IJMOT-2006-8-207 © 2007 ISRAMT

INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY,

VOL. 2 , NO. 1 , JANUARY 2007

A Stacked Structure Broadband Circular Polarization Microstrip S-Ring Antenna

Huan-Cheng Lien* and Huei-Chiou Tsai

Wu Feng Institute of Technology, 117 Chian-Ku Rd., Sec.2, Ming-Hsiung (621), Chiayi,Taiwan,R.O.C

E-mail: lhcaa@mail.wfc.edu.tw

Abstract-A single-feed S-Ring Antenna (SRA) by stacked substrate structure for circularly polarized application was studied in this paper. The antenna is designed for Left-Hand Circular Polarization (LHCP) at a center frequency of 2.1GHz. The design of the proposed antenna is aimed at obtaining wider bandwidth and better circular polarization (CP) Axial-Ratio (AR) for Global Positioning System (GPS) applications. By a mixed stacked different dielectric substrate structure to enhance the bandwidths of the impedances and other characteristics such as including AR bandwidth, CP gains and antenna efficiency of the proposed antenna has been compared and analyzed. The experimental results verify that the antenna has an impedance bandwidth (VSWR.≦ 2) of 23.5% and a 3-dB AR bandwidth more than 23%.

Key Words: S-Ring Antenna; stacked substrate structure

I. INTRODUCTION This paper describes a method for designing a circularly polarized SRA with stacked structure patch in which the antenna is fed at one point from the back of the substrate. In communication systems, to develop small size, lightweight, low profile, broad bandwidth, and proper polarization are fundamental demands in antenna design for the miniaturization of communication equipment. To achieve the miniaturization of antenna have been proposed and investigated. Wireless communications have progressed very rapidly in recent years. To meet the

miniaturization requirement, the antennas employed in wireless communications must have their dimensions reduced accordingly. Planar antennas, such as microstrip antenna have the attractive features of low profile, small size, are very promising for satisfying this design consideration. Planar antennas are also very attractive for applications in GPS. Therefore, planar antenna designs techniques for achieving compact and broadband CP have attracted much attention from antenna researchers. For overcoming multipath fading problem the majority of current Global Positioning Satellite (GPS) receivers operating typically use a CP antenna. Therefore, develop broadband CP techniques to enhance the bandwidths of the microstrip antennas is very important. If a microstrip patch such as using two equal amplitude mutually vertical linearly polarized waves with the phase difference of 90 degrees relative to each other, a circularly polarized electromagnetic wave can be obtained. Single-feed circularly polarized antennas are currently receiving much attention. Single-feed circularly polarized microstrip antennas allow a reduction in the complexity, weight, and have the additional advantage of small size and produce a completely planar antenna. Microstrip patch antennas designed to radiate CP with a single-feed are described in [1-3], however these kinds of antennas inherently have a narrow bandwidth [4]. To enlarge the impedance and AR bandwidth of a patch antenna, several methods to obtain a larger bandwidth, such as by [5] concept enhanced impedance bandwidth, by using [6] broadened the AR bandwidth, or stacked patch in

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INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY,

VOL. 2, NO. 1, JANUARY 2007

different dielectric layers [7-8], and based on special feeding techniques. To broaden the impedance bandwidth and the AR bandwidth, a novel broadband SRA is proposed and developed, with the new feeding topology have been designed using the IE3D software from Zeland [9] and realized. In this paper, the microstrip is specified as a perfect electric conductor and the substrates are specified as lossless. The effective wavelength is defined as

2

1

3

2

Feed point

r1r2

(a)

1

4

6

5

(b)

eog ελλ /= , with the effective dielectric

constant given by 2/)1( re εε += . The present design of SRA is making use of a difference permittivity substrate to situate the feed line and the radiate element, respectively. The design objective of the stacked structure patch antenna is to generate a circularly polarized and improve the bandwidth of the conventional patch antenna.

II. ANTENNA DESIGN AND STRUCTURE The antenna structure and the substrate material mainly affect antenna properties such as radiation efficiency, pattern, directivity etc. There are several structures for a circularly polarized antenna with single-feed. To improve upon the impedance and the AR bandwidth, a compact stacked configuration SRA is used as shown in figure 1. The present stacked structure design of SRA is employ a higher permittivity of substrate to situate the bottom layer for the rectangular microstrip feed line and the ground plane, and use a lower permittivity substrate on the top layer for the radiate element, which the higher permittivity

substrate is FR4 with rε of 4.4, loss tangent 0.022, thickness 1.524 mms, and the lower

permittivity substrate is RO with rε of 3.38, loss tangent 0.0025 and thickness 0.508 mms. For the stacked configuration SRA, it is designed for LHCP at a center frequency of 2.1 GHz, the resonance frequency can be approximately

calculated by equating the length from feed point to the end along the ring of the SRA to λ/4. Figure 1: (a) Top-view and (b) Side-view of the proposed antenna To overcome its inherent limitation of narrow impedance bandwidth due to feed networks with quarter-wave transformer or hybrid circuit, in the present paper, the impedance transformer is consists of two elements of the feed line and a cylindrical conductor. The single feed is accomplished by means of an end launch SMA connector that is located on the z-axis, the inner conductor of the coaxial probe pass rectangular feed line by vertical a cylindrical conductor is electrically connected to the upper S-Ring radiating element, while the total length of the impedance transformer is slight less than quarter-wavelength. The radius of the cylindrical connector and the length of the 50 ohms rectangular feed line should be carefully chosen. The present of SRA by adjusting the space of layer separation increase bandwidth. The greater the separation distance the greater the increase in bandwidth. The bandwidth is further increase by selecting a material to fill the separation distancing, which has a low dielectric constant (i.e., ideally 1=the dielectric constant of air).

IJMOT-2006-8-207 © 2007 ISRAMT

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INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY,

VOL. 2 , NO. 1, JANUARY 2007

When the feed point is chosen correctly, by properly adjusting the dimensions of the S-Ring radiating element, the spacing of the open S-Ring gap, and the distances of the radiating element relative to the bottom rectangular microstrip feed line (i.e., the length of the cylindrical conductor), it is possible to cause one of the generated signals to either lead or lag the phase of the other by 90 degrees. So that excited with equal amplitudes and a 90o phase shift, thereby generating a CP signal.

III. SIMULATION AND MEASUREMENT RESULTS

By properly adjusting above the mentions relative elements sizes, then we can obtain a better and wider bandwidth CP antenna. Figure 1 depicts the detailed antenna structure of the SRA, while the simulative optimized parameters of the SRA are list in table I, respectively. The antenna with the proposed geometry in figure 1 is fabricated and measured. The Return Losses (RL) of the antenna was measured by using an HP8720D vector network analyzer. Figure 2 shows the variation of simulated and measured RL with frequency, the optimum impedance bandwidth of the simulated and measured 10 dB RL bandwidths were 23.8% and 23.5% has been obtained when the S-Ring gap is 8.5mm (~0.07λo). Good agreement between measurement and simulation could be noticed, confirming the wide frequency behavior of the new SRA feeding topology. The AR results (simulated and measured) are shown together in figure 3 as a comparison. The measured results AR value less than 3 dB is in excess of 23 % bandwidth in the range of 1.9 GHz to 2.45 GHz was approximate simulation purpose, while the CP gain showed in figure 4 is measured over 5.5 dBi across a frequency band between 1.9 to 2.4 GHz.

Return loss [S(1,1)]

-26-24-22-20-18-16-14-12-10

-8-6-4

1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6

Frequency (GHz)

dB

-26-24-22-20-18-16-14-12-10-8-6-4

Measurement Simulation

Figure 2: Variation of Return Loss with frequency. Table I: The re l a t ive paramete r s o f the SRA configurations (Un i t : mm) 1 S-Ring radiating element r1= 28, r2= 34 2 Air gap 3 Slot Radius = 4,6,4 4 Cylindrical conductor Length = 14.4, Radius = 0.85 Rectangular feed line Length = 16.36, Width= 3 6 SMA connector

The simulated of antenna and radiating efficiency is more than 60% over the matching bandwidth showed in figure 5, while figure 6 depicts the LHCP far field patterns of the antenna.

Axial Ratio

0123456789

101112

1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6Frequency (GHz)

dB

0123456789101112

Simulation Measurement

Figure 3: Variation of Axial Ratio with frequency.

IJMOT-2006-8-207 © 2007 ISRAMT

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INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY,

VOL. 2 , NO. 1, JANUARY 2007

Maximum Gain

0

12

3

4

56

7

1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6Frequency (GHz)

dBi

0

12

3

4

56

7

simulated Measurement

Figure 4: Variation of circular polarization gain with frequency.

Figure 5: The antenna and radiating efficiency. Figure 6: The LHCP far field patterns of the antenna.

IV. CONCLUSIONS We described a method for designing circularly polarized stacked structure antennas with a new feeding topology. The excitation was done at a point from the back of the antenna substrate. The propose impedance transformer permits a better purity of the CP and a wider impedance bandwidth with respect to conventional antennas. The proposed antennas design is not limited to the improvement on the impedance and AR bandwidths of the conventional but also the radiation characteristics such as CP gain.

REFERENCES [1] R. James and P. S.Hall, Handbook of Microstrip

Antennas.London, U.K.:Peter Peregrinus, 1981. [2] P C Sharma and K C Gupta,”Analysis and

optimised design of single feed circularly polarised microstrip antenna,” IEEE Trans. Antennas Propagat., vol. AP-31, No.6, pp.949-955, 1983.

[3] T. C. Edwards, “Foundations for Microstrip Circuit Design,” John Wiley & Sons, USA, 1981.

[4] D. M. Pozar and D. H. Schaubert, Eds., Microstrip Antennas: The Analysis and Design of Microstrip Antennas and Arrays. Piscataway, NJ: IEEE Press, 1995.

[5] C. H. Chen, A. Tulintseff and R. M. Sorbello, “Broadband two-layer microstrip antenna”, in IEEE AP-S International Symposium Digest, 1984, pp. 251-254.

[6] R. B. Waterhouse, “Stacked Patches Using High and Low Dielectric Constant Material Combinations”, IEEE Trans. Antennas and Propagat., vol.47, pp. 1767-1771, Dec. 1999.

[7] Q. GARCÍA GARCÍA, ‘Broadband stacked annular ring’ In proceedings of ICAP’95. Eindoven 1995.

[8] JAMES J. R., HALL P.J. (Eds ; ‘Handbook of Microstrip Antennas’; (Peter Pereg＞ rinus Ltd IEE London 1989).

[9] IE3D 7.0, Zeland Software Inc., Freemont, USA, 2000.

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