Pencil-Beam Full-Space Scanning 2D CRLH Leaky-Wave Antenna ...

Pencil-Beam Full-Space Scanning 2D CRLH Leaky-Wave Antenna Array Hoang V. Nguyen∗† , S. Abielmona† , A. Rennings‡ and C. Caloz† † Poly-Grames

´ Research Center, Ecole Polytechnique de Montr´eal 2500 Chemin Polytechnique, Montr´eal, 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 composite right/left-handed (CRLH) transmission line (TL) leakywave antenna (LWA) array is proposed. The proposed array achieves the same performance of conventional phase array antennas in a simpler manner. Moreover, array elements are fed by a CRLH infinite-wavelength series power divider and therefore the separation between array elements and the number of array element can be extended arbitrary. The CRLH series power divider eliminates the requirement of complex and high loss feeding networks commonly found in conventional arrays. Experimental results of each system components are presented along with analytical and simulation results of the complete array system. Index Terms— Antenna array, composite right/left-handed, leaky-wave, pencil-beam, frequency-scanning, phase-scanning.

I. I NTRODUCTION The steady and constant rise of wireless users has fuelled an increase in wireless applications. This increase has created a complex and crowded frequency spectrum in a constantly evolving wireless landscape. Omni-directional antennas used in many of today’s wireless devices cause interference, lowering signal quality, reducing data rates, and limiting bandwidth. Thus, a need exists for an antenna system having a narrow beamwidth to reduce interference, while also offering the flexibility of full-space scanning. The conventional approach to realize two-dimensional (2D) beam scanning array is to form a 2D array of radiating elements [1]. This 2D array requires complex and lossy feeding networks which in many cases are the bottle neck of 2D scanning array. Leaky-wave antennas (LWA) have a distinct advantage of a much simpler feeding network than conventional arrays [2]. Several works on LWAs have been reported for 2D beam scanning. Allen et al. [3] used the heterodyne mixing technique along with a delay line network to frequency- and phase-scan, respectively. This approach restricts the array’s phase-scanning capability to a limited scan range, where the LO frequency is required to be in the guidedwave region of the CRLH TL so as not to interfere with RF radiation. In another approach, Lai et al. [4] used 2D CRLH TL array with orthogonal power feeding to control the main beam azimuthal radiation direction. This approach requires variable input powers to the antenna, which is inconvenient or not available in some applications. In addition, the main 1-4244-1449-0/07/$25.00 © 2007 IEEE

beam can only be scanned within a limited azimuthal direction between 00 and 900 . Beam steering in a CRLH TL array has also been recently reported in a Butler matrix configuration [5], where eight quadrature branch-line couplers were employed. The use of couplers restricts the array’s operational bandwidth, while also providing only discrete phase shifts for discrete scan angles. In this work, we propose a novel 2D CRLH TL LWA array capable of phase and frequency scanning a pencilbeam 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 network with distinct advantages over traditional series or corporate feeding networks. The applications of such an array include RFID systems, smart antennas, and DOA estimation. II. P ROPOSED S TRUCTURE AND P RINCIPLE OF O PERATION The novel, efficient pencil-beam 2D scanning array is shown in Fig. 1. It consists of a series power divider feeding an entire system of phase shifters connected to mixers, which subsequently feed into an array of LWAs.

Uniform (closed) CRLH Series Power Divider

Frequency-Scanned Leaky-Wave Anntenas RF

ϕ

50Ohm

d RF

2ϕ

50Ohm

RF (3.5-4.3GHz) RF

50Ohm

3ϕ RF

50Ohm

4ϕ

IF (0.5-1.3GHz)

z

θ x φ

LO (3GHz)

y

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

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

139

5

4

3

1

2

MIM Capacitor Shorted stub Fig. 2. Prototype of the closed CRLH series power divider, along with a unit cell perspective view. −2

S11

−4 −6

S21

−8 −10 −12

S51

−14 2.85

III. C OMPONENT P ROTOTYPES

2.9 2.95 3 3.05 frequency (GHz)

(a) Magnitude

phase (degree)

magnitude (dB)

from backfire to endfire without phase shifters, (3) inexpensive electronic scanning, and (4) highly directive in φ=00 plane due to its large aperture [2]. However, the 1D CRLH LWA has a fan beam in the transverse plane. In order to obtain a pencilbeam, the aperture size in the transverse plane needs to be increased, and that is done by arraying several LWAs in the yz-plane, to compose a 2D LWA array shown in Fig.1. The CRLH LWA must operate in the fast-wave region in order to radiate efficiently. Thus at the mixer, an injected LO signal with fLO =f0 must be converted to the RF frequency (fRF = fIF + fLO ), which is within the leakage range of the LWAs. 2D scanning is accomplished by employing frequencyand phase-scanning in the xz- and yz-plane, respectively. The frequency-scanning in the xz-plane is achieved by tuning the fIF , while maintaining fLO = f0 at all times, hence varying fRF . On the other hand, phase-scanning in the yz-plane is achieved by adjusting the phase shifters so that the phase difference between each LWA is ϕ. For diagonal scanning, both fIF and the phase shifters must be tuned simultaneously.

160 140 S51 120 100 80 60 40 20 S21 0 −20 −40 2.85 2.9 2.95 3 frequency (GHz)

3.05

(b) Phase

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

A. Series Power Divider The series power divider/combiner is shown in Fig.2. It consists of an open-ended CRLH resonator operating at the infinite-wavelength frequency (f0 = 2.955 GHz), where β(f0 ) = 0. Since the electrical length of the resonator is exactly zero at f0 , an equal phase/magnitude state exists all along the CRLH resonator structure. The infinite-wavelength series power divider presents several advantages over conventional planar feeding networks. It is compact and low-loss compared to traditional lossy and bulky feeding networks, provides equal and in-phase power to an arbitrary number of elements compared to the corporate network which is limited to 2N elements, allows arbitrary spacing between element feeds to avoid grating lobes while optimizing gain, and does not suffer from magnitude imbalance (reducing directivity) of conventional series feeding networks [6], [7], [8]. The CRLH series power divider is implemented in metalinsulator-metal technology for capacitors and shorted stubs for inductors on Rogers Duroid 3003 (r = 3). Due to the inherent leakage of a CRLH TL, the series power divider is enclosed in a metallic box, as shown in Fig.1, to eliminate potentially interfering radiation. Furthermore, a quarter-wave transformer at f0 is used to match the series power divider to 50Ω. The measured scattering parameters for the closed CRLH series 1-to-4 power divider are shown in Fig. 3. The magnitude response in Fig. 3a shows the intersection point of S21 and S51 , indicating an equal magnitude of −10 dB at the transition frequency f0 = 2.955 GHz. Similarly, the phase response in Fig. 3b shows the intersection point of S21 and S51 , indicating an equal phase of 200 also at f0 = 2.955 GHz. S11 in Fig. 3a also indicates that the series power divider is well matched at f0 = 2.955 GHz.

B. Phase-shifter and Mixer The variable phase shifter is implemented by loading a TL with a variable capacitor in shunt [9]. The phase√shifter is shown in Fig. 4. The phase of a TL, given by β = ω LC, will be varied accordingly by choosing the appropriate capacitor value to obtain the required phase shift. Placing two capacitors separated by λg /4, reduces the reflections from the shunt capacitors, thus minimizing insertion loss. Finally, coplanar waveguide is used here to eliminate the grounding via’s inductance. Triple-balanced commercial diode mixers (Mini-circuits MCA-50MH) were chosen with an IF range of 0.1 − 1.5 GHz, with good isolation between all ports, and a conversion loss between 6 − 8 dB across the frequency range of interest. In order to reduce the external coupling between the IF and RF ports, the IF input feed was placed at the substrate’s bottom layer, while the mixer’s RF output is directly connected to the LWA at the substrate’s top layer [10] C. Leaky-wave Antenna The CRLH TL LWA element is implemented in planar microstrip technology. The inset of Fig. 5 shows a small scale prototype consisting of 6 unit cells, each having a series interdigital capacitor and a shunt stub inductor. The measured scattering parameters of the prototype are shown in Fig. 5. Within the leaky-wave passband of 3 − 4.5 GHz, the CRLH TL LWA is well matched to the system impedance. Low S21 level indicates power leakage as a wave transverses the CRLH TL LWA.

140

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

Ground Input

λ/4

λ/4

fRF = 3.5 GHz

Output

λ/4

Phase shift

C

3.9 GHz

4.3 GHz

#1

#2

#3

#4

#5

#6

ϕ= 30o

Ground

φ

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

o

0

o

o

90

-90

#7

0

#8

θ

#9

S21

−5

o

-30

magnitude (dB)

−10 −15 −20 −25

Fig. 6. Theoretical (array factor approach, Eq. [1]) radiation pattern of the 2D LWA array shown in Fig. 1, showing frequency-scanning in Θ and phasescanning in φ for various frequencies and phase shifts. The white dashed lines indicate the φ-plane cut of the beam peak.

S11

−30 −35

unit cell

p

−40 −45

2

2.5

3

3.5 4 frequency (GHz)

4.5

5

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

IV. C OMPLETE S YSTEM R ESULTS In this section, the frequency- and phase-scanning capabilities of the array will be demonstrated. The far-field radiation pattern of the LWA can be easily and efficiently computed based on the array factor (AF) approach in [11]. The mathematical expression to compute the radiation pattern of the array factor approach of a 2D LWA array system is given by:

AF (θ, φ) =

M 

Im1 ej(m−1)(k0 px sin θ cos φ+ϕxm ) ·

m=1 N 

and φ = 90o planes. The nine radiation plots in Fig. 6 illustrate frequency- and phase-scanning for three different frequencies fRF of 3.5 GHz (left-hand/backward), 3.9 GHz (broadside), and 4.3 GHz (right-hand/forward), and three different phase shifts 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 to frequency and excited phase shift. For the proof-of-concept, the prototype uses an array of only 4 LWA elements. The radiation patterns of a 4-element CRLH TL LWA array obtained with Eq. 1 and with full-wave simulations are shown in Fig. 7 for plots 1, 5, and 9 of Fig. 6. Frequency-scanning and phase scanning are clearly apparent in Fig. 7 as the beam peak scans from backward to forward direction for three frequencies of 3.5, 3.9 and 4.3 GHz and phase of −300 , 00 , and 300 . The full-wave simulated beam peaks shown in Fig. 7 are slightly off from those computed by AF approach due potentially to the coupling between LWA elements which is not accounted for in the AF approach.

(1) j(n−1)(k0 py sin θ sin φ+ϕyn )

e

n=1

where px and py represent the distances between M and N elements along the x and y directions, respectively; ϕxm = (n − 1)k0 p sin θM B and Im1 = I0 e−α(n−1)p represent the phases and magnitude,respectively, acquired by each unit cell along a single LWA in the x-direction for frequency-scanning, while ϕyn represents the phases acquired by each LWA along the y-direction for phase-scanning. Fig. 6 shows the frequency and phase scanning of the radiation pattern for an array of 10

V. C ONCLUSION A novel pencil-beam, full-space scanning 2D CRLH TL LWA array has been presented. Each component of the array has been verified experimentally and the 2D frequencyscanning and phase-scanning capabilities of the complete system has been confirmed theoretically. In comparison to the conventional phase array antenna, the proposed array has several advantages such as (1) full-space 2D scanning with frequency and phase, (2) arbitrary number of and distance between array elements, and (3) simple, low cost and miniaturize feeding networks. The proposed 2D scanning array find many

141

Array factor Full-wave sim. –300

–600

00 θ

300

f0=3.9GHz ϕ=0 #5

f0=3.5GHz ϕ = +30 #1

0dB −8dB

600

f0=4.3GHz ϕ = −30 #9

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

applications in wireless communications, smart antennas and RFID systems.

[4] A. Lai, K. M. K. H. Leong, and T. Itoh, “Leaky-wave steering in a two-dimensional metamaterial structure using wave interaction excitation,” in Proc. Int. Microwave Symposium (IMS), San Francisco, USA, June 2006, pp. 1643-1646. [5] T. Kaneda, A. Sanada, and H. Kubo, “2D Beam scanning planar antenna array using composite right/left-handed leaky wave antennas,” IEICE Trans.Electron., vol. E89-C, no. 12, December 2006, pp. 1904-1911. [6] A. Lai, K. M. K. H. Leong, and T. Itoh , “A Novel N-port series divider using infinite wavelength phenomena,” Proc. of Int. Microwave Symposium, Long Beach, California, U.S.A, June, 2005. [7] M. A. Antoniades and G. V. Eleftheriades , “A broadband series power divider using zero-degree metamaterial phase-shifting lines,” IEEE Microw. and Wireless Components Lett., vol. 15, no. 11, November 2005, pp. 808810. [8] H. V. Nguyen and C. Caloz , “An arbitrary N-port series power divider,”IEEE Electronic Lett. (submitted). [9] D. M. Pozar, Microwave Engineering, 2nd Ed., Wiley, 1998 [10] F. P. Casares-Miranda, C. Viereck, C. Camacho-Penalosa, and C. Caloz, “Vertical microstrip transition for multilayer microwave circuits with decoupled passive and active layers,” IEEE Microwave and Wireless Comp. Lett., vol.16, no.7, July 2006, pp 401-403. [11] C. Caloz and T. Itoh, “Array factor approach of leaky-wave antennas and application to 1-D/2-D composite right/left-handed (CRLH) structures ,” IEEE Microwave and Wireless Comp. Lett., vol.14, no.6, July 2006, pp 274-276.

R EFERENCES [1] C. A. Balanis, Antenna theory: analysis and design, 2nd ed., John Wiley & Sons Inc., 1997. [2] C. Caloz, and T. Itoh, Electromagnetic metamaterials transmission line theory and microwave applications, Wiley and IEEE-Press, 2005. [3] C. A. Allen, K. M. K. H. Leong, and T. Itoh, “2-D Frequencycontrolled beam-steering by a leaky/guided wave transmission line array,”in Proc. Int. Microwave Symposium (IMS), San Francisco, USA, June 2006, pp. 457-460.

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