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An Omnidirectional Planar Microstrip Antenna Randy Bancroft and Blaine Bateman Abstract—A new omnidirectional printed planar microstrip antenna is described. The radiation pattern of this antenna is analyzed numerically with the finite-difference time-domain (FDTD) method and compared with measurement. The length of the antenna can be scaled to increase or decrease gain. The driving point is unbalanced and may be fed directly with a coaxial cable. The impedance at the driving point of the antenna can be to provide a varied by adjusting the width of the radiating elements match. Index Terms—Microstrip antenna, omnidirectional, printed antenna.
I. INTRODUCTION Many 802.11b (2.40–2.50 GHz) and/or 802.11a (5.15–5.35 GHz) wireless applications require an antenna with an omnidirectional pattern. When scaling a conventional antenna solution above 1 GHz, such as the coaxial colinear (COCO) antenna [1] one encounters efficiency degradation which is not significant at frequencies of a few hundred megahertz. An alternative planar omnidirectional antenna solution requires a two layer board for implementation and a matching network [2]. In this paper, we will describe an omnidirectional microstrip antenna which does not suffer from these limitations and its radiation pattern. II. OMNIDIRECTIONAL MICROSTRIP ANTENNA (OMA) The geometry of an OMA is presented in Fig. 1 [3]. The antenna consists of top and bottom traces. The bottom trace starts with a wide trace (W2 ) of length L, then alternates between a narrow and wide sections until a wide section terminates the antenna on the bottom side. The top trace begins with a narrow trace which is shorted to the center of the first wide trace on the bottom. The traces on the top layer alternate from wide to narrow complementing the narrow to wide traces on the bottom layer. The last upper trace is narrow and shorted to the center of the last wide bottom trace. The length L of each section is approximately 0:2750 . The width (W1 ) of the narrow line sections is chosen such that it forms a 50
microstrip line with its opposite side viewed as a groundplane. The wide sections (W2 ) are approximately five times as wide as a narrow trace. III. SEVEN SECTION OMA DESIGN A seven section OMA was designed to operate at 2.45 GHz on 0.762 mm (0.030”) Sheldahl Comclad laminate material. The relative dielectric constant of the substrate is r = 2:6 with a 0:0025 tan . The dimensions of the antenna are: W1 = 2:06 mm, W2 = 16:25 mm L = 36:58 mm. Shorting pins located on either end of the antenna have a 0.5 mm radius. The antenna is fed with a probe at the junction of the first narrow line and the next wide section meet (Fig. 1). The dielectric material extends out 2.0 mm from each side and 2.0 mm from each end.
Manuscript received April 30, 2003; revised October 6, 2003. The authors are with Centurion Wireless Technologies,Westminster, CO 80031 USA (e-mail:
[email protected]). Digital Object Identifier 10.1109/TAP.2004.832338
Fig. 1. Geometry of omnidirectional microstrip antenna.
The driving point impedance may be adjusted to provide a 50 input impedance by varying the value of W2 The current at either end of the antenna reflect from a short at the end of a section of microstrip line which is approximately one quarter wavelength. This keeps the current on each of the wide sections of the antenna all approximately in phase to produce an omnidirectional pattern. The finite-difference time-domain method (FDTD) was used to compute the expected radiation patterns. [4] A sinusoidal 2.586, 2.400, and 2.215 GHz source implemented for 16 periods was utilized to compute the radiation patterns of the antenna over its 2:1 VSWR bandwidth. The patterns are presented in Fig. 2(a)–(c) (2.586 GHz), Fig. 3(a) and (b) (2.400 GHz), Fig. 4(a) and (b) (2.215 GHz) with corresponding measured radiation patterns. The measured patterns are slightly squinted downward compared with the FDTD analysis and diverge from prediction at the low end of the band. It appears the attached feeding cable slightly affects the phase relationship along the array and is the cause of this beam squint and reduction of pattern bandwidth. The small coaxial cable used to feed the array was impractical to model with FDTD. The maximum gain was predicted to be 6.4 dBi versus 4.6 dBi measured at 2.586 GHz. The antenna sidelobes are approximately 011 dB below the main lobe. The optimum match for the antenna is at 2.4 GHz with a 371 MHz 2:1 VSWR impedance bandwidth. The normalized bandwidth is 15.45% which is very good for a printed antenna. The antenna’s driving point is unbalanced and may be directly fed with a coaxial cable.
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(c) Fig. 2. (a) y –z plane radiation patterns of OMA computed using FDTD analysis (dashed) and measured (solid) 2.586 GHz. (b) x–y plane radiation patterns of OMA computed using FDTD analysis (dashed) and measured (solid) 2.586 GHz. (c) x–z plane radiation patterns of OMA computed using FDTD analysis (dashed) and measured (solid) 2.586 GHz.
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Fig. 3. (a) y –z plane radiation patterns of OMA computed using FDTD analysis (dashed) and measured (solid) 2.400 GHz. (b) x–y plane radiation patterns of OMA computed using FDTD analysis (dashed) and measured (solid) 2.400 GHz.
IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 52, NO. 11, NOVEMBER 2004
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Fig. 4. (a) y –z plane radiation patterns of OMA computed using FDTD analysis (dashed) and measured (solid) 2.215 GHz. (b) x–y plane radiation patterns of OMA computed using FDTD analysis (dashed) and measured (solid) 2.215 GHz.
IV. CONCLUSION An omnidirectional microstrip antenna was described which can be scaled to control its gain properties and has a very omnidirectional pattern. The fabrication process is simple and low cost.
Broad-Band Double-Layered Coplanar Patch Antennas With Adjustable CPW Feeding Structure K. F. Tong, K. Li, T. Matsui, and M. Izutsu
REFERENCES [1] T. J. Judasz and B. B. Balsley, “Improved theoretical and experimental models for the coaxial colinear antenna,” IEEE Trans. Antennas Propagat., vol. AP-37, pp. 289–296, Mar. 1989. [2] Iwasaki and Hisao, “A microstrip array antenna with omnidirectional pattern fed by CPW,” in Proc. IEEE Int. Antennas and Propagation Symp. Dig., vol. 34, July 1996, pp. 1912–1915. [3] “U.S. Patent Pending,” U.S. Patent Application 60/461 689. [4] K. S. Yee, “Numerical solution of initial boundary value problems involving Maxwell’s equations in isotropic media,” IEEE Trans. Antennas Propagat., vol. AP-14, pp. 302–307, 1966.
Abstract—In this paper, we have presented the double-layered coplanar patch antennas of enhanced impedance bandwidth and adjustable conductor-backed coplanar waveguide feed lines. The proposed structure retains the advantage of laying the coplanar patch and coplanar waveguide (CPW) feed line on the same surface, which makes direct integration with other devices easier. In addition, the substrate thickness of the radiating patch can be adjusted to achieve a wider impedance bandwidth while the dimensions of the CPW feed line are kept unchanged. Simulation has been done by using commercial electromagnetic (EM) simulation software. Four testing antennas, which have centre frequency at about 10 GHz, were designed. The four testing antennas had the same total thickness, but different thickness combinations. From the measured return loss, gain, and radiation patterns of the antennas, it was demonstrated that different thickness combinations do not affect the characteristics of the antennas seriously. Therefore, the dimensions of the CPW feed structure of the antennas can be adjusted individually and can be selected for different applications. Index Terms—Broad-band antenna, coplanar waveguides feed line, planar antenna.
I. INTRODUCTION Coplanar waveguide (CPW) fed antennas [1]–[6] have advantages such as low radiation loss, less dispersion and uni-planar config-
Manuscript received June 23, 2003; revised November 26, 2003. The authors are with the National Institute of Information and Communications Technology, Tokyo 184-8795, Japan (e-mail:
[email protected]). Digital Object Identifier 10.1109/TAP.2004.834392
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