7/29/2019 06348512

1/2

Long Slot Array (LSA) Antenna Integrated with

Compact Broadband Coupled Microstrip Impedance

Transformer

Loon Y. Lee , Hyoung-sun Youn and Magdy F. Iskander

Hawaii Center for Advanced Communication (HCAC)

College of Engineering, University of Hawaii

Honolulu, HI, 96822

hsyoun@hawaii.edu

AbstractIn this paper, a miniaturized coupled microstrip

impedance transformer has been developed and integrated with

the long slot array (LSA) antenna as a feeding structure. The

proposed impedance transformer offers low cost capability by

employing printed circuit technology and compact size circuitries

can be accomplished by a miniaturization technique, i.e.

meandering. Conventional microstrip impedance transformerswhich can be designed to cover the frequency band of interest

(225 MHz 450 MHz) are too large to be directly implemented

onto the LSA due to their limited space on the metallic strip

section between feeds. To overcome this issue, miniaturization

particularly the meandered transmission line is applied and the

compact size (2.1 cm X 3.1 cm) impedance transformer has been

developed. The developed impedance transformer provides less

than 1 dB insertion loss over the operation frequency range and it

is small enough to be directly implemented on the LSA. The

developed impedance transformers were implemented to each

port of the planar LSA as a feeding structure and their feasibility

was tested by using HFSS. Simulation result shows that the LSA

integrated with the developed impedance transformer provided

very similar radiation pattern and gain as the LSA excited by

ideal lumped ports.

I. INTRODUCTIONRecently, researches on developing wideband antennas

have been conducted to replace the conventional phased arrayemploying discrete elements that are costly to fabricate. Forthis reason, a continuous long slot aperture array antenna hasbeen introduced and showed impressive performanceespecially at low band applications [1,2]. The long slot array(LSA) antennas without the ground plane provided extremelybroad bandwidth. However, the bandwidth of the LSA isgreatly reduced by placing a ground plane to obtainunidirectional radiation pattern [2]. To overcome this issue, in

our previous research, a hybrid Ferrite/EBG ground planewhich provides low profile with ultrawide bandwidth (40:1)has also been developed and tested in [3]. A cylindrical versionof the LSA integrated with cylindrical hybrid EBG/Ferriteground plane was evolved from the planar LSA antenna toobtain omni-directional radiation pattern while maintaining theultra-wide bandwidth characteristics [4]. One major advantageof the LSA does not require a balun, since it supportsunbalanced feeding. However, impedance of the radiatingapertures usually range from 200 up to 377 depending on

the models physical structure and the resistive loading on theground plane. Therefore, for the LSA antenna to be connectedto the typical 50 ohm RF cable, the LSA is necessary to beintegrated with wideband impedance transformers. For easierfabrication and lower cost, an impedance transformer whichcan be directly fabricated on the LSA structure is preferable.For this reason, a miniaturized coupled microstrip impedancetransformer has been developed and integrated with the planerLSA as a feeding structure is described in this paper. The LSAintegrated with the transformer was designed to operate in 225 450 MHz band. The developed microstrip impedancetransformer can be directly implemented on metallic strips ofthe LSA by using the printed circuit technology and it has 4:1ratio that matches 200 of radiation aperture impedance to the50 of typical RF source impedance, i.e. coaxial cable.Performances of the LSA integrated with the impedancetransformer were analyzed by HFSS simulation. Simulationresults were compared to the LSA with the exact physicalparameters excited by ideal lumped ports. Both cases providedvery similar radiation pattern and gain over the bandwidth,which indicates that the proposed feeding structure using theimpedance transformer effectively excites EM wave throughslots of the LSA. In following section, detail dimensions of thedeveloped impedance transformer and its simulation results arepresented.

II. BROADBAND COUPLED MICROSTRIP TRANSMISSIONLINE IMPEDANCE TRANSFORMER

In most microwave applications, the conventional quarter-wave impedance transformers are still widely used due to theirease of implementation, but they typically have bandwidthlimitation. For this reason, an impedance transformer using themulti-section or tapered line has been developed for wideband

applications. However, these impedance matching networksusually require larger space especially for operations at lowfrequencies. On the other hand, an impedance transformerusing the coupled transmission line [5] provides broadbandwidth characteristics and a compact size can be achievedby the meandering miniaturization [6]. Therefore, this paperadopted the design of the coupled microstrip transmission lineimpedance transformer from [6] to develop a broadband,compact size impedance transformer. The impedancetransformer in [6] was designed to cover the frequencies from

978-1-4673-0462-7/12/$31.00 2012 IEEE

7/29/2019 06348512

2/2

0.52 GHz to 1.72 GHz with the center frequency at 1.1 GHz.By increasing the length of the middle section of the impedancetransformer, its operation frequencies can be shifted toward tolower frequency while maintaining the bandwidth.Subsequently, the straight transmission line in the impedancetransformer is miniaturized by using meandering shape asshown in Figure 1 (a). Physical dimensions of the developedminiaturized impedance transformer are described in Figure 1(a). The circuitry was modeled on a substrate (thickness = 0.8,r = 2.55) and its length between 50 and 200 ports is 2.1cm while the width of the circuit including the meanderedtransmission line is around 3.1 cm. Figure 1(b) plots simulatedinsertion loss and return loss of the impedance transformer.This provides insertion loss less than 1 dB over the bandwidth.Figure 2 illustrates the implementation of the transformer onthe LSA. Backing PEC plate of the substrate is used as a PECstrip for the LSA. Developed impedance transformers areimplemented on opposite side of the substrate. The 200- portin Fig. 1(a)) of the impedance transformer is placed on intoeach feeding position of the LSA and these ports are connectedto the PEC backing side of next substrate (PEC strip) using thinwire as shown. A 50- coaxial cable is directly connected tothe 50- port of each impedance transformer. Each coaxialcable was excited by the wave port provided in the HFSSsimulation. Since spacing between feeders in the LSA is 6 cm,the developed impedance transformer can be implemented oneach port without coupling effect or interference. As it can beseen multiple impedance transformers are implemented in onePEC strip. Characteristics of the LSA integrated with theimpedance transformers were simulated by HFSS. To evaluateits performance, simulation results were compared with thoseof the LSA excited by ideal lumped ports which were used in[4].

III. RESULTS AND CONCLUSIONSFigure 3 plots the simulated radiation pattern and gain of

the LSA with ideal 200- lumped ports and the LSA integratedwith the micro strip impedance transformers at 450 MHz. Asone can see, both cases produce omni-directional radiationpattern with about 1 dBi gain. Note that similar results wereobserved over the operational bandwidth. These results indicatethat the proposed feeding structure consisting of impedancetransformer connected to a 50- coaxial cable behave properlyas the ideal lumped port. In current model, each impedancetransformer is connected with independent coaxial cable fed bythe wave port in order to test feasibility of directimplementation of the impedance transformer. However, byintegrating microstrip power splitters next to the impedancetransformer, the number of coaxial cable connected the LSAcan be reduced and this task is in progress.

REFERENCES

[1] A. Neto and J.j. Lee, Infinite bandwidth long slot array antennas, IEEEAntennas Wireless Prog. Lett., vol. 4, pp. 75-78, 2005

[2] A. Neto, J.J. Lee, Ultrawide-band Properties of Long-Slot Arrays,IEEE Trans. On Ant. And Prop. Vol. 54, No. 2, pp. 534-543, Feb. 2006

[3] J.M. Bell, M.F. Iskander, Experimental Analysis of an UltrawidebandHybrid EBG/Ferrite Ground Plane, IEEE Trans. On Inst. And Meas.,Vol. 58, No. 8, pp. 2899-2905, Aug. 2009

[4] H.S. Youn, L. Lee, N. Celik, M.F. Iskander, Design of a CylindricalLong-Slot Array Antenna Integrated with Hybrid EBG/Ferrite GroundPlane" has been accepeted by the IEEE Antennas and WirelessPropagation Letters, 2010

[5] T. Jensen., V. Zhurbenko, V. Krozer, and P. Meincke, Coupledtransmission line as impedance transformer, [j]. IEEE Trans.Microwave Theory & Tech., 2007 vol. 55, no.12: 2957-2965.

[6] X. Zhou, X.G. Liu, H.P. Guo, L.X. Shao, Design of broadbandimpedance transformer using coupled microstrip transmission line,Microwave, Ant. Prop. And EMC Tech. for Wireless Comm. Internation

Symp., 2009, pp. 994 997

Figure 1. Miniaturized coupled Microstrip impedancetransformer (Top), and simulation results of the developedimpedance transformer (bottom).

Figure 2. The long slot array antenna integrated withimpedance transformers. 50 sides of the impedancetransformers are directly connected with coax cable.

Figure 3. Radiation pattern of a PLSA excited by ideal 200lumped port (left), and the proposed feeding structure withimpedance transformer connected with 50 coax cable (right).