NOVEL COMPACT DESIGN OF CPW-FED CIRCULAR PATCH ANTENNA

Jui-Han Lu

Department of Electronic Communication Engineering National Kaohsiung Institute of Marine Technology

Kaohsiung, Taiwan 81 1, R.O.C. Email: jhlu@mail.nkimt.edu.tw

Introduction

Due to low radiation leakages and easily integrated with active devices in the same side of the planar substrate, several related designs of CPW-fed microstrip antenna with different coupling slots have been studied [l-51. However, very few designs for compact microstrip antenna with CPW-fed line have been published in the open literature. The only compact design with CPW-fed line that has been proposed uses meandered slots in the antenna’s ground plane [6]. In this article, first we demonstrate that by increasing the vertical arm’s length of the U-shaped coupling slot connected with coplanar waveguide (CPW) feed line (see Fig. la), the fundamental resonant frequency of the circular microstrip antenna can significantly be lowered. Moreover, by perpendicularly bending vertical portion of the U-shaped coupling slot back to back (see Fig. Ib), which can be considered as a pair of introduced rectangular slots connected with the vertical portion of the U-shaped slot, a modified U-shaped slot is presented in this study. It is found that the proposed design causes more significantly lowering of the antenna’s fundamental resonant frequency. Prototypes of the proposed design have been successfully implemented, and experimental results are described in detail in this Letter.

Antenna Design and Results

Fig. l a shows the geometry of a compact circular microstrip antenna with CPW-fed U-shaped slot, which consists of a pair of vertical slots of dimension of Ls x Ws and a horizontal slot of dimension of L x W connected with the vertical slots. The CPW feed line, designed to be with a 50 il characteristic impedance, has a signal strip (center conductor) of width Wc and a gap Gc between the signal and the coplanar ground. The circular microstrip antenna has a radius of R and is printed on a substrate of thickness h and relative permittivity E,.

And good impedance matching of the proposed compact design can be

@7803-733@8/02 $17.00 0 2002 IEEE 34

obtained by properly adjusting the distance, d, along the resonant direction (y axis) of the fundamental operating frequency of the circular patch, which is away from the horizontal portion of the U-shaped coupling slot. In Fig. lb , a pair of rectangular slots of dimension of L, x W, is introduced and connected with the vertical portion of the U-shaped slot, which can be considered as a modified U-shaped slot in this study.

Prototypes of the proposed antenna were constructed and experimentally studied. It is first found that when the vertical arm’s length of the U-shaped coupling slot is increased, the fundamental operating frequency of the circular microstrip antenna can be significantly decreased. Moreover, from IE3D simulation results, the surface current path in the ground plane can also be lengthened more by introducing a pair of rectangular slots connected with the vertical portion of the U-shaped slot, which causes more significantly lowering of the antenna’s fundamental resonant frequency. By properly adjusting the length and width of the introduced rectangular slot, the fundamental operating frequency of the circular microstrip antenna can be lowered significantly. Fig. 2 shows the measured return loss for the proposed compact circular microstrip antennas with the regular and modified U-shaped coupling slot. It is noted that, for the case of circular patch antenna with W, equal to 14 mm, the resonant frequency is at 1328 MHz, which is only about 0.65 times that of the reference antenna. This suggests that an antenna size reduction as large as about 58% can be achieved for the proposed antenna at a fixed operating frequency. Fig. 3 shows typical measured radiation patterns for the compact circular microstrip antennaoperating at 1300 MHz. It is seen that, due to the coupling slot dimension increased to enhance back radiation, the front to back ratio (F/B) of the proposed compact antenna is found to be about 2dB, which is 4 dB less than that of the regular circular microstrip antenna at the fixed operating frequency. Fig. 4 shows the measured antenna gains in the broadside direction for the proposed compact and regular antenna near 1300 MHz band. It is seen that the gain difference between the proposed compact and regular microstrip antenna is less than 0.4 dB. The compact design can provide similar antenna gain with that of the conventional circular microstrip antenna..

Conclusions

A new compact circular microstrip antenna with a U-shaped coupling slot has been experimentally studied. By increasing the vertical arm’s length and width of the U-shaped coupling slot, the resonant frequency of the circular microstrip antenna is significantly lowered about from 2048 MHz to 1328 MHz, which can lead to about 58% antenna size reduction. And, the proposed compact design provides

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similar antenna gain compared with that of the regular circular microstrip antenna.

References

M. I. Aksun, S. L. Chuang and Y . T. Lo, "Coplanar waveguide-fed microstrip antennas," Microwave Opt. Technol. Lett., vol. 4, 1991, pp. 292-295. W. Menzel and W. Grabherr, "A microstrip patch antenna with coplanar feed line," IEEE Microwave Guided Wave Lett., vol. 1 ,

L. Giauffret, J. M. Laheurte and A. Papiernik, "Study of various shapes of the coupling slot in CPW-fed microstrip antennas," IEEE Trans. Antenna and Propagation, vol. 45, 1997, pp.

S. M. Deng, M. D. Wu and P. Hsu, "Impedance characteristics of microstrip antennas excited by coplanar waveguides with inductive or capacitive coupling slots," IEEE Microwave Guided Wave Lett., vol. 5 , 1995, pp. 391-393. L. Giauffret and J. M. Laheurte, "Parametric study of the coupling aperture in CPW-fed microstrip antennas," IEE Proc. Microw. Antennas Propag., vol. 146, 1999, pp. 169-174. M. C. Pan, "Design of compact rectangular microstrip antenna with a CPW feed," 2001 IEEE AP-S Int. Symp. Dig., Vol. 4, 2001,

199 1, pp. 340-342.

642-647.

pp. 82-85.

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Fig. 1 Geometry of a CPW-fed compact circular microstrip antenna with (a) U-shaped coupling slot. (b) U-shaped coupling slot with vertical slot perpendicularly bended.

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Fig. 2 Measured return loss for the proposed CPW-fed compact circular antenna in Fig. 1 with Ls = 14.0 mm, L2 = 6.0 mm, E,

= 4.7, h = 1.6 mm, R = 20.0 mm, Wc = 5.5 mm, Gc = 0.5 mm, L = 11.0 mm, W = 2.6 mm, Ws = 1 mm, ground-plane size =

75 mm x 75 mm. (a) U-shaped coupling slot with d = 0 mm. (b) modified U-shaped coupling slot with d = 6.0 mm;

Fig. 3 Measured E-plane (y-z plane) and H-plane (x-z plane) radiation patterns for the proposed antenna given in Fig. 2b for Wz = 14 mm.

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Fig. 4 Measured gain against frequency for the compact and regular circular microstrip antennas given in Fig. 3.

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