posite right/left-handed (CRLH) transmission line (TL) leaky-wave antenna (LWA) array is proposed. The proposed arrayachieves the same performance of conventional phase arrayantennas in a simpler manner. Moreover, array elements arefed by a CRLH infinite-wavelength series power divider andtherefore the separation between array elements and the numberof array element can be extended arbitrary. The CRLH seriespower divider eliminates the requirement of complex and highloss feeding networks commonly found in conventional arrays.Experimental results of each system components are presentedalong with analytical and simulation results of the complete arraysystem.

Index Terms— Antenna array, composite right/left-handed,leaky-wave, pencil-beam, frequency-scanning, phase-scanning.

I. INTRODUCTION

The steady and constant rise of wireless users has fuelledan increase in wireless applications. This increase has createda complex and crowded frequency spectrum in a constantlyevolving wireless landscape. Omni-directional antennas usedin many of today’s wireless devices cause interference, lower-ing signal quality, reducing data rates, and limiting bandwidth.Thus, a need exists for an antenna system having a narrowbeamwidth to reduce interference, while also offering theflexibility of full-space scanning.

The conventional approach to realize two-dimensional (2D)beam scanning array is to form a 2D array of radiatingelements [1]. This 2D array requires complex and lossyfeeding networks which in many cases are the bottle neckof 2D scanning array. Leaky-wave antennas (LWA) have adistinct advantage of a much simpler feeding network thanconventional arrays [2]. Several works on LWAs have beenreported for 2D beam scanning. Allen et al. [3] used theheterodyne mixing technique along with a delay line networkto frequency- and phase-scan, respectively. This approachrestricts the array’s phase-scanning capability to a limited scanrange, where the LO frequency is required to be in the guided-wave region of the CRLH TL so as not to interfere with RFradiation. In another approach, Lai et al. [4] used 2D CRLHTL array with orthogonal power feeding to control the mainbeam azimuthal radiation direction. This approach requiresvariable input powers to the antenna, which is inconvenientor not available in some applications. In addition, the main

beam can only be scanned within a limited azimuthal directionbetween 00 and 900. Beam steering in a CRLH TL array hasalso been recently reported in a Butler matrix configuration[5], where eight quadrature branch-line couplers were em-ployed. The use of couplers restricts the array’s operationalbandwidth, while also providing only discrete phase shifts fordiscrete scan angles.

In this work, we propose a novel 2D CRLH TL LWAarray capable of phase and frequency scanning a pencil-beam in full-space. The structure is compact and low-profile,offers continuous frequency- and phase-scanning capability,utilizes a new low-loss and flexible series feeding networkwith distinct advantages over traditional series or corporatefeeding networks. The applications of such an array includeRFID systems, smart antennas, and DOA estimation.

II. PROPOSED STRUCTURE AND PRINCIPLE OF OPERATION

The novel, efficient pencil-beam 2D scanning array is shownin Fig. 1. It consists of a series power divider feeding anentire system of phase shifters connected to mixers, whichsubsequently feed into an array of LWAs.

50Ohm

50Ohm

50Ohm

50Ohm

RFRF

RFRF

RFRF

RFRF

LO (3GHz)

Uniform(closed) CRLHSeriesPowerDivider

Frequency-Scanned Leaky-Wave Anntenas

z

x

y

dϕ

2ϕ

3ϕ

4ϕ

IF (0.5-1.3GHz)

θ

φ

RF (3.5-4.3GHz)

Fig. 1. Pencil-beam full-space 2D scanning CRLH leaky-wave antenna array.

The CRLH LWA element is the core radiating part of thearray. Several advantages of a CRLH LWA are (1) simple, low-cost, and miniature feeding structures, (2) frequency-scanning

1391-4244-1449-0/07/$25.00 © 2007 IEEE

from backfire to endfire without phase shifters, (3) inexpensiveelectronic scanning, and (4) highly directive in φ=00 plane dueto its large aperture [2]. However, the 1D CRLH LWA has afan beam in the transverse plane. In order to obtain a pencil-beam, the aperture size in the transverse plane needs to beincreased, and that is done by arraying several LWAs in theyz-plane, to compose a 2D LWA array shown in Fig.1.

The CRLH LWA must operate in the fast-wave region inorder to radiate efficiently. Thus at the mixer, an injected LOsignal with fLO =f0 must be converted to the RF frequency(fRF = fIF + fLO), which is within the leakage range of theLWAs. 2D scanning is accomplished by employing frequency-and phase-scanning in the xz- and yz-plane, respectively. Thefrequency-scanning in the xz-plane is achieved by tuning thefIF , while maintaining fLO = f0 at all times, hence varyingfRF . On the other hand, phase-scanning in the yz-plane isachieved by adjusting the phase shifters so that the phasedifference between each LWA is ϕ. For diagonal scanning,both fIF and the phase shifters must be tuned simultaneously.

III. COMPONENT PROTOTYPES

A. Series Power Divider

The series power divider/combiner is shown in Fig.2. Itconsists of an open-ended CRLH resonator operating at theinfinite-wavelength frequency (f0 = 2.955 GHz), whereβ(f0) = 0. Since the electrical length of the resonator isexactly zero at f0, an equal phase/magnitude state exists allalong the CRLH resonator structure. The infinite-wavelengthseries power divider presents several advantages over conven-tional planar feeding networks. It is compact and low-losscompared to traditional lossy and bulky feeding networks,provides equal and in-phase power to an arbitrary number ofelements compared to the corporate network which is limitedto 2N elements, allows arbitrary spacing between elementfeeds to avoid grating lobes while optimizing gain, and doesnot suffer from magnitude imbalance (reducing directivity) ofconventional series feeding networks [6], [7], [8].

The CRLH series power divider is implemented in metal-insulator-metal technology for capacitors and shorted stubs forinductors on Rogers Duroid 3003 (εr = 3). Due to the inherentleakage of a CRLH TL, the series power divider is enclosedin a metallic box, as shown in Fig.1, to eliminate potentiallyinterfering radiation. Furthermore, a quarter-wave transformerat f0 is used to match the series power divider to 50Ω.

The measured scattering parameters for the closed CRLHseries 1-to-4 power divider are shown in Fig. 3. The magnituderesponse in Fig. 3a shows the intersection point of S21 andS51, indicating an equal magnitude of −10 dB at the transitionfrequency f0 = 2.955 GHz. Similarly, the phase response inFig. 3b shows the intersection point of S21 and S51, indicatingan equal phase of 200 also at f0 = 2.955 GHz. S11 in Fig. 3aalso indicates that the series power divider is well matched atf0 = 2.955 GHz.

12345

MIM Capacitor

Shorted stub

Fig. 2. Prototype of the closed CRLH series power divider, along with aunit cell perspective view.

2.85 2.9 2.95 3−14−12−10

−8−6−4−2

frequency (GHz)

mag

nitu

de (d

B)

3.05

S11

S21

S51

(a) Magnitude

2.85 2.9 2.95 3−40−20

020406080

100120140160

frequency (GHz)

phas

e (d

egre

e)

3.05

S21

S51

(b) Phase

Fig. 3. Measured scattering parameters of the closed CRLH series powerdivider. (a) Magnitude response. (b) Phase response.

B. Phase-shifter and Mixer

The variable phase shifter is implemented by loading a TLwith a variable capacitor in shunt [9]. The phase shifter isshown in Fig. 4. The phase of a TL, given by β = ω

√LC, will

be varied accordingly by choosing the appropriate capacitorvalue to obtain the required phase shift. Placing two capacitorsseparated by λg/4, reduces the reflections from the shuntcapacitors, thus minimizing insertion loss. Finally, coplanarwaveguide is used here to eliminate the grounding via’sinductance.

Triple-balanced commercial diode mixers (Mini-circuitsMCA-50MH) were chosen with an IF range of 0.1−1.5 GHz,with good isolation between all ports, and a conversion lossbetween 6 − 8 dB across the frequency range of interest. Inorder to reduce the external coupling between the IF and RFports, the IF input feed was placed at the substrate’s bottomlayer, while the mixer’s RF output is directly connected to theLWA at the substrate’s top layer [10]

C. Leaky-wave Antenna

The CRLH TL LWA element is implemented in planarmicrostrip technology. The inset of Fig. 5 shows a smallscale prototype consisting of 6 unit cells, each having a seriesinterdigital capacitor and a shunt stub inductor. The measuredscattering parameters of the prototype are shown in Fig. 5.Within the leaky-wave passband of 3 − 4.5 GHz, the CRLHTL LWA is well matched to the system impedance. Low S21

level indicates power leakage as a wave transverses the CRLHTL LWA.

140

C

λ/4 λ/4 λ/4

Ground

Ground

Input Output

Fig. 4. Coplanar waveguide loaded-line phase shifter, with prototype shownin inset.

2 2.5 3 3.5 4 4.5 5−45

−40

−35

−30

−25

−20

−15

−10

−5

0

frequency (GHz)

mag

nitu

de (d

B)

S21

S11

unit cell p

Fig. 5. Measured S11 and S21 of a CRLH TL LWA prototype, shown ininset, consisting of 6 unit cells with p = 8.34 mm.

IV. COMPLETE SYSTEM RESULTS

In this section, the frequency- and phase-scanning capabil-ities of the array will be demonstrated. The far-field radiationpattern of the LWA can be easily and efficiently computedbased on the array factor (AF) approach in [11]. The math-ematical expression to compute the radiation pattern of thearray factor approach of a 2D LWA array system is given by:

AF (θ, φ) =M∑

m=1

Im1ej(m−1)(k0px sin θ cos φ+ϕxm)·

N∑

n=1

ej(n−1)(k0py sin θ sin φ+ϕyn)

(1)

where px and py represent the distances between M and Nelements along the x and y directions, respectively; ϕxm =(n − 1)k0p sin θMB and Im1 = I0e

−α(n−1)p represent thephases and magnitude,respectively, acquired by each unit cellalong a single LWA in the x-direction for frequency-scanning,while ϕyn represents the phases acquired by each LWA alongthe y-direction for phase-scanning. Fig. 6 shows the frequencyand phase scanning of the radiation pattern for an array of 10

(N = 10) LWAs, each consists of 22 unit cells (M = 22).The graphs in Fig. 6 show pencil beam, in both the φ = 0o

= 3.5 GHz 4.3 GHz

0o

30o

-30o

3.9 GHz

ϕ=

Phase shift

θ-90o 90o

φ

#1 #2 #3

#4 #5 #6

#7 #8 #9

fRF

Fig. 6. Theoretical (array factor approach, Eq. [1]) radiation pattern of the2D LWA array shown in Fig. 1, showing frequency-scanning in Θ and phase-scanning in φ for various frequencies and phase shifts. The white dashed linesindicate the φ-plane cut of the beam peak.

and φ = 90o planes. The nine radiation plots in Fig. 6 illustratefrequency- and phase-scanning for three different frequenciesfRF of 3.5 GHz (left-hand/backward), 3.9 GHz (broadside),and 4.3 GHz (right-hand/forward), and three different phaseshifts of −300, 00 and +300. Plots 1, 5, and 9 in Fig. 6 indicate(θ,φ)=(−300,2250), (00,00), and (+300,450). As can be seen,the pencil-beam peak is scanned in full space with respect tofrequency and excited phase shift.

For the proof-of-concept, the prototype uses an array ofonly 4 LWA elements. The radiation patterns of a 4-elementCRLH TL LWA array obtained with Eq. 1 and with full-wavesimulations are shown in Fig. 7 for plots 1, 5, and 9 of Fig. 6.Frequency-scanning and phase scanning are clearly apparentin Fig. 7 as the beam peak scans from backward to forwarddirection for three frequencies of 3.5, 3.9 and 4.3 GHz andphase of −300, 00, and 300. The full-wave simulated beampeaks shown in Fig. 7 are slightly off from those computedby AF approach due potentially to the coupling between LWAelements which is not accounted for in the AF approach.

V. CONCLUSION

A novel pencil-beam, full-space scanning 2D CRLH TLLWA array has been presented. Each component of the ar-ray has been verified experimentally and the 2D frequency-scanning and phase-scanning capabilities of the completesystem has been confirmed theoretically. In comparison tothe conventional phase array antenna, the proposed array hasseveral advantages such as (1) full-space 2D scanning withfrequency and phase, (2) arbitrary number of and distance be-tween array elements, and (3) simple, low cost and miniaturizefeeding networks. The proposed 2D scanning array find many

141

Array factorFull-wave sim. 00

–300 300

0dB

−8dB

f0=4.3GHz

f0=3.9GHz

θ

ϕ = +30#1

f0=3.5GHzϕ = −30#9

ϕ = 0#5

–600 600

Fig. 7. Theoretical (array factor approach, Eq.[ 1]) and full-wave simulatedradiation patterns for various radiation patterns of Fig. 6. Radiation patterns1, 5, and 9 of Fig. 6 for a transverse plane cut of φ = 45o.

applications in wireless communications, smart antennas andRFID systems.

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