CPW balun for printed balanced antennas

Fig. 2 CPW balun for printed balanced antennas

a Layoutb Multim

To feeand quCPW-to-These banicantinput impto design

Balun theory: The multimodal behaviour of CPWs can be used to build

0design procedure for most of these baluns is not available [48], andtherefore the designers have to rely on time-consuming parametricelectromagnetic simulations to tune them.

ELECTRONICS LETTERS 22nd May 2014 Vol. 5odal circuit model

d printed balanced antennas (such as dipoles, folded dipolesasi-Yagi and bow-tie antennas) in CPW circuits,slotline (or to-coplanar-strips (CPS)) baluns are used [38].luns may electromagnetically interact with antennas with sig-backward radiation, modifying their radiation pattern andedance, and thus the matching of the circuit, so it is difcultthe balun independently of the antenna. Besides, an analyticalA.M. Snchez, M. Rib, L. Pradell, J. Anguera andA. Andjar

In coplanar waveguide (CPW) circuits, printed balanced antennas mustbe excited through baluns. These baluns often feature coplanar-to-slot-line transitions that must be tuned and matched by electromagneticoptimisation, and may interfere with the antenna backward radiation.A new CPW balun for printed balanced antennas is presented and mod-elled. As its multimodal circuit model makes clear, it integrates animpedance-matching network within its structure that can be analyt-ically designed. The balun has been experimentally tested, exhibitinggood agreement with its circuit model and a weak electromagneticinteraction with the antenna radiation.

Introduction: The coplanar waveguide (CPW) is a uniplanar trans-mission line suitable for high-frequency designs since it allows asimple connection of series and shunt elements. It exhibits a multimodalbehaviour since it can propagate simultaneously the CPW even and oddmodes (see their denitions in Fig. 1a). Both modes interact in mostCPW transitions and asymmetries [1, 2]. The even mode is the modecommonly used in CPW designs, whereas the odd mode is oftenregarded as spurious and suppressed by means of air bridges.However, the odd mode provides a further degree of freedom todesign new CPW circuits. In particular, it can excite a printed balancedantenna connected to the lateral CPW ground planes.

Y0e = 4 Re [YA] Im [YA] + B1 = 0

a

1:2

b

Io1

Ie1Ie2 Ie2

Io2Ie1/2

Ie1/2

Vo1/2Vo1 Vo2

Ve2Ve1

Io1 Io2

Vo1/2 Vo2/2

Vo2/2

Ie2/2

Ie2/2

Ve2Ve1

Ve1

Io1 Io2

Ve2

Fig. 1 CPW asymmetric shunt-short-circuit transition

a Layout with voltage and current denitions for even (subscripts e) and odd(subscripts o) modesb Multimodal circuit model

l1

l1

l1

l2

l2 l3

l3

YA

Z 0o,b o Z 0o

,b ojB1

jB3VoA

1:2

a

b

x

y

z

balanced antenna

VoA/2

VoA/2

V+elN

V+elN

V+elN Z0e,be Z0o,boCPW baluns for printed balanced antennas based on the asymmetricshunt-short-circuit transition shown in Fig. 1a. This transition is mod-elled by the multimodal model shown in Fig. 1b [1, 2], which connesthe contribution of each mode in a different port. Using this transition,the balun shown in Fig. 2a can be proposed and modelled as shown inFig. 2b, where YA is the antenna admittance and Z0e, e, Z0o and o arethe characteristic impedance and propagation constant of the even andodd modes, respectively.The circuit model shown in Fig. 2b allows, in addition to a rigorous

computation of the system input reection coefcient, a simple ex-planation of the balun operating principle. The exciting even-modewave (VeIN

+ ) propagates until the asymmetric shunt-short-circuit tran-sition, where it generates transmitted and reected even- andodd-mode waves, according to the circuit model in Fig. 1b. The reectedeven-mode wave propagates backwards to the input port. The reectedodd-mode wave propagates backwards to a distance l1 until it is short-circuited by the air bridge, thus generating a stub with the input admit-tance jB1. The transmitted even-mode wave is open-circuited by the gapat the CPW centre strip. Finally, the transmitted odd-mode wave reachesthe antenna terminals and, along with its reection at the open-circuitedstub of length l3 (with the input admittance jB3 for the odd mode),generates the antenna odd-mode excitation voltage VoA.This balun, besides performing the required unbalancedbalanced

transformation, offers multiple possibilities of impedance matchingsince it behaves as the versatile and easy-to-design double-stubimpedance-matching network. This matching network is containedwithin the balun structure, which in turn is embedded in the feedingline of the antenna. The balun almost does not have conductors parallelto the antenna axis, and thus it weakly interferes with the antenna back-ward radiation. This fact allows an independent design of both theantenna (electromagnetic) and balun (analytic), since the antennaradiation diagram and input impedance are not affected by the balun.The balun in Fig. 2 can be modied in several ways to satisfy particu-

lar design needs. Since the even mode, due to its voltage conguration,cannot excite the antenna, it may be allowed to propagate beyond theasymmetric shunt-short-circuit transition by placing the CPW centre-strip gap away from the transition; it will generate a further open-circuited stub. Beyond this gap, the odd mode can be transformed intoa modally compatible slotline (or CPS) mode by removing the centrestrip of the CPW. Finally, the stub of length l3 may be loaded with ashort circuit. A compact particular case, used below, occurs when l2 =l3 = 0, since then a perfect match is achieved if

Y0e = 4Re YA[ ] Im YA[ ] + B1 = 0

Experimental validation: The balun in Fig. 3 for a printed-dipoleantenna was designed (with l2 = l3 = 0) using the circuit model inFig. 2b, fabricated on a RogersTM 3003 substrate (with R = 3.02 andthickness 0.8 mm) and measured. The antenna was designed to beslightly sub-resonant at the design frequency f0 = 1.9 GHz so as toexhibit a capacitive behaviour easily compensable with the (inductive)odd-mode stub of length l1.

22.5 mml1=11.1 mm

2.4 mm

1 mm1.1 mm

xy

z

Fig. 3 Fabricated printed-dipole antenna with CPW balun

Fig. 4 compares the measured reection coefcient of the fabricatedantenna with that computed with the circuit model in Fig. 2b. Thegood agreement between them validates both the proposed balun struc-ture and its multimodal circuit model. The antenna with the balun fea-tures an antenna efciency of 72% at f0, and a gain of 2.5 dB (thesimulated gain of the dipole alone is 2.2 dB). Fig. 5 shows its measuredradiation diagrams. As can be seen, they are dipole-like with lowcross-polar-eld levels. Therefore the balun exhibits very weak

No. 11 pp. 785786

electromagnetic interaction with the antenna, even in the presence ofstrong backward radiation as in this case.

frequency, GHz

|S 11|,

dB

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.020

16

12

8

4

0

circuit modelmeasurement

Fig. 4 Comparison between measured and simulated (using multimodalcircuit model of Fig. 2b) reection coefcient of antenna with CPW balunof Fig. 3

90

90

0180

5101520

90

90

0180

51015

ba

Eq/EMAX, dBEf/EMAX, dB

qq

Fig. 5 Measured radiation diagrams of antenna of Fig. 3

a = 0b = 90

Conclusion: In this Letter, a new CPW balun for balanced printedantennas such as dipoles, folded dipoles or quasi-Yagi antennas hasbeen presented, modelled and tested. It is embedded in the antenna

antenna, even in the presence of strong backward radiation, andallows good antenna-efciency levels.

Acknowledgment: This work was funded by the research projectTEC2010-20318-C02/TCM from the Spanish Ministerio de Ciencia eInnovacin.

The Institution of Engineering and Technology 201418 March 2014doi: 10.1049/el.2014.0907

A.M. Snchez and M. Rib (La Salle Universitat Ramon Llull,Barcelona 08022, Spain)

L. Pradell (Universitat Politcnica de Catalunya (UPC), Barcelona08034, Spain)

E-mail: pradell@tsc.upc.edu

J. Anguera and A. Andjar (Fractus, Sant Cugat del Valls 08190,Spain)

References

1 Rib, M., and Pradell, L.: Circuit model for mode conversion in co-planar waveguide asymmetric shunt impedances, Electron. Lett., 1999,35, (9), pp. 713715

2 Contreras, A., Rib, M., Pradell, L., and Blondy, P.: Uniplanar bandpasslters based on multimodal immitance inverters and end-coupled slotlineresonators, IEEE Trans. Microw. Theory Tech., 2013, 61, (1), pp. 7788

3 Lin, Y.-S., and Chen, C.H.: Lumped-element impedance-transforminguniplanar transitions and their antenna applications, IEEE Trans.Microw. Theory Tech., 2004, 52, (4), pp. 11571165

4 Tilley, K., Wu, X.-D., and Chang, K.: Coplanar waveguide fed coplanarstrip dipole antenna, Electron. Lett., 1994, 30, (3), pp. 176177

5 Sor, J., Qian, Y., and Itoh, T.: Coplanar waveguide fed quasi-Yagiantenna, Electron. Lett., 1999, 36, (1), pp. 12

6 Ding, Y., Jiao, Y.C., Fei, P., Li, B., and Zhang, Q.T.: Design of a multi-band quasi-Yagi-type antenna with CPW-to-CPS transition, IEEEAntennas Wirel. Propag. Lett., 2011, 10, (1), pp. 11201123

RSfeeding line, and integrates also an easy-to-design double-stub matchingnetwork within its structure. The balun is based on the interactionbetween the exciting CPW even (unbalanced) mode and the CPW odd(balanced) mode, and can be accurately modelled by a multimodalcircuit which explains its principle of operation and allows its analyticdesign. The good agreement between the measurement and the circuitsimulation validates the balun topology and its multimodal circuitmodel. The balun features a weak electromagnetic interaction with the

ELECTRONICS LETTE7 Kan, H.K., Waterhouse, R.B., Abbosh, A.M., and Bialkowski, M.E.:Simple broadband planar CPW-fed quasi-Yagi antenna, IEEEAntennas Wirel. Propag. Lett., 2007, 6, (1), pp. 1820

8 Hong, Y.-P.: Fat dipole antenna with broadband balun usingCPW-to-slotline eld transformation, Electron. Lett., 2013, 49, (10),pp. 635636

22nd May 2014 Vol. 50 No. 11 pp. 785786